Rotational Spectra and Structures of Diethanolamine And

ROTATIONAL SPECTRA AND STRUCTURES OF DIETHANOLAMINE AND

2-AMINOPHENOL

A thesis submitted To Kent State University in partial fulfillment of the requirements for the Degree of Master of Science

Gretchen Renee Laubacher

August 2011

Gretchen Renee Laubacher B.S., University of Dayton, 2009 M.S., Kent State University, 2011

Approved by

Michael Tubergen , Advisor

Michael Tubergen , Chair, Department of Chemistry

John R.D. Stalvey , Dean, College of Arts and Sciences

Table of Contents

Page List of Figures v List of Tables vii Acknowledgments ix Chapter 1. Introduction………………………………………………………… 1 2. Theory of Rotational Spectroscopy………………………………… 2 2.1 Mechanics of Rotating Bodies 2 2.2 Angular Momentum 5 2.3 Classes of Rigid Rotors 7 2.3.1 Symmetric Top Molecules 10 2.3.2 Asymmetric Top Molecules 11 2.4 Centrifugal Distortion 17 2.5 Nuclear Hyperfine Structure 19 2.6 Electronic Structure Calculations 21 2.6.1 Molecular Orbital Ab initio Models 21 2.6.2 Electron Correlation Methods 26 2.7 Structural Information 27 2.7.1 Comparison with High Level Theoretical Models 27 2.7.2 Kraitchman Isotopic Substitution 28 3. Experimental………………………………………………………… 31 3.1 History of Microwave Spectroscopy 31 3.2 Instrument 32 3.2.1 Background Information 32 3.2.2 Fabry-Perot Resonant Cavity 33 3.2.3 Supersonic Jet Expansion 34 3.2.4 Pulsed Radiation and Fourier Transformation 38 3.3 Experimental Initiation 38 3.4 Spectrometer Used for Current Research 39 3.4.1 Microwave Circuit 39 3.4.2 Sensitivity of Instrument and Basic Components of 42 Instrument 3.5 Synthetic Methods 44 3.6 Computational Methods 47 4. Diethanolamine……………………………………………………… 48 4.1 Background Information 48 4.2 Theoretical Modeling 50 4.2.1 Previous Theoretical Modeling 50 4.2.2 Current Theoretical Modeling for this Research 51

iii

4.3 Spectra and Hamiltonian Fitting 63 4.3.1 Spectrum A of Diethanolamine 63 4.3.2 Spectrum B of Diethanolamine 65 4.3.3 Discussion of Structure 68 4.4 Diethanolamine Isotopic Study 72 4.4.1 Structural Modeling of Deuterated Isotopomers 73 4.4.2 Spectra and Hamiltonian Fitting 74 4.4.2.1 Deuterated-Diethanolamine Spectrum A 74 4.4.2.2 Deuterated-Diethanolamine Spectrum B 77 4.4.2.3 Deuterated-Diethanolamine Spectrum C 80 4.4.2.4 Deuterated-Diethanolamine Spectrum D 82 4.4.3 Discussion 85 5. 2-aminophenol………………………………………………………. 92 5.1 Background Information 92 5.2 Theoretical Modeling 93 5.3 Spectrum and Hamiltonian Fitting 98 5.4 Discussion of Structure 101 6. Conclusion………………………………………………………… 103 References 105

LIST OF FIGURES

Figure Page 1. Illustration of a spherical top molecule, carbon tetrachloride 8 2. Illustration of a prolate symmetric top molecule, chloromethane 8 3. Illustration of an oblate symmetric top molecule, benzene 9 4. Illustration of an asymmetric top molecule, water 9 5. Correlation diagram for asymmetric rotor state energies between the 16 oblate and prolate limits 6. Schematic illustrating randomness to directed flow seen in a supersonic 36 jet expansion 7. Schematic of instrument used in this research 41 8. Representative sample of 606-505 transition for diethanolamine 43 9. Picture of instrument used in this research 45 10. Deuteration sites for diethanolamine 46 11. Structure of diethanolamine 48 12. Structure of N-nitrosodiethanolamine 49 13. Structure of diethanolamine at B3LYP/6-311++G(d,p) level 50 14. Molecular structure and definitions of torsional angles for 55 Diethanolamine 15. Conformer ec1 of diethanolamine from MP2/6-311++G(d,p) level 61 16. Conformer ec2 of diethanolamine from MP2/6-311++G(d,p) level 62 17. Conformer ec3 of diethanolamine from MP2/6-311++G(d,p) level 62 18. Conformer ec4 of diethanolamine from MP2/6-311++G(d,p) level 63 19. Rotational transition of the 616-515 transition of Spectrum A for 67 Diethanolamine 20. Rotational transition of the 606-505 transition of Spectrum A for 67 Diethanolamine 21. Rotational transition of the 616-515 transition of Spectrum B for 70 Diethanolamine 22. Rotational transition of the 615-514 transition of Spectrum B for 71 Diethanolamine 23. Rotational transition of the 717-616 transition of d-DEA Spectrum A for 76 Diethanolamine 24. Rotational transition of the 716-615 transition of d-DEA Spectrum A for 76 Diethanolamine 25. Rotational transition of the 707-606 transition of d-DEA Spectrum B for 77 Diethanolamine 26. Rotational transition of the 817-716 transition of d-DEA Spectrum B for 77 Diethanolamine

27. Rotational transition of the 615-514 transition of d-DEA Spectrum C for 81 Diethanolamine 28. Rotational transition of the 818-717 transition of d-DEA Spectrum C for 81 Diethanolamine 29. Rotational transition of the 616-515 transition of d-DEA Spectrum D 84 for diethanolamine 30. Rotational transition of the 717-616 transition of d-DEA Spectrum D 84 for diethanolamine 31. Deuteration sites on Conformer ec1 of diethanolamine 87 32. Deuteration sites on Conformer ec2 of diethanolamine 90 33. Structure of 2-aminophenol 92 34. Lowest energy cis-conformer of 2-aminophenol determined by 94 Soliman et al. 35. The trans conformer determined from Soliman et al. 94 36. The gauche conformer determined from Soliman et al. 95 37. Cis Conformer determined from RHF/6-311++G(d,p) level 97 38. Trans Conformer determined from RHF/6-311++G(d,p) level 97 39. 515-404 rotational transition of 2-aminophenol 100 40. 505-414 rotational transition of 2-aminophenol 100

LIST OF TABLES Table Page 1. Four classes of nonlinear rigid rotors 7 2. Asymmetric rotor energy levels of J=1 and 2 15 3. Configuration and relative energies of diethanolamine conformers 52-54 at RHF/6-31G(d) level 4. Configuration and relative energies of diethanolamine conformers 56-58 at RHF/6-311++G(d,p) level 5. Configuration and relative energies for four lowest energy 60 conformers at MP2/6-311++G(d,p) level 6. Relative dipole moments and rotational constants for conformers at 60 MP2/6-311++G(d,p) level 7. Experimental rotational constants determined for Spectrum A of 64 Diethanolamine 8. Rotational transitions of Spectrum A of diethanolamine 66 9. Experimental rotational constants for Spectrum B of 68 Diethanolamine 10. Rotational transitions for Spectrum B of diethanolamine 69 11. %ΔIrms values for conformers compared with Spectrum A 71 12. %ΔIrms values for conformers compared with Spectrum B 72 13. Possible deuteration sites for Conformers ec1 and ec2 73 14. Rotational transition frequencies and relative quantum numbers 75 for d-DEA Spectrum A 15. Experimental rotational constants for d-DEA Spectrum A 77 16. Experimental rotational constants for d-DEA Spectrum B 77 17. Rotational transition frequencies and relative quantum numbers 78 for d-DEA Spectrum B 18. Rotational transition frequencies and relative quantum numbers 80 for d-DEA Spectrum C 19. Experimental rotational constants for d-DEA Spectrum C 82 20. Rotational transition frequencies and relative quantum numbers 83 for d-DEA Spectrum D 21. Experimental rotational constants for d-DEA Spectrum D 85 22. %ΔIrms and rotational constants for possible deuterated 86 -diethanolamine molecules with d-DEA Spectrum A 23. Possible deuterated-diethanolamine molecules with deuterated 87 label and relative deuteration site on Conformer ec1 24. %ΔIrms and rotational constants of possible deuterated 88 diethanolamine molecules with d-DEA Spectrum B 25. %ΔIrms and rotational constants of possible deuterated- 89 diethanolamine molecules with d-DEA Spectrum C

vii

26. Possible deuterated diethanolamine molecules with deuterated 90 label and relative deuteration site on Conformer ec2 27. %ΔIrms and rotational constants of possible deuterated 91 diethanolamine molecules with d-DEA Spectrum D 28. Relative energies for the cis, trans and gauche conformers at 95 various levels of theory 29. Principal dipole moments and relative energy for the Cis and 98 Trans Conformers determined from RHF/6-311++G(d,p) level of theory 30. Quantum numbers and relative frequencies for 2-aminophenol 99 Spectrum 31. Rotational constants of experimental spectrum with their relative 99 uncertainties 32. Results of %ΔIrms for Cis and Trans Conformers with compared 102 with Experimental Spectrum

viii

ACKNOWLEDGMENTS

My sincere appreciation goes to God, whose inspiration and divine direction made it possible for me to accomplish this task.

My inestimable appreciation goes to my family members for their moral advice and financial support throughout my studies. Without them, I would have been forever stressed. Thank you: mom, dad and siblings for putting up with me in my less than cheerful moments.

My profound gratitude goes to my advisor, Dr. Michael Tubergen, for making this thesis possible through his wealth of academic experience in microwave spectroscopy. I joined the Tubergen laboratory with no prior knowledge of microwave spectroscopy, but was never made to feel inferior. Everything I know is owed to Dr. Tubergen. Thank you.

My appreciation also goes to Andy Conrad, Heather Seedhouse, Katelin Byerly, and

Ashley Fox. Without Andy’s help and guidance throughout my studies here at Kent

State, I would have been lost. His patience and helpful advice was greatly appreciated.

Heather Seedhouse was a joy to work with, and through her I made a great friend both in the lab and out of the lab. Together Katelin Byerly and I accomplished the 2- aminophenol project.

1. Introduction

When I entered graduate school in the fall of 2009, I knew I wanted to do research on environmental molecules. I began interviewing the possible research advisors in order to decide which professor I would work with. Upon talking to Dr. Tubergen, I discovered that his research dealt with gas-phase molecules. Many environmental molecules are gases, as many are volatile organic compounds. Therefore, I chose to do my research with Dr. Tubergen and work with gas- phase molecules using microwave spectroscopy.

Microwave spectroscopy involves the rotational transitions of gas-phase molecules.

After doing a great deal of research on possible gas-phase molecules to study, I chose the molecule diethanolamine. Diethanolamine is an environmentally hazardous compound that is actually a liquid at room temperature. It can easily be vaporized by heating the sample. I chose diethanolamine because it had a number of case studies that illustrated the potential dangers of the molecule. Some of these dangers involved lung issues with workers at the production facilities. By studying diethanolamine using microwave spectroscopy, the rotational spectra of the molecule can be determined. These rotational spectra are unique to diethanolamine, and could eventually be inputted into a databank containing the rotational spectra of environmental molecules. The future goal of microwave spectroscopy is to develop the instrument as a means of remote sensing for these environmental molecules near production plants.

2. Theory of Rotational Spectroscopy

2.1 Mechanics of Rotating Bodies

In microwave spectroscopy, the rotational energy level transitions are identified through the absorption of the microwave radiation, which falls between 1 GHz and 1000

GHz in frequency. For a molecule, the microwave frequencies that are equal to the energy between two rotational states will be absorbed. The energy transitions are a direct result of the dipole components of the molecule; without a dipole moment, the molecule will not have an observed rotational spectrum.1 From this rotational spectrum, structure characteristics, such as bond lengths and atomic distances can be identified for the molecule.

For microwave spectroscopy, it is useful to begin the theory behind the process by starting with the classical expressions of angular momentum and rotational energy. In addition to examining angular momentum and rotational energy, it is also worthwhile to examine the classic moments of inertia.1 The classical angular momentum of a rigid system of particles is represented by Equation 2.1 below:

(2.1) where ω is the angular velocity and I is the moment of inertia tensor. I is expressed as:

(2.2)

Where

(2.3)

(2.4)

(2.5)

(2.6)

(2.7)

(2.8)

Here m represents the mass of an atom, and x, y and z are the atom’s coordinates in a rectangular coordinate system, with the origin at the center of mass. From this, the matrix I of the rotational inertia tensor is established:

(2.9)

So overall, the classical rotational kinetic energy contains three diagonal terms and six off-diagonal terms. The inertia tensor can be diagonalized by rotating the xyz axes into a new body-fixed coordinate system with orthonormal axes a, b and c, which are known as the principal axes; from this, the principal moments of inertia are developed.2 By convention, Ia is the smallest moment, while Ic is the largest. In the case where a, b, and 4

c coordinates represent the principal axis system, the components of angular momentum become:1

(2.10-2.12)

Through these new coordinates, the rotational kinetic energy is also diagonalized:3

(2.13)

(2.14)

So, from this, the classical Hamiltonian becomes:2

(2.15)

Where A, B, and C are the three rotational constants. Therefore, the three rotational constants, with units of Hertz (Hz), are defined as:3

(2.16)

(2.17)

(2.18)

Through microwave spectroscopy, the structural information of a molecule is studied. The structural information is found in the principal moments of inertia, Ia, Ib, and Ic. The principal moments of inertia are derived from the spectroscopic constants A,

B and C: 5

(2.19)

(2.20)

(2.21)

With

(2.22)

Where Ia is in units atomic mass units- and the rotational constants are in the units megahertz (MHz). In order to relate the moments of inertia to the structure of the molecule, one must first assign the rotational spectrum of the molecule. Assigning the spectrum involves relating the observed frequencies to the quantum numbers of the transition. To obtain further data on the structure of the molecule, often isotopic studies of the molecule are performed.1

2.2 Angular Momentum

The Hamiltonian operator is expressed in terms of angular momentum; thus, a short introduction to angular momentum is useful for better understanding of the basis for the solution of the Schrödinger equation. The angular momentum operators for the space-fixed axis system are defined as:

(2.23)

(2.24)

(2.25)

(2.26)

Where PA, PB and PC are component operators of angular momentum in the space-fixed axis system, P2 is the operator conjugate to the square of the total angular momentum,

1/2 1 2 i=(-1) , and ħ=h/2π. The operators P and PC commute, and thus have a common set of orthogonal eigenfunctions, which are designated by ΨJ,M. Through manipulations

2 involving the raising and lowering operators, the quantized eigenvalues of P and PC are obtained: 2J(J+1) and M respectively. In these cases, J is the quantum number for angular momentum, having the values of 0, 1, 2, 3,… and M is the component of angular momentum along the z axis of the space-fixed axis system, having the values of J, J-1, J-

2,…,-J When considering the component of angular momentum Pc (lower case subscript

2 c) in the body-fixed axis system, the components P , PC, and Pc also commute, giving the

1 2 eigenfunctions designated as ΨJ,K,M. In this case, the eigenvalues for P and PC are the same, while the those for Pc are found to be K where K=J, J-1, J-2,…,-J and is the component of angular momentum along the z axis of the body-fixed axis system.

Overall, these eigenvalues provide the building blocks for the solution to the Schrödinger equation for symmetric and asymmetric top molecules.

2. 3 Classes of Rigid Rotors

There are four classes of nonlinear rigid rotors:2

Table 1: Four Classes of Nonlinear Rigid Rotors

spherical top (Fig. 1)

prolate symmetric top (Fig. 2)

oblate symmetric top (Fig. 3)

asymmetric top (Fig. 4)

Since there are these separate classes of molecules based on the magnitude of the principal moments of inertia, there are different approaches to solving the Schrödinger equation. For the spherical tops, where Ia=Ib=Ic, there is no dipole moment, and thus no observable microwave spectrum. Therefore it is unnecessary to discuss the quantum mechanical properties of the spherical top molecules.

Fig. 1: An Illustration of Spherical Top Molecule, Carbon Tetrachloride

Fig. 2: An Illustration of a Prolate Symmetric Top Molecule, Chloromethane

Fig. 3: An Illustration of an Oblate Symmetric Top Molecule, Benzene

Fig. 4: An Illustration of an Asymmetric Top Molecule, Water

2.3.1 Symmetric Top Molecules

Symmetric top molecules have two equal rotational moments of inertia, with a third unique moment associated with rotation about the axis of highest symmetry.1

Prolate symmetric tops tend to be cigar-shaped, while oblate symmetric tops tend to resemble disks. For prolate symmetric top molecules, the Hamiltonian operator simplifies to:1

(2.27)

In this equation, the Hamiltonian operator is in terms of angular momentum, where the eigenvalues are already known. Therefore, the solution to the Schrödinger equation for rigid prolate symmetric top molecules becomes:1

(2.28)

Here, the rotational energies are expressed in the J and K quantum numbers. The equation can be simplified further to:1

(2.29)

2 2 2 2 Where A=h /8π Ia and B=h /8π Ib. Due to the positive value of (A-B), the energy of a level increases with increasing K. The selection rules and absorption frequencies for the symmetric top molecule are given below:1,4

and (2.30)

(2.31) 11

For the oblate symmetric top molecule, the Hamiltonian is expressed as:

(2.32)

As before, the known eigenvalues of this Hamiltonian lead to any easy solution to the

Schrödinger equation, where the rotational energy levels for the oblate symmetric top are:3

(2.33)

Here again, B and C represent the rotational constants, while J and K are the quantum numbers for angular momentum and the component of angular momentum in the body- fixed axis system, respectively.1 For oblate symmetric top molecules, the energy level decreases with increasing K, since (C-B) has a negative value.1 However, for the asymmetric top, where none of the rotational moments of inertia are equal, no simplification of the Hamiltonian is possible.1,2

2.3.2 Asymmetric Top Molecules

1 The parameter, , is used to describe the behavior of the asymmetric top:

(2.34)

Where A, B, and C are the rotational constants corresponding to the a, b, and c axes, respectively. In a prolate symmetric-top, , has the value of -1, while for the oblate symmetric-top, =+1. The more asymmetric a molecule is, the closer  is to zero.1

1 In the asymmetric top molecule, where Ia≠Ib≠Ic, the Hamiltonian is expressed as: 12

(2.35)

In this equation, the rotational constants A, B, and C, are expressed in units of angular momenta. By rearranging the Hamiltonian expression, easier calculations can be made:1

(2.36)

Where

(2.37)

(2.38) and  is defined as previously. From here, the solution of the Schrodinger equation can be expressed through the use of the linear combination of the symmetric top wavefunctions:

(2.39)

Where the cn terms are numerical constants and Ψn is a member of the orthonormal set of symmetric rotor wavefunctions. From here, the Schrodinger equation now can take the following form:

(2.40)

By multiplying the equation by the complex conjugate and then integrating, the equation simplifies to:

(2.41)

For orthonormal basis wavefunctions, the value of ∫Ψm*Ψndτ will always equal zero when m≠n and will always equal one when m=n. A delta function can be used to simplify the equation:

(2.42)

Where δm,n=1 when m=n and δm,n=0 when m≠n. This set of linear equations has a non- trivial solution only if the determinant of the coefficients equals zero as described by

(2.43)

To solve this equation for the values of E, the matrix elements must be found. These values are the eigenvalues of the asymmetric top Hamiltonian operator, and from these one can calculate the ratios of the coefficients (cn).

Another way to find the energies of a system is by using a matrix. The matrix below represents the action of the asymmetric top Hamiltonian on an infinite set of symmetric top wavefunctions:

(2.44)

The matrix can be diagonalized to generate the eigenvalues of the asymmetric top wavefunctions.1 14

The wave functions of the asymmetric top can also be expressed as the linear combination of symmetric top wave functions. Here, the new function can illustrate the oblate and prolate character of the molecule:

(2.45)

For asymmetric tops, a loss of degeneracy in K leads to an increase in complexity of the rotational energy levels (Fig. 5).2 For a totally asymmetric molecule, the loss of degeneracy in K for the J=1 state is expressed by the K values equal to -1, 0, and +1. For totally symmetric molecules, there is no splitting of K levels. The prolate case is shown to increase in energy as K increases, while the oblate case decreases in energy as K increases.

A closed energy expression for partial asymmetry character can be developed using the expression .1 The energy for a slightly asymmetric near oblate top is expressed as:1

(2.46)

Where

(2.47)

(2.48)

For the near prolate asymmetric case 15

(2.49)

Where

(2.50)

(2.51)

Table 2 gives the asymmetric top energy levels with J=1 and 2.5

Table 2: Asymmetric Rotor Energy Levels of J=1 and 2

JKAKC E(A,B,C)

000

110

111

101

220

221

211

212

202

Fig. 5: Correlation diagram for asymmetric rotor state energies between the oblate and prolate limits.

2.4 Centrifugal Distortion

Centrifugal distortion results from the rotation of a non-rigid system, and further complicates the matter of the rotational energy level separations. As the molecule rotates faster and faster, the bonds are stretched further and further. Since the moments of inertia are inversely related to the rotational constants, as the bonds are further stretched, the rotational constants are decreased. So as the molecule spins faster and faster, the bond length increases, the rotational constants decrease, and the spacing between the rotational energy levels decrease.

While the centrifugal distortion must be taken into account when considering rotational transitions, the effect is small. Through the use of distortion terms, the energy levels for non-rigid systems will be more accurately defined. Perturbation theory is used to express the energy levels in these distorted systems.1 Using perturbation theory, the

Hamiltonian operator is expressed as:

(2.52)

1 Where represents the Hamiltonian for the rigid rotor top:

(2.53)

1 And d represents the perturbation due to the rotational distortion of the molecule:

(2.54)

Where α and β become a, b, or c in the body-fixed axis system. Angular momenta are represented by Pα and Pβ, while τααβ are molecular constants, which are dependent on the inverse elements of the matrix of the corresponding force constants.

The energy expression using the first order perturbation theory becomes:

(2.55)

Where Er represents the unperturbed energy and Ed represents the energy as a result of the centrifugal distortion.1 As an example, the energy levels of a distortable oblate symmetric top molecule are:

(2.56)

1,4 Where DJ, DJK, and DK are quartic distortion constants. The frequency of transition for symmetric top molecules is then:4

(2.57)

Perturbation theory can also be used to arrive at the energy solution of an asymmetric top molecule:

(2.58)

1 Where the d coefficients are all distortion constants and Er is energy of the rigid rotor.

Overall, first order perturbation theory accounts for the changes caused by centrifugal distortion. 19

2.5 Nuclear Hyperfine Structure

Nuclear hyperfine structure in molecular rotational spectra may be due to the combination of a few interactions. The magnetic and electric interactions of the molecular fields with the nuclear moments may cause the hyperfine structure. However, for hyperfine structure to arise, the molecule must have quadrupole moments. Since spherically symmetric molecules with nuclear spins of 0 or ½ do not have quadrupole moments, hyperfine structure is not seen in these molecules.1 Nonspherical molecules generate a quadrupole moment, due to the nonspherical distribution of nuclear charge.

The electric field interactions arise from this nonspherical distribution of charge. For gases, the rotational state of the molecule determines the field gradients of the nucleus; therefore, the nuclear quadrupole interaction differs for each rotational state and leads to a hyperfine structure of the rotational levels.1

The symbol χ represents the quadrupole coupling constants, and is defined as:

(2.59)

Where e is the electron charge, Q is the nuclear quadrupole moment and q is the charge distribution in the molecule.1

Furthermore, Q gives the measure of deviation of nuclear charge from a spherical shape, and is expressed as:

(2.60)

2 2 2 2 1 Where ρ is the nuclear charge density, zn is the nuclear axis and ρ =xn +yn +zn . When

Q=0, the molecule would have a completely spherical distribution of nuclear charge, resulting in no quadrupole interaction. Positive Q values result in a cigar shape distribution of nuclear charge, while negative Q values result in a disk-like distribution of nuclear charge.1

The molecular charge distribution is represented by q, and is a function of the magnitude of rotation in the system. In the principal-axis system, q is expressed as:1

(2.61)

1 Where  are the direction cosines of the principal axes, and qaa, etc are constants.

The matrix elements of the Hamiltonian operator can be derived using the expressions Q and q:1

(2.62)

The general expression for a system with one quadrupole nucleus is determined by evaluating the bracketed terms:1

(2.63)

Where

(2.64) 21

In this energy expression, F, a new quantum number, is introduced. F represents the total angular momentum, which is a result of the coupling of the nuclear spin, I, and the molecular rotational momentum, J. F can have values in the range:1

(2.65)

2.6 Electronic Structure Calculations

2.6.1 Molecular Orbital Ab initio Models

Ab initio calculations model the wavefunctions using only the fundamental constants and atomic number of the nuclei. In molecular orbital theory, one-electron functions known as orbitals, are used to approximate the wavefunctions of molecular quantum mechanics problems. A molecular orbital, ψ(x,y,z), is a function of the

Cartesian coordinates x, y and z of an electron; the square of the molecular orbital, ψ2, refers to the probability distribution of the electron in space.6 Electronic spin must also be considered; it assumes the values of s=± ½. The complete wavefunction for a single electron is defined as the product of a molecular orbital, ψ(x,y,z), and a spin function,

α(s); this product is termed a spin orbital, χ(x,y,z,s).

Other properties of molecular orbital wavefunctions are worth noting. The orbitals must be orthogonal:6

(2.66)

where Sij is the overlap integral and ψi* is the complex conjugate. The orbitals can also

2 6 be normalized by multiplying the individual ψi by a constant (N ):

(2.67)

The full many-electron orbital wavefunction for a closed-shell ground state n- electron system doubly occupying n/2 orbitals is represented as ψ, which is built from the spin orbitals with a Slater determinant:

(2.68)

Here, the molecular orbitals, ψi, are built from a set of one-electron functions called basis functions, which are represented below in Equation 2.69:6

(2.69)

Here, cμi represents the molecular orbital expansion coefficients, and i are the one- electron basis functions that constitute the basis set. The accuracy of the molecular orbtials improves as the number of basis functions increases,6 but the increase in basis functions comes at a high computational cost. To alleviate such high computational cost, the optimal basis set must be selected.

In this work, the basis functions used were Gaussian-type basis functions. A

Gaussian function is one where there is a radial dependence of where r is the distance from the function’s center (normally the atom’s position in the molecule) and α 23

is a constant determining the size of the function.6,7 As an example, the s-type basis

6 function μ is composed of primitive s-type Gaussians:

(2.70)

where dμs are fixed coefficients and gs are primitive Gaussian functions. In this work,

Pople split valence sets are used, which are designated by a k-nlG or k-nlmG naming scheme. Here, the k refers to the number of primitive Gaussian-type orbitals for the core orbitals, while nl (split valence) and nlm (triple split valence) indicate the number of

Gaussians used to describe the valence orbitals. In the split valence basis, n describes the number of Gaussians for the inner part of the valence orbitals and l for the outer part.

Likewise, for the triple split valence, n refers to the inner Gaussians, l with the middle

Gaussians, and m with the outer Gaussians in describing the valence orbitals.8

Diffuse and polarization functions can be added to these basis sets. Diffuse functions, such as s- and p-type functions, are indicated by a + or ++ prior to G in the basis set label. A single + refers to the use of one set of diffuse s- and p-functions added to the heavy atoms; while a ++ indicates both a set of diffuse s- and p-functions to the heavy atoms, as well as an additional diffuse s-function added to the hydrogens. In this work, the largest basis set used was the 6-311++G(d,p) basis set, which includes six

Gaussians for the core orbitals, three Gaussians for the inner valence orbitals, one

Gaussian for the middle valence orbitals, and one Gaussian for the outer valence orbitals.

In addition, this basis set has a diffuse sp-function on the heavy atoms, a diffuse s- 24

function for the hydrogens, a d-type polarization function on the heavy atoms, and a p- type polarization function on the hydrogens.

In order to solve the electronic Schrödinger equation, the expansion coefficients of the basis functions must be determined. The Hartree-Fock self-consistent field method, which is based on the variational method in quantum mechanics, is used to find them.6,8 Given any antisymmetric normalized function of the electronic coordinates Φ, the expectation value of the energy belonging to the function is:6

(2.71)

If Φ= ψ, then Φ will satisfy the Schrödinger equation and the expectation value of the energy E’ will be the exact energy E:6

(2.72)

If Φ is any other normalized antisymmetric function, the expectation value of the energy

E’ is greater than the exact energy E:6

(2.73)

By adjusting the expansion coefficients to minimize the expectation value of energy E’, the expansion coefficients cμi of the basis set can then be determined; this is accomplished by an iterative minimization procedure using the Hartree-Fock equation for a single spin orbital χi: 25

(2.74) where εi is the spin orbital energy and f1 is the Fock operator:

(2.75)

Where, h1 is the core Hamiltonian, Ju is the Coulomb operator that accounts for

Coulombic repulsions between electrons, and Ku is the exchange operator that accounts for the modification of this energy due to spin correlation. In practice, a starting set of orbitals is used to form the Fock operator; the Hartree-Fock equations are then solved to obtain a new set of spin orbitals. The process is repeated as many times as necessary until convergence criteria are met.

In this work, molecular geometry optimization problems are the computational problems that need to be solved. Gradient methods, where the energy with respect to nuclear coordinates is minimized to determine a local minimum, determine the molecular geometries. A stationary point is found by varying the molecular geometry until the forces acting on a nucleus by electrons and other nuclei are zero.

While Hartree-Fock ab initio methods consider the Coulombic interactions, they do so by approximating electron-electron interactions by an average interaction. As a result, this method does not consider instantaneous Coulombic interactions between electrons.8 To account for these effects, electron correlation methods are used.

2.6.2 Electron Correlation Methods

Møller-Plesset perturbation theory, which is used in this work, incorporates instantaneous electron correlation in the electronic structure calculations. The correlation energy is defined as the difference in energy between the lowest possible energy in a given basis set and the energy obtained using Hartree-Fock methods.8 Møller-Plesset perturbation theory determines the correlation energy by applying a perturbation to a zero-order electronic Hamiltonian, 0, which is the sum of the one electron Fock operators:8

(2.76)

The perturbation (1) is given by:8

(2.77) where is the original wavefunction approximation (here often Hartree-Fock wavefunctions are used). Therefore, the perturbation is readily obtained via the difference between and 0.

The ground state Hartree-Fock energy (EHF) of a normalized wavefunction ψ0 is:

(2.78)

The Hartree-Fock energy is improved with the second-order perturbation, which is defined as: 27

(2.79)

0 where ψJ is a multiply excited wavefunction that is an eigenfunction of with

(0) eigenvalue of EJ . This second-order correction to energies is defined as second-order

Møller-Plesset perturbation theory, more commonly symbolized as MP2.

2.7 Structural Information

2.7.1 Comparison with High Level Theoretical Models

The spectral patterns of molecules can be predicted through application of the quantum mechanical relationships. When the microwave radiation interacts with the gas phase molecule, rotational transitions occur. After a number of rotational transitions are observed, the rotational constants and centrifugal distortion constants can be fit to the measured frequencies. Using these constants, the moments of inertia and bond distances can be obtained. The most direct and simplest way of characterizing molecular structures from rotational spectra is to assign the rotational spectra to a theoretical structure that is determined through high level ab initio calculations. The root mean square difference in the moments of inertia, ΔIrms, showcases the differences between the moments of inertia of the theoretical structure compared to the moments of inertia to the experimental structure. This method of assignment is based upon minimizing the ΔIrms value. The

ΔIrms value is defined below in Equation 2.80: 28

(2.80)

Where Ia,exp and Ia,theo are the experimental and theoretical moments of inertia, respectively. Since the ΔIrms value is biased towards larger values for the moments of inertia, the root mean square average of the percent relative differences in moments of

9 inertia (%ΔIrms) is preferred:

(2.81)

Assignment of spectra to theoretical structures is often a matter of simply selecting the theoretical structure with the smallest %ΔIrms value. However, in the event where the assignment is not obvious from the %ΔIrms value, additional information may need to be taken into account. In this case, dipole moments, relative energies, and quadrupole coupling constants are also considered in the assignment process. Additional information on the molecular structure can be determined through isotopic substitution.

2.7.2 Kraitchman Isotopic Substitution

A convenient method to calculate the position of an atom in a molecule is the

Kraitchman method.1 The Kraitchman method uses the changes in moments of inertia that are the result of a single atom isotopic substitution. In this method, the principal axis coordinates of the atom are determined, which allows for the determination of the 29

structure of the molecule. When an isotopic substitution of an atom is made, the elements of the intertial tensor, I, are changed:1

(2.82)

Where m+Δm is the mass of the isotopic atom, and m is the original mass of the atom. M represents the total mass of the parent molecule, and μ represents the reduced mass of the isotopic substitution:

(2.83)

Likewise, the other elements of I become:

(2.84)

(2.85)

(2.86)

(2.87)

(2.88)

As before, diagonalization of the inertial matrix will yield the principal moments of inertia for the substituted molecule.

When using Kraitchman’s equations, one must know the moments of inertia for the unsubstituted system and the moments of inertia for the isotopically substituted system. For a molecule with known moments of inertia Ia, Ib and Ic for the parent 30

isotopomer and Ia’, Ib’, and Ic’ for the substituted isotopomer, the a coordinate is given as:1

(2.89)

Where

(2.90)

In the equations above, ΔIa=Ia’-Ia. Similar expressions are used for the calculation of the principal axis coordinates b and c.

3. Experimental

3.1 History of Microwave Spectroscopy

Spectroscopy is the study of transitions between energy states of molecules.

Microwave spectroscopy, a subset of molecular spectroscopy, is the study of a rotational spectrum of a molecule. These transitions are the result of the absorption and emission of electromagnetic radiation and are unique for each specific molecule. This specificity allows for the unique identification of every spectrum as well as for the direct determination of the molecular structure.

Microwave spectroscopy dates back to the period during World War II.

Microwave technology was important for radar during World War II, and this resulted in an increase in the study of microwave radiation. In the beginning, the measurements were made in the centimeter wave region with oscillators and detectors.1 Since its inception in 1946, microwave spectroscopy has provided the most accurate available equilibrium geometries for many polar molecules.2 In order to obtain a rotational spectrum of the molecule, the molecule must have a permanent dipole moment, which enables the electric field of the microwave radiation to exert a torque on the molecule, causing it to rotate more quickly or slowly.

Initially, relatively small and easily amenable molecules were studied using microwave spectroscopy. Through these studies, rotational structure and hyperfine

structure were identified.10 During this period from about the 1950s until the 1970s, organic molecules of increasing complexity were studied. In addition, with the advent of the space race in the 1960s, the study of molecules in the interstellar medium with microwave spectroscopy began; NH3 was the first stable polyatomic molecule to be identified in the interstellar medium in 1968.11 After the interrogation of the small molecules with microwave spectroscopy, a slower period developed. In 1976, Ekkers and Flygare paved the way for a huge advancement with the use of high-power pulse trains of microwave single selected frequency to excite transitions, which greatly improved signal-to-noise ratios; conventional methods used continuously swept sources.12

In 1979, Terry J. Balle and Willis H. Flygare developed the cavity Fourier- transform microwave (FTMW) spectrometer.13 With the addition of the use of the supersonic jet expansion, new opportunities were brought forth.14 The cooling brought about by the supersonic jet expansion allows for the study of a vast range of weakly bound molecules and complexes. These improvements provided the basis for the instrument that was ultimately exploited throughout the world.

3.2 Instrument

3.2.1 Background Information

The technique of pulsed-nozzle Fourier-transform microwave spectroscopy has been used extensively, with minor modifications and improvements. The Balle-Flygare 33

spectrometer was first reported in publication in Review of Scientific Instrumentation in

1980.15 Overall, the instrument contains four basic components: a Fabry-Perot resonant cavity, a pulsed supersonic free jet expansion system, a pulsed microwave circuit and data-analysis through Fourier transform methods. The four components work together in the Fourier-transform microwave spectrometer to produce a rotational spectrum. First, the sample is introduced into a vacuum chamber as a pulse of high pressure gas; following this, the molecules expand within the vacuum chamber. The chamber is composed of two Fabry-Perot cavity mirrors, where the molecules can interact with a microwave pulse. The gas molecules become rotationally excited when the frequency of the microwave pulse matches the frequency of an allowed rotational transition.

Rotationally excited dipoles induce a current in an antenna; this signal is then Fourier- transformed.

3.2.2 Fabry-Perot Resonant Cavity

As mentioned earlier, the 1st component is a Fabry-Perot resonant cavity, which is comprised of the space between two concave mirrors that face one another. The microwave radiation enters the cavity, traverses the distance between the two mirrors, and is reflected back. A standing wave is established when resonance is achieved:16

(3.1)

where L is the distance between the two mirrors and λ is the wavelength of the microwave radiation. The resonant condition can be tuned to various microwave 34

frequencies by changing the mirror separation. This type of cavity increases the intensity, since the radiation is repeatedly reflected back and forth between the two mirrors. The concave mirrors reduce the diffractional losses by focusing the radiation in a beam waist, thereby increasing the time it takes for radiation to dissipate.16

3.2.3 Supersonic Jet Expansion

The 2nd component of the Balle-Flygare system is the pulsed supersonic jet expansion, which is used for delivery of the gaseous sample. The method of the supersonic jet expansion originates from the Maxwell-Boltzmann Distribution of

Velocities, which has the form:

(3.2)

Where T is the absolute temperature, m is the mass of the molecule, v is the velocity, and

K is the Boltzmann constant. The equation gives the fraction of gas molecules with a given velocity. A Maxwell-Boltzmann velocity distribution changes with temperature.

An increase in temperature leads to a broader curve, since at higher temperatures a greater percentage of molecules are traveling at a higher velocity, than in the low temperature case.

A small hole links the gas reservoir to the chamber. If the gas pressure in the reservoir is sufficiently high, then the departing molecules will undergo many collisions as they pass through the hole. A change in the molecular velocities will occur during expansion through the nozzle, which is illustrated in Fig. 6. As enthalpy is transferred 35

from random motion to directed flow, the speed of sound (a) decreases and the flow velocity (μ) increases. The local speed of sound is proportional to √T, and therefore it decreases as the expansion proceeds. The Mach number, represented as M, relates the flow velocity and the local speed of sound:

(3.3)

As the flow velocity, μ, increases, and the local speed of sound, a, decreases, M increases. After the throat of the nozzle, M becomes greater than unity, hence the term supersonic.15

Translational cooling occurs as a direct result of the supersonic expansion. At low temperatures, molecules move slowly and the number of collisions is low. In supersonic jet expansion, fast moving molecules collide with slow ones, accelerating the slow molecules and decelerating the fast; this leads to the narrow velocity distribution.

This narrow velocity distribution is maintained until the molecules collide with a wall.

In the early stages of expansion, there are collisions that cool the vibrational and rotational degrees of freedom. The efficiency of cooling is of the order:

(3.4) which is determined by the efficiency of energy transfer among these motions, which depends in part, on the densities of the different types of energy levels.

Fig. 6: Schematic illustrating randomness to directed flow seen in a supersonic jet expansion

For this process to be useful, however, the molecules need to be in the gas phase with little or no condensation. There are a few ways that this is achieved. First, the molecule is in the cold bath for a short time. As the expansion proceeds, the translational temperature decreases, as does the molecular density. When the densities are too low for collisions to occur, the rotational and vibrational coolings stop, since they rely on the collisions. Second, a seeded-mixed gas is used for the expansion, containing the molecules of interest with excess carrier gas, such as helium. Most of the collisions are between the carrier gases. Since rare gases have weak interatomic forces, it is difficult to get a three-body collision for condensation to occur; a three body-collision is necessary for nuclei to seed condensation.15

Since supersonic jets limit condensation, they are useful in the study of molecular gas phase spectra. At room temperatures, a large number of quantum states are populated, which leads to an enormous number of individual lines which makes understanding the spectra difficult. Cooling the sample in a supersonic jet limits the populations to the lowest energy levels. This greatly reduces the number of lines and greatly simplifies spectral assignment. The use of the supersonic free jet has allowed for the study of van der Waals molecules with microwave spectroscopy, as well as the observation of very weak hydrogen-bonded complexes that are unstable at ordinary temperatures.1

3.2.4 Pulsed Radiation and Fourier Transformation

The final components of the Balle-Flygare system involve the use of pulsed radiation to stimulate rotational transitions, followed by the Fourier transform of the resulting time dependent data.15 The cooled molecules from the supersonic jet are irradiated with a pulse of microwave radiation. If there is a rotational transition in the frequency range, the molecules will become coherently polarized. Once polarization occurs, the molecules emit coherent polarized radiation at the frequency of the rotational transition. This radiation can be detected and plotted versus the time of decay.

3.3 Experimental Initiation

An experiment is initiated by first tuning the movable mirror to a position in which the cavity is in resonance with the microwave radiation. A frequency is chosen which coincides with a molecular transition frequency for the species of interest. Once the cavity is tuned to the appropriate frequency, the pulsed molecular beam valve is opened for several hundred microseconds to admit a sample to the cavity. The complexes are produced in a pulsed supersonic jet expansion of a gas mixture into a vacuum chamber. The rotational temperature of the sample in jet expansion is on the order of a few Kelvin, and the molecules are stabilized in nearly collision-free environment. Interaction with a pulse of microwave radiation (typically 1-3 μs in duration) causes the dipole moments of the complexes to align, resulting in a macroscopic polarization of the ‘ensemble’ of complexes. After the pulse, relaxation occurs, and the decay of this polarization with time (free induction decay or FID) is 39

recorded. To obtain a spectrum as a function of frequency, a Fourier transform is applied to the time domain signal. The entire process is then repeated as many times as necessary to obtain reasonable signal to noise for the transition in question. The analysis of the resulting spectra results in the determination of rotational constants which are related to the principal moments of inertia of the complex. From these constants, the molecular geometry of the complex can be determined.17

3.4 Spectrometer Used for Current Research

3.4.1 Microwave Circuit

The circuit of the microwave spectrometer18 is illustrated below in Fig. 7. The spectrometer was modeled after the NIST minicavity FTMW spectrometer.17 The microwave power is provided by the microwave synthesizer, an Agilent Technologies

E8247C PSG CW synthesizer, with a range of radiation from 1-20 GHz. The output (ν) is then passed through a Sierra Microwave Technologies SPDT pin diode switch S1

(SFD0526-011). During the irradiation half of the microwave circuit, the switch is closed

(S1a). The microwave output is then directed into a Miteq single sideband mixer SSBM

(SM0226LC1A). The microwave synthesizer provides a 10 MHz output, which is tripled to 30 MHz by a Techtrol Cyclonetics frequency multiplier (FXA217-30). During the irradiation pulse, the 30 MHz sideband is passed into the SSBM mixer by a Minicircuits

SPST pin diode switch S2 (ZYSW-2-50DR). The output of the SSBM mixer (ν+30

MHz) is amplified by 18 dB by a Miteq low-noise amplifier A1 (AFSM3-02001800-40-

8P-C). It then proceeds into the resonant cavity through a Sierra SPDT diode switch S3 40

(SFD0526-011). An L-shaped antenna directs the microwave radiation into the cavity. A typical irradiation pulse is 0.8 μs.18

After the irradiation pulse, a small amount of time is given to allow for the cavity ringing to decay (2-3 μs). After this period, the detection branch of the circuit, by closing switch S3b, is activated. Any free induction signal is then amplified by a Miteq low- noise amplifier A2 (JS4-10002600-22-5A; 33 dB gain, 2.2 dB noise figure). The amplified signal is then passed through an image rejection mixer, IR (Miteq

IR0226LC1A). During detection, switch S1b is also closed, allowing the generator frequency (ν) to pass into the local oscillator port of the image rejection mixer. The IF output of the image rejection mixer is the free induction decay signal superimposed on the 30 MHz sideband frequency. A Minicircuits band-pass filter (SIF-30) then filters the output, which is then amplified by a Miteq Au-1494 amplifier, A3 (56 dB gain). It is then directed into a National Instruments 100 MHz, 8-bit digitizing board (NI 5112).

Here, 40000 channels are digitized at a rate of 100 MHz, resulting in a digital frequency resolution of 2.5 kHz. The FID is typically recorded for 400 μs. Two Stanford Research

Systems Digital Delay Generators (DG535) control the timing of the gas pulse, irradiation pulse and detection pulse; the delay generators are phase locked to the Agilent frequency generator through the 10 MHz reference signal. The low frequency range of the spectrometer is 10.5 GHz and is governed by decreasing cavity Q. The low-noise

Fig. 7: Schematic of Instrument used in the Research

amplifier A1 falls off above 20 GHz; however, the spectrometer’s upper limit could be extended to 26 GHz by replacing amplifier A1 with an amplifier rated for higher frequencies.18

The frequency and digital delay generators are controlled through custom

LabVIEW software and a National Instruments GPIB board (778032-01). The LabVIEW software also controls signal averaging, displays and saves the Fourier transformed spectral data, and performs frequency scans by stepping the microwave source and cavity position. The frequency is swept, while the voltage output is recorded on a Herotek Inc.

Schottky detector (DHM265AA) using the second channel of the digitizing board; this process adjusts the cavity position. A National Instruments PCI-6601 counter-timer board powers the motorized micrometer until the cavity frequency is resonant with the irradiation frequency.18

3.4.2 Sensitivity of Instrument and Basic Components of Instrument

The instrument is sensitive enough to detect the 5/2 ← 5/2 and 7/2 ← 7/2 components of the 1 ← 0 transition of 17O13CS in natural abundance (0.0003927% natural abundance) in a sample made with 2% OCS in Ar within 1000 shots. In Fig. 8 below, a representative signal from the 606-505 transition of diethanolamine is shown; this signal was the result of averaging 500 shots at 12584.6 MHz. The appearance of the line is that of split Doppler doublets. This signal is more complicated because of the partially resolved hyperfine structure arising from the I=1 14N nucleus. The vacuum system is capable of maintaining pressure below 10-4 torr; pulse repetition rates can be up to 15 s-1, 43

while the instrument can scan 450 MHz in 6 hours averaging 100 shots per scan segment.18

Fig. 8: Representative sample of 606-505 transition for Diethanolamine

The instrument used in this research is illustrated below in Fig. 9. The chamber that houses the Fabry-Perot cavity consists of a six-way cross formed by a 15.5 in long, 8 in diameter tube, held together with four 6 in. diameter ports. The vacuum chamber is pumped by a Varian VHS-6 diffusion pump (2400 L s-1); a two-stage Edwards E2M30 rough pump backs up the diffusion pump. Two 7.5 in. diameter spherical diamond-tip machined aluminum mirrors form the cavity. The mirrors are mounted on four guide rails; one mirror is connected to a motorized micrometer, while the other forms an integral end flange to the instrument. The motorized micrometer, which allows for 44

adjustments to tune the cavity to different frequencies, has a 2 in. range of travel, while the mirrors are nominally separated by 30 cm. On the back of the stationary mirror, a reservoir nozzle is mounted, which connects to the carrier gas supply. The valve can be heated, using Watlow band heaters (STB1A1A3-A12) and an Omega CN8201 temperature controller, to the appropriate temperature necessary for the molecule being studied. With the addition of heat, samples are able to be vaporized and combined with the carrier gas before passing through the valve and into the vacuum chamber. The expansion passes through a 0.182 in diameter hole and into the resonant cavity.

3.5 Synthetic Methods

In addition to obtaining rotational spectra of normal species of molecules, further information on the structural information of a molecule can be gathered by obtaining the rotational spectra of multiple isotopomers. By substituting an atom with an isotope, the mass of the molecule is changed. A change in the mass of the molecule results in a change in the moments of inertia:1

(3.5)

where, Ib is one of the moments of inertia, mα represents the mass of an atom, and aα and bα are the coordinates of the atom. With these new moments of inertia, further structural information on bond angles and lengths can be determined.

Fig. 9: Picture of Instrument used in this Research

The project on diethanolamine (DEA) (Chapter 4) required spectral data from the

2H isotopomer, and therefore required a synthesis to obtain this isotopomer. The process to synthesize deuterated diethanolamine isotopomers was relatively straightforward. In general, deuterated diethanolamine was obtained through a vacuum distillation method.

Diethanolamine, which is a viscous liquid at room temperature, was mixed with D2O

(deuterium oxide) in various ratios. The first mixture consisted of a 1:4 ratio of

DEA:D2O, which was then allowed to sit at room temperature for 22 days, before the extraction process. To extract off the water from the now deuterated diethanolamine, the mixture was hooked up to a vacuum line.

This process created a mixture of various combinations of deuterated diethanolamine, with the deuteration sites at the H1, H2 and H3 positions illustrated below in Fig. 10. Other samples of diethanolamine deuterated isotopomers were produced in a similar fashion; a second mixture resulted in less deuteration of diethanolamine, as it was 1:1:1 sample of DEA:D2O:H2O. A third mixture was of a 1:1 sample of DEA:D2O.

Fig. 10: Deuteration Sites for Diethanolamine 47

3.6 Computational Methods

Electronic structure calculations are used to characterize the molecular structure.

Structural optimization calculations of starting structures of a molecule are run at relatively low level of theory, typically RHF/6-31G(d). The newly optimized structures are then further used as an input for high level structure optimizations; typically these high level optimizations are at the MP2/6-311++G(d,p) level, which are run using the

Itanium-2 and Glenn clusters at the Ohio Supercomputer Center. Partial optimizations, where one or more structural parameter is held constant, were also used. The ab initio packages used in this work are GAUSSIAN9819 and GAUSSIAN0320.

4. Diethanolamine

4.1 Background Information

The main goal of my research was to investigate the structural characteristics of diethanolamine (DEA) using microwave spectroscopy. Fig. 11 below shows the structure of diethanolamine. It is used in a variety of applications, including purifications, surfactants, detergents, and cosmetics.21 Diethanolamine is classified as a Department of

Transportation environmentally hazardous substance.22 The potency of diethanolamine is brought about by the reaction of diethanolamine with nitrates, which are commonly found in cosmetic formulae. When diethanolamine reacts with nitrates, a potent carcinogen is formed: N-nitrosodiethanolamine (NDELA). The structure of N- nitrosodiethanolamine is illustrated below in Fig. 12. A recent study demonstrated the formation of NDELA from DEA.23 In this study, rats were dermally administered DEA, while sodium nitrate was added to their drinking water; NDELA was a by-product of this experiment. N-nitrosodiethanolamine is easily absorbed in the skin, and can lead to stomach, esophagus, liver and bladder cancers.

Fig. 11: Structure of Diethanolamine

Fig. 12: Structure of N-nitrosodiethanolamine

In addition to the formation of the potent carcinogen N-nitrosodiethanolamine, diethanolamine also has other hazards. DEA is shown to inhibit the absorption of choline in baby mice.24 Choline is required for brain development and maintenance. It is unknown how diethanolamine affects the choline levels in humans. In addition, diethanolamine was shown to cause potential acute, chronic and subchronic toxicity properties for aquatic species.25 Due to the hazardous nature of diethanolamine, microwave spectroscopy can be used to obtain spectral information for further study. For example, the spectral transitions obtained in this research may be used to detect diethanolamine in the environment through remote sensing methods. The structural information obtained through this research may help us understand its intermolecular interactions and also can be used to follow-up on past studies of the molecule.

4.2 Theoretical Modeling

4.2.1 Previous Theoretical Modeling

Studies on the structure of diethanolamine have been carried out in the past. Many of these past studies have focused on conformational studies of the molecule by using ab initio calculations. One study involved modeling the gas phase basicities of diethanolamine and other alkanolamines for comparison to experimental measurements.26

Calculations for the ab initio studies were carried out at the B3LYP and MP2 levels of theory. The B3LYP is a hybrid functional in which the exchange energy is combined with the exact energy from Hartree-Fock theory. The name arises from the use of Beck’s exchange functional (B) and the correlation part from Lee (L), Yang (Y) and Parr (P); the

3 arises from the use of three parameters used to describe the hybrid functional. The most stable diethanolamine structure generated at the B3LYP/6-311++G(d,p) level is depicted below in Fig. 13. While this study did report a number of conformers for many of the molecules studied, diethanolamine was not extensively studied due the large number of possible conformers.

Fig. 13: Structure of diethanolamine at B3LYP/6-311++G(d,p) level26

Another study on the conformational structure of diethanolamine used ab initio methods to predict pKa values of amines.27 For this study, a conformational study of diethanolamine was performed at a very low level (HF/3-21G*). A further study used density functional theory calculations to determine vibrational frequencies and structural information for diethanolamine.28 The B3LYP/6-311++G(d,p) basis set was used for this study, and resulting optimized geometry of diethanolamine was very similar to that obtained by Da Silva.26

4.2.2 Current Theoretical Modeling for this Research

In order to more fully obtain a conformational study of diethanolamine, more conformers needed to be studied. Previous research on the conformational structure of diethanolamine was not extensive enough, resulting in large gaps in the studies. While not all possible starting configurations of diethanolamine were considered in this research, a total of 82 heavy atom configurations were studied. Heavy atom conformers are related through rotations about the carbon, oxygen and nitrogen bonds. The 82 heavy atom conformers were modeled in the program Avogadro.29 Relatively low levels of theory were first used to study the configurations of diethanolamine. The level of theory first used was the RHF/6-31G(d) level of theory. Following convergence of the heavy atom conformers at this level of theory, an energy ordering table was generated; the energy ordering of diethanolamine is given in Table 3.

Table 3: Configuration and relative energies of diethanolamine conformers at RHF/6- 31G(d) level

Starting Optimized Configuration Conformer (τ1 τ 2 τ 3- τ 4 τ 5 τ 6) ΔE (KJ/mol) sv1 gg'a-ag'a 0.00 sv2 gg'a-ag'a 0.00 sv3 gg'a-ag'g' 2.31 sv4 gg'a-ag'g' 2.31 sv5 gg'a-ag'g' 2.31 sv6 gga-agg' 2.31 sv7 g'gg-ag'a 2.88 sv8 gg'g-g'gg' 4.55 sv9 gg'g-g'gg' 4.55 sv10 gg'a-gga 4.89 sv11 gg'a-gga 4.89 sv12 ag'g'-agg' 4.89 sv13 g'gg-ag'g 7.27 sv14 gaa-agg' 8.01 sv15 gg'a-aag' 8.01 sv16 gg'a-aag' 8.01 sv17 g'gg-aaa 10.14 sv18 gg'a-g'aa 10.45 sv19 g'gg-g'gg' 10.89 sv20 g'gg-aag' 10.93 sv21 gga-ag'g' 11.28 sv22 gag-agg' 12.16 sv23 gg'a-gaa 12.41 sv24 gg'a-gaa 12.41 sv25 gg'g-g'g'a 12.44 sv26 gg'a-gg'g' 13.05 sv27 ggg'-agg' 13.05

sv28 gag'-agg' 13.29 sv29 gag'-agg' 13.29 sv30 g'gg-g'ga 13.62 sv31 ag'g-g'gg' 13.80 sv32 gg'a-aga 13.90 sv33 gg'a-aga 13.90 sv34 gga-aag' 13.93 sv35 gaa-ag'g' 13.93 sv36 gg'g'-ag'a 15.07 sv37 gg'g'-ag'a 15.07 sv38 gg'g'-gga 15.72 sv39 gag-ag'a 16.21 sv40 g'gg-gaa 16.53 sv41 gga-g'aa 16.92 sv42 g'gg'-gg'a 17.16 sv43 ggg'-ggg' 17.66 sv44 gg'g'-gg'g' 17.66 sv45 gag-ag'g' 18.27 sv46 g'gg-gg'g' 18.61 sv47 gga-gaa 18.66 sv48 gag'-ag'a 18.70 sv49 gag'-ag'a 18.70 sv50 gag'-gaa 18.72 sv51 gaa-aag' 19.29 sv52 ggg'-g'ga 20.00 sv53 ag'g-gg'g' 20.00 sv54 gga-ggg' 20.63 sv55 gag'-ag'g' 20.92 sv56 ggg'-ag'g' 21.57 sv57 gga-gg'g' 21.57 sv58 gaa-g'aa 22.31

sv59 gag-aaa 22.33 sv60 gag-aaa 22.33 sv61 gg'g'-aaa 22.43 sv62 gga-aga 22.89 sv63 gaa-ggg' 23.34 sv64 gaa-gaa 24.12 sv65 gag'-aaa 24.43 sv66 gga-g'g'a 26.34 sv67 gaa-gg'g' 26.72 sv68 ggg'-aag' 26.72 sv69 gg'g'-g'aa 26.98 sv70 gaa-aga 27.14 sv71 gg'g'-g'ga 27.32 sv72 gag'-g'aa 28.06 sv73 gag-gaa 29.10 sv74 gag-ggg' 29.27 sv75 gaa-g'g'a 30.51 sv76 gag-aga 31.40 sv77 gag-gg'g' 31.89 sv78 ggg'-g'ag' 31.89 sv79 ggg'-gaa 35.10 sv80 gag'-ggg' 35.17 sv81 gg'g'-gaa 35.53 sv82 ggg'-g'g'a 40.69

The conformers are denoted as sv1, sv2, sv3,…etc. as a naming scheme; the symbol sv was chosen due to the use of the split valence level: RHF/6-31G(d). Upon examining this table, it is noted that a number of the starting configurations optimize to the same structure. These can be observed by examining the torsional angle configurations as well as the relative energies in Table 3. The energies in this table are relative to the lowest energy conformer (which has a ΔE=0.00 KJ/mol). Torsional angle configurations are included in Table 3 to give representation of the optimized structures. The placement of the torsional angles in the molecule diethanolamine is depicted in Fig. 14; τ is used to represent the torsional angles. The torsional angle configurations are noted as either being gauche (g and g’) or anti (a); g is noted for a torsional angle around 60°, g’ around

-60°, and a around 180°.

τ1= C10-O9-C8-C6 τ2= O9-C8-C6-N τ 3= C8-C6-N-C4 τ 4= C6-N-C4-C3 τ 5= N-C4-C3-O2 τ 6= C4-C3-O2-H1

Fig. 14: Molecular structure and definitions of torsional angles for diethanolamine

The RHF/6-31G(d) optimized conformers were further studied at the RHF/6-

311++G(d,p) level of theory, which is a relatively high level. At this level of theory, more conformers converged together, resulting in less unique conformers. Table 4 presents the results of this level of theory for the relative energies and new conformer labels. The energy ordering changed slightly from the lower level of theory to

Table 4: Configuration and relative energies of diethanolamine conformers at RHF/6- 311++G(d,p) level

Starting Optimized Configuration Conformer (τ 1 τ 2 τ 3- τ 4 τ 5 τ 6) ΔE (KJ/mol) tsv1 gg'a-ag'a 0.00 tsv2 gg'a-ag'a 0.00 tsv3 gg'a-ag'a 0.00 tsv4 g'gg-ag'a 3.02 tsv5 gg'a-ag'g' 3.43 tsv6 gg'a-ag'g' 3.43 tsv7 gg'a-ag'g' 3.43 tsv8 gga-agg' 3.43 tsv9 gg'a-gga 5.12 tsv10 gg'a-gga 5.12 tsv11 ag'g'-agg' 5.12 tsv12 g'gg-agg' 5.27 tsv13 g'gg-ag'g 8.12 tsv14 gaa-agg' 8.42 tsv15 gg'a-aag' 8.42 tsv16 gg'a-aag' 8.42 tsv17 g'gg-aaa 9.45 tsv18 gg'a-g'aa 9.85 tsv19 g'gg-aag' 11.18 tsv20 gg'g-g'gg' 11.60 tsv21 gg'g-g'gg' 11.60 tsv22 gga-ag'g' 11.61 tsv23 gg'a-gaa 12.16 tsv24 gag-agg' 12.42 tsv25 gg'a-aga 13.25 tsv26 gg'a-aga 13.25 tsv27 gga-aag' 13.53 tsv28 gaa-ag'g' 13.53

tsv29 gag'-agg' 13.74 tsv30 gag'-agg' 13.74 tsv31 gg'g'-ag'a 13.94 tsv32 gg'g'-ag'a 13.94 tsv33 gg'a-gg'g' 14.56 tsv34 ggg'-agg' 14.56 tsv35 gg'a-g'g'a 14.63 tsv36 gag-ag'a 14.65 tsv37 g'gg-g'ga 14.68 tsv38 ag'g-g'gg' 15.04 tsv39 gga-g'aa 15.79 tsv40 g'gg-gaa 15.96 tsv41 gag'-ag'a 17.25 tsv42 gag'-ag'a 17.25 tsv43 gag-ag'g' 17.92 tsv44 gaa-aag' 18.06 tsv45 gg'g'-gga 18.69 tsv46 g'gg'-gg'a 18.75 tsv47 gga-ggg' 19.59 tsv48 g'gg-gg'g' 19.85 tsv49 gga-gaa 20.09 tsv50 gg'g'-aaa 20.15 tsv51 gag-aaa 20.15 tsv52 gag-aaa 20.15 tsv53 gaa-g'aa 20.19 tsv54 gag'-ag'g' 20.58 tsv55 ggg'-ag'a 20.59 tsv56 ggg'-ggg' 20.76 tsv57 gg'g'-gg'g' 20.76 tsv58 gga-aga 20.79 tsv59 ggg'-ag'g' 21.92 tsv60 gga-gg'g' 21.92

tsv61 gaa-ggg' 21.96 tsv62 gaa-gaa 22.26 tsv63 gag'-aaa 22.46 tsv64 ggg'-g'ga 22.71 tsv65 ag'g-gg'g' 22.71 tsv66 gga-g'g'a 24.28 tsv67 gaa-aga 24.46 tsv68 gg'g'-g'aa 24.68 tsv69 gag'-g'aa 25.93 tsv70 gg'g'-g'ga 25.94 tsv71 gaa-gg'g' 26.54 tsv72 ggg'-aag' 26.54 tsv73 gag-gaa 27.02 tsv74 gaa-g'g'a 27.52 tsv75 gag-ggg' 27.80 tsv76 gag-aga 28.50 tsv77 gag-gg'g' 31.45 tsv78 ggg'-g'ag' 31.45 tsv79 gag'-gaa 32.73 tsv80 gg'g'-gaa 32.93 tsv81 gag'-ggg' 33.49 tsv82 ggg'-gaa 34.03

the higher level of theory, but most of the changes were the result of previously unique conformers at the RHF/6-31G(d) level converging into the same conformer at the RHF/6-

311++G(d,p) level. The naming scheme for the conformers is denoted as Conformer tsv1, tsv2, tsv3,…etc. which is based on the use of the triple split valence level: RHF/6-

311++G(d,p).

The four lowest unique conformers in energy (Conformers tsv1, tsv4, tsv5 and tsv9) from the RHF/6-311++G(d,p) level of theory calculations (from Table 4) were further studied at an even higher level of theory: MP2/6-311++G(d,p). The laboratory

PC did not have enough memory to carry out calculations at this level of theory.

Therefore, these calculations were performed using the Itanium-2 cluster at the Ohio

Supercomputer Center. The results of the MP2 optimizations for the four lowest energy conformers are given in Table 5. The naming scheme for the conformers at this level of theory is denoted as ec1, ec2, ec3,… etc, which is based off of the use of electron correlation in the MP2/6-311++G(d,p) level of theory. It can be seen how the energy ordering changed slightly from the RHF/6-311++G(d,p) level to the MP2/6-311++G(d,p) level of theory for these conformers. Conformer ec3 (which corresponds to Conformer tsv9 at the RHF/6-311++G(d,p) level) and Conformer ec4 (tsv5) switched places in the energy ordering. The relative energies also slightly changed, with Conformer ec2 being closer in energy to Conformer ec1 at the MP2 level of theory than at the high RHF level of theory. Table 6 below provides the dipole moments in the principal axis system for the four lowest energy conformers, as well as their rotational constants. The four lowest conformers from this level of theory were used to generate rotational transition

predictions from RRFIT calculations. The RRFIT program is a rigid rotor fitting program developed by Professor C. Glaubitz.30

Table 5: Configuration and Relative Energies for Four Lowest Energy Conformers at MP2/6-311++G(d,p) level

Starting Optimized Configuration ΔE Conformer (τ1 τ 2 τ 3- τ 4 τ 5 τ 6) (KJ/mol) ec1 gg'a-ag'a 0.00 ec2 g'gg-ag'a 0.82 ec3 gg'a-gga 2.81 ec4 gg'a-ag'g' 3.33

Table 6: Relative dipole moments and Rotational Constants for 4 lowest energy conformers of Diethanolamine at MP2/6-311++G(d,p) level

Conformer Conformer Conformer Conformer Parameter ec1 ec2 ec3 ec4 A/MHz 5982.1666 6218.8855 6060.9834 6501.3305 B/MHz 1074.1382 1135.5108 1054.5069 1151.4174 C/MHz 1048.6936 1096.8395 1037.1762 1084.4778

μa/D 3.2538 -3.03 2.5443 4.0177

μb/D 0.7957 0.3713 3.071 -1.1024

μc/D 0.088 1.0458 0.7188 1.5878 ΔE/kJ mol-1a 0.00 0.82 2.81 3.33

a Relative to Conformer ec 1, includes zero- point energy correction.

Furthermore, the four lowest energy conformers at the MP2/6-311++G(d,p) level of theory were modeled using MacMolPlt, a 3D structure program.31 The modeling conformers are shown in Fig. 15, 16, 17 and 18. From these structures, a few interesting features are worth noting. Each conformer contains intramolecular hydrogen bonding networks. For example, in Conformer ec1, the amine-H forms a hydrogen bond with the

right-hand hydroxyl-O, and the left-side hydroxyl-H forms a hydrogen bond with the N.

Similar hydrogen bonding networks are also seen in the other three conformers.

Conformer ec1 and Conformer ec4 are nearly identical conformers; however, the right side hydroxyl of Conformer ec4 is rotated counterclockwise relative to the right hydroxyl of Conformer ec1. Other than this rotation in the hydroxyl, Conformer ec1 and

Conformer ec4 are identical. This slight rotation in Conformer ec4 is enough to greatly increase the energy relative to Conformer ec1.

Fig. 15: Conformer ec1 of Diethanolamine from MP2/6-311++G(d,p) level

Fig. 16: Conformer ec2 of Diethanolamine from MP2/6-311++G(d,p) level

Fig. 17: Conformer ec3 of Diethanolamine from MP2/6-311++G(d,p) level

Fig. 18: Conformer ec4 of Diethanolamine from MP2/6-311++G(d,p) level

4.3 Spectra and Hamiltonian Fitting

Diethanolamine required some conditioning before any signal would be observed.

After adjusting the temperature of heating over the course of a few weeks, the ideal temperature to heat the sample was determined to be around 150°C. Since the sample is a viscous liquid at room temperature, the sample was inputted into the valve; surrounding the valve is a thermocouple cuff that could then be heated to the proper temperature (in this case 150° C).

4.3.1 Spectrum A of Diethanolamine

Spectral predictions from RRFIT were used to guide the initial search for rotational transitions. Two separate spectra were found for the normal species of diethanolamine. The largest projection of the dipole moment in the principal axis system

(a, b, or c) corresponds to the highest intensity lines in the spectrum. The first spectrum assigned will be labeled as Spectrum A of diethanolamine. For Spectrum A, a total of 20 a-type lines were assigned. As the lines were found, they were inputted into the RRFIT predictions to further improve the experimental determination of the rotational constants as well as the predictions for the remaining lines.

The 20 a-type lines resulted in the determination of the experimental rotational constants. The final values of these rotational constants were obtained from fitting the observed transition frequencies using the program SPFIT.32 Table 7 gives the rotational constants of Spectrum A with their relative uncertainties.

Table 7: Experimental Rotational Constants determined for Spectrum A

Spectrum A Rotational Constant Value (MHz) A 5949.4(161) B 1065.3450(54) C 1033.0029(54)

Table 8 lists the frequencies of the 20 a-type transitions and their quantum numbers. In this table, Obs-Calc refers to the differences between the observed frequency and the calculated frequency. The calculated frequency, which is minimized during the fitting procedure, is the frequency calculated from the fitted values of A, B and C, and any distortion constants. No distortion constants were determined for diethanolamine. The nearly symmetric structure of diethanolamine results in hyperfine structure that is difficult to assign. One week of work was dedicated to obtaining the quadrupole

structure of the rotational transitions; this was attempted by averaging a large number of shots at the frequency of a rotational transition. However, the quadrupole structure was never resolved, even after using 20,000-shots to obtain the rotational transitions.

Therefore, instead of assigning the quadrupole for each transition, the frequency of the transition was estimated to be the center of the transition. A few typical transitions are shown below in Fig. 19 and 20. As depicted in the figures, the transitions are not perfect doublets. Instead, the transitions contain a number of shoulders and small peaks due to the nuclear quadrupole interactions and the presence of N-atom in diethanolamine. The

14N nucleus has a spin equal to one, and coupling of nuclear spin and molecular rotation cause hyperfine structure to occur. Typically, the spectral transitions could easily be seen above the background in 300 shot scans.

4.3.2 Spectrum B of Diethanolamine

In addition to Spectrum A being assigned for diethanolamine, a second spectrum was also observed. Like the observation of Spectrum A, Spectrum B was also found by examining the RRFIT predictions for the lowest energy conformers, and by using the highest dipole moment projection as a starting point. For Spectrum B, a total of 21 a- type rotational transitions were measured. The rotational constants for Spectrum B are given in Table 9. A listing of these 21 a-type transitions is given in Table 10. As before, the rotational constants for Spectrum B were determined using the program SPFIT. A few example spectral transitions are illustrated below in Fig. 21 and 22. Most rotational transitions were easily seen above the background in 300-shot scans.

Table 8: Rotational Transitions of Spectrum A

Spectrum A Frequency Obs-Calc Quantum Number (MHz) (MHz)

616-515 12491.875 0.082

606-505 12584.602 0.120

625-524 12589.524 0.158

624-523 12595.098 0.126

615-514 12685.97 0.152

717-616 14573.217 0.051

707-606 14679.545 0.073

726-625 14687.326 0.137

725-624 14696.237 0.084

716-615 14799.581 0.083

818-717 16654.232 -0.039

808-707 16773.337 -0.015

827-726 16784.88 0.055

826-725 16798.237 -0.019

817-716 16912.811 -0.072

919-818 18734.928 -0.149

909-808 18865.885 -0.087

928-827 18882.167 -0.080

927-826 18901.233 -0.172

918-817 19025.735 -0.192

Intensity (arbitrary Intensity (arbitrary units)

12491.4 12491.6 12491.8 12492 12492.2 12492.4 Frequency (MHz)

Fig. 19: Rotational Transition of the 616-515 transition of Spectrum A for Diethanolamine at 12491.875 MHz in 5000 shots

Intensity (arbitrary Intensity (arbitrary units)

12584.2 12584.4 12584.6 12584.8 12585 Frequency (MHz)

Fig. 20: Rotational Transition of the 606-505 transition of Spectrum A for Diethanolamine at 12584.602 MHz in 5000 shots

Since only a-type transitions were initially obtained, b-type transitions were searched for. Over the course of two weeks, b-type transitions of diethanolamine were searched for on the spectrometer. A b-type transition would greatly improve the A rotational constant, since b-type transitions give more information on A. No such b-type transition was ever located during this period; this may be the result of the relatively weak μb of diethanolamine (see Table 6).

Table 9: Experimental Rotational Constants for Spectrum B

Spectrum B Rotational Constant Value (MHz) A 6180.1(183) B 1127.0816(74) C 1086.6090(74)

4.3.3 Discussion of Structure

In order to assign Spectrum A and Spectrum B to the possible conformers of diethanolamine, it is best to examine a number of variables. Often times, it is helpful to examine the relative energies of the likely conformers. The lowest energy conformers are most likely to be seen in microwave spectroscopy. In addition, it is also helpful to examine the dipole moments of the possible conformers. The highest dipole moment should match up with the most intense lines in the spectra; for example, a large μa dipole moment should correspond to intense a-type transitions. Another commonly used method to determine assignment is the %ΔIrms (see Equation 2.81) which was previously discussed. This equation relates the moments of inertia for the experimental spectra

Table 10: Rotational Transitions for Spectrum B

Spectrum B Frequency Obs-Calc Quantum Number (MHz) (MHz)

505-404 11063.825 0.175 616-515 13158.939 0.075 606-505 13273.848 0.128 625-524 13281.25 0.144 634-533 13283.266 -0.239 624-523 13289.663 0.080 615-514 13401.786 0.136 717-616 15351.128 0.012 707-606 15482.429 0.083 726-625 15494.088 0.077 725-624 15507.537 -0.023 716-615 15634.361 0.050 818-717 17542.863 -0.106 808-707 17689.288 -0.013 827-726 17707.721 0.088 826-725 17726.661 -0.266 817-716 17866.411 -0.111 919-818 19734.112 -0.262 909-808 19894.192 -0.172 928-827 19918.712 -0.220 918-817 20097.881 -0.329

Intensity (arbitrary Intensity (arbitrary units)

13158.6 13158.8 13159 13159.2 13159.4 Frequency (MHz)

Fig. 21: Rotational Transition of the 616-515 transition of Spectrum B for Diethanolamine at 13158.919 MHz in 5000 shots

Intensity (arbitrary Intensity (arbitrary unit)

13401.4 13401.6 13401.8 13402 13402.2 Frequency (MHz)

Fig. 22: Rotational Transition of the 615-514 transition of Spectrum B for Diethanolamine at 13401.786 MHz in 1000 shots

compared to the moments of inertia calculated from an ab initio model structure. The lower the %ΔIrms, the better the fit.

Upon examining Spectrum A, it becomes evident that it best matches up with

Conformer ec1. Conformer ec1 is the lowest energy conformer, making it the most likely to be found by the microwave spectrometer. Furthermore, the ab initio dipole moments for Conformer ec1 are: μa=3.254 D, μb=0.796 D, and μc= 0.088 D. Since μa is the largest dipole moment, and only a-type transitions were found for Spectrum A, this further confirms Spectrum A relates to Conformer ec1. In addition, the %ΔIrms was determined for Spectrum A when compared against the four lowest energy conformers. The values for the %ΔIrms values comparing Spectrum A and the four lowest energy conformers are depicted below in Table 11. From this table, it is evident that the lowest %ΔIrms value relating Spectrum A belongs to Conformer ec1. While Conformer ec3 also has a relatively low %ΔIrms value for Spectrum A, it is also 2.81 KJ/mol higher in energy than

Conformer ec1 (see Table 5). The combination of the low energy, the high μa dipole moment, and the low %ΔIrms compared with Spectrum A, results in an assignment of

Spectrum A with Conformer ec1.

Table 11: %ΔIrms values for Conformers compared with Spectrum A

Conformer %ΔIrms A ec1 1.05 ec2 5.53 ec3 1.29 ec4 7.12

In a similar fashion, Spectrum B was identified as Conformer ec2. Conformer ec2 has the second lowest energy, 0.82 KJ/mol higher in energy than Conformer ec1.

Conformer ec2 also has a relatively high μa dipole moment; μa=-3.030 D, μb=0.371 D,

μc=1.05 D. Since only a-type transitions were found for Spectrum B, this spectrum matches up nicely with Conformer ec2. The %ΔIrms value was also calculated comparing the moments of inertia for the four lowest energy conformers to the moments of inertia determined experimentally for Spectrum B. A table illustrating the %ΔIrms values is depicted below in Table 12. The %ΔIrms value was lowest for Conformer ec2, which further illustrates the assignment of Spectrum B with Conformer ec2.

Table 12: %ΔIrms values for Conformers compared with Spectrum B

Conformer %ΔIrms B ec1 3.81 ec2 0.74 ec3 4.71 ec4 2.98

4.4 Diethanolamine Isotopic Study

Often isotopic studies are completed in order to obtain further structural information on the molecule. For this study, diethanolamine was deuterated; the experimental process of deuteration was described in Section 3.5. An isotopic substitution results in a mass change of the molecule. As a result, the mass change leads to a change in the moment of inertia, as is depicted below in Equation 4.1:

(4.1)

where Ib the moment of inertia in the principal axis b, mα is the mass of the atom, and aα and bα are the coordinates of the atom. Since rotational transitions depend on the moment of inertia, a change in the moment of inertia will result in shifts in the rotational transitions.

A total of 14 possible deuterated molecules could be formed (7 belonging to

Conformer ec1, and 7 belonging to Conformer ec2). The deuteration sites are depicted in

Fig. 10 in Section 3.5 and tabulated below in Table 13.

Table 13: Possible Deuteration Sites for Conformers ec1 and ec2

Conformer ec1 Deuteration Sites Conformer ec2 Deuteration Sites H1 H1 H2 H2 H3 H3 H1,H2,H3 H1,H2,H3 H1, H2 H1, H2 H1, H3 H1, H3 H2, H3 H2, H3

4.4.1 Structural Modeling of Deuterated Isotopomers

In order to generate theoretical models for deuterated diethanolamine, the program STRGN was used.33 STRGN uses the ab initio model structure of the normal species to calculate rotational constants of isotopically substituted forms. From the starting structures, the atoms are substituted to model the 14 possible combinations. For this study, the ab initio calculations for the normal species of Conformer ec1 and

Conformer ec2 were used at both the RHF/6-31G(d) and RHF/6-311++G(d,p) levels of theory. STRGN then calculates new rotational constants for each isotopomer. From these new rotational constants, the program RRFIT can again be used to generate rotational transition predictions. The calculated rotational transition predictions of the normal species of diethanolamine for the conformers were compared against their observed positions; a shift of the predicted rotational transition compared to the actual position was noted. By applying this known shift to the new isotope rotational transition predictions, a clearer picture of where the rotational transitions are located is obtained.

With these new shifted rotational transition predictions for the isotopic diethanolamine structures, the search for the rotational transitions was begun on the microwave spectrometer.

4.4.2 Spectra and Hamiltonian Fitting

Rotational transition spectra were obtained by locating the frequencies of individual rotational transitions; upon the observation of a new rotational transition, it was added to the RRFIT prediction to further improve the fit. A total of four separate spectra were obtained for deuterated diethanolamine. The results of these spectra are presented below.

4.4.2.1 Deuterated-Diethanolamine Spectrum A

The first spectrum was obtained by using a 1:4 mixture of diethanolamine:deuterium oxide, and is noted as deuterated-diethanolamine Spectrum A

(d-DEA Spectrum A). For this spectrum, a total of 15 a-type rotational transitions were obtained. Table 14 presents the quantum numbers and relative frequencies for these 15 a-type transitions. A few example rotational transitions are depicted below in Fig. 23 and

24 illustrating the hyperfine structure of the rotational transitions. The rotational transitions were typically visible in about 300-shot scans, and at this level were easily visible above the background. The rotational constants obtained from these 15 a-type rotational transitions are given in Table 15.

Table 14: Rotational Transition Frequencies and relative quantum numbers for d-DEA Spectrum A d-DEA Spectrum A Quantum Number Frequency (MHz) Obs-Calc (MHz)

606-505 12184.400 0.054 625-524 12189.658 0.086 634-533 12191.589 0.318 624-523 12195.600 0.031 615-514 12286.315 0.083 717-616 14106.350 -0.013 707-606 14212.481 0.002 726-625 14220.990 0.257 725-624 14230.305 -0.017 716-615 14333.281 0.012 818-717 16120.612 -0.119 808-707 16239.346 -0.078 827-726 16251.608 -0.087 826-725 16265.909 -0.153 817-716 16379.886 -0.099

Intensity (arbitrary Intensity (arbitrary unit)

14106 14106.2 14106.4 14106.6 14106.8 Frequency (MHz)

Fig. 23: Rotational Transition of the 717-616 transition of d-DEA Spectrum A for

Diethanolamine at 14106.350 MHz in 1000 shots Intensity (arbitrary Intensity (arbitrary unit)

14332.8 14333 14333.2 14333.4 14333.6

Frequency (MHz)

Fig. 24: Rotational Transition of the 716-615 transition of d-DEA Spectrum A for Diethanolamine at 14333.281 MHz in 800 shots

Table 15: Experimental Rotational Constants for d-DEA Spectrum A

d-DEA Spectrum A Rotational Constant Value (MHz) A 5617.0(243) B 1032.0776(80) C 999.6463(83)

4.4.2.2 Deuterated-Diethanolamine Spectrum B

The second spectrum was obtained by using a 1:1:1 mixture of diethanolamine:deuterium oxide:H2O and is referred to as deuterated-diethanolamine

Spectrum B (d-DEA Spectrum B). This mixture was used in order to obtain a less deuterated diethanolamine molecule. For this spectrum, a total of 15 a-type rotational transitions was obtained. The rotational constants obtained from these 15 a-type rotational transitions and their relative uncertainties are given in Table 16. Table 17 gives the 15 a-type rotational transitions with their assigned quantum numbers and frequencies. Fig. 25 and 26 illustrate some examples of the rotational transitions obtained for this spectrum. Like before, the rotational transitions were typically visible in about 300-shot scans, and at this level were easily visible above the background.

Table 16: Experimental Rotational Constants for d-DEA Spectrum B

d-DEA Spectrum B Rotational Constant Value (MHz) A 5737(40) B 1056.5109(116) C 1024.9216(116)

Table 17: Rotational Transition Frequencies and relative quantum numbers for d-DEA Spectrum B

d-DEA Spectrum B Frequency Obs-Calc Quantum Number (MHz) (MHz)

616-515 12392.575 0.009 606-505 12483.075 0.053 625-524 12487.981 0.103 624-523 12493.484 0.033 615-514 12582.117 0.044 717-616 14457.393 -0.012 707-606 14561.112 -0.004 726-625 14569.447 -0.033 725-624 14577.646 -0.054 716-615 14678.433 -0.033 818-717 16521.826 -0.152 808-707 16637.990 -0.116 827-726 16649.397 -0.115 826-725 16662.742 -0.123 817-716 16774.406 -0.160

Intensity (arbitrary Intensity (arbitrary unit)

14560.8 14561 14561.2 14561.4 Frequency (MHz)

Fig. 25: Rotational Transition of the 707-606 transition of d-DEA Spectrum B for Diethanolamine at 14561.112 MHz in 1000 shots

Intensity (arbitrary Intensity (arbitrary unit)

16774 16774.2 16774.4 16774.6 16774.8 Frequency (MHz)

Fig. 26: Rotational Transition of the 817-716 transition of d-DEA Spectrum B for Diethanolamine at 16774.406 MHz in 1000 shots

4.4.2.3 Deuterated-Diethanolamine Spectrum C

The third spectrum of deuterated diethanolamine, known as deuterated- deithanolamine Spectrum C (d-DEA Spectrum C) was obtained by using unassigned lines previously obtained in the scans when searching for d-DEA Spectrum A and d-DEA

Spectrum B. Due to the large overlap of these spectra, rotational transitions in the same scan could belong to different spectra. A total of 13 a-type rotational transitions was obtained for d-DEA C. Table 18 gives the quantum numbers and relative frequencies for these 13 a-type rotational transitions. Examples of the rotational transitions for this spectrum are depicted below in Fig. 27 and 28. The rotational constants obtained from these 13 a-type rotational transitions and their relative uncertainties are given in Table 19.

Table 18: Rotational Transition Frequencies and relative quantum numbers for d-DEA Spectrum C

d-DEA Spectrum C Quantum Frequency Obs-Calc Number (MHz) (MHz)

606-505 12276.279 0.079 625-524 12281.356 0.120 615-514 12377.521 0.109 717-616 14213.997 0.054 707-606 14319.781 0.046 726-625 14327.717 0.026 725-624 14336.934 0.000 716-615 14439.670 -0.001 818-717 16243.661 -0.048 808-707 16362.014 -0.111 827-726 16373.920 -0.034 826-725 16387.735 -0.066 817-716 16501.545 -0.081

Intensity (arbitrary Intensity (arbitrary unit)

12377.2 12377.4 12377.6 12377.8 Frequency (MHz)

Fig. 27: Rotational Transition of the 615-514 transition of d-DEA Spectrum C for

Diethanolamine at 12377.521 MHz in 1000 shots Intensity (arbitrary Intensity (arbitrary unit)

16243.2 16243.4 16243.6 16243.8 16244 Frequency (MHz)

Fig. 28: Rotational Transition of the 818-717 transition of d-DEA Spectrum C for Diethanolamine at 16243.661 MHz in 2000 shots

Table 19: Experimental Rotational Constants for d-DEA Spectrum C

d-DEA Spectrum C Rotational Constant Value (MHz) A 5744.5(151) B 1039.6265(45) C 1007.3702(47)

4.4.2.4 Deuterated-Diethanolamine Spectrum D

The fourth spectrum was obtained by using a 1:4 mixture of diethanolamine:deuterium oxide and is referred to as deuterated-diethanolamine

Spectrum D (d-DEA Spectrum D). For this search, rotational transitions as a result of the isotopic substitutions were searched around the DEA Spectrum B transitions. It took a great amount of conditioning in order to obtain any rotational transitions. For this spectrum, a total of 19 a-type rotational transitions was obtained. Table 20 gives the 19 a-type rotational transitions with their assigned quantum numbers and frequencies. Fig.

29 and 30 give some examples of the rotational transitions obtained for this spectrum.

The rotational transitions were typically visible in about 500-shot scans, and at this level were easily visible above the background. The rotational constants obtained from the 19 a-type rotational transitions and their relative uncertainties are given in Table 21.

Table 20: Rotational Transition Frequencies and relative quantum numbers for d-DEA Spectrum D

d-DEA Spectrum D Quantum Frequency Obs-Calc Number (MHz) (MHz)

616-515 12682.527 0.109

606-505 12785.600 0.153

625-524 12791.870 0.172

624-523 12798.967 0.094

615-514 12899.669 0.127

717-616 14795.436 0.039

707-606 14913.378 0.123

726-625 14923.232 0.102

735-634 14926.500 0.112

725-624 14934.606 0.006

716-615 15048.756 0.088

818-717 16907.945 -0.092

808-707 17039.677 0.034

827-726 17054.315 -0.007

826-725 17071.364 -0.143

817-716 17197.300 -0.114

919-818 19020.091 -0.202

909-808 19164.341 -0.080

918-817 19345.488 -0.233

Intensity (arbitrary Intensity (arbitrary unit)

12682 12682.2 12682.4 12682.6 12682.8 Frequency (MHz)

Fig. 29: Rotational Transition of the 616-515 transition of d-DEA Spectrum D for

Diethanolamine at 12682.527 MHz in 3000 shots Intensity (arbitrary Intensity (arbitrary unit)

14795 14795.2 14795.4 14795.6 14795.8 Frequency (Mhz)

Fig. 30: Rotational Transition of the 717-616 transition of d-DEA Spectrum D for Diethanolamine at 14795.436 MHz in 500 shots

Table 21: Experimental Rotational Constants for d-DEA Spectrum D

d-DEA Spectrum D Rotational Constant Value (MHz) A 5852.6(164) B 1084.1494(60) C 1047.9543(60)

4.4.3 Discussion

While there were small differences between the observed frequencies and calculated frequencies of the four spectra obtained for deuterated-diethanolamine, exact assignment of the spectra to specific isotopomers is not easily determined. While the d-

DEA Spectra A, B, and C can be assigned to Conformer ec1 and d-DEA Spectrum D can be assigned to Conformer ec2, it is more complicated to assign the spectra to a specific deuteration site. This complication is due to the lack of b-type rotational transitions for both the normal species of diethanolamine, as well as the deuterated diethanolamine.

Without any b-type rotational transitions, the A rotational constant has a great deal of uncertainty (as is illustrated in the rotational constants obtained experimentally, see

Tables 7, 9, 15, 16, 19 and 21). Without knowing the value of the A rotational constant, it is difficult to assign spectra that have very similar B and C rotational constants.

For d-DEA Spectrum A, the experimentally determined rotational constants were:

A=5617 MHz, B= 1032.0776 and C=999.6463 MHz. Table 22 gives the %ΔIrms values of the possible Conformer ec1 deuteration sites to the experimentally determined d-DEA

Spectrum A. In addition, this table also lists the ab initio calculated rotational constants for the 7 possible deuterated isotopes of Conformer ec1 of diethanolamine. The possible

deuteration combinations of ec1 are presented in Fig. 31 and Table 23. The most likely possible deuteration combinitions that belong to Conformer ec1 are DEA4 and DEA6

(due to the low %ΔIrms values). However, upon examining the ab initio calculated rotational constants for these two possible conformers, it becomes obvious that assignment to a specific deuteration site is impossible. Due to the very similar calculated

B and C rotational constants for DEA4 and DEA6, it is impossible to determine which one results in d-DEA Spectrum A. With the A rotational constant of d-DEA Spectrum A being not clearly defined, it is impossible to make an exact assignment. As a result, d-

DEA Spectrum A is the result of deuteration of Conformer ec1, and likely is the result of either triply or doubly deuteration.

Table 22: %ΔIrms and Rotational Constants of Possible Deuterated Diethanolamine Molecules with d-DEA Spectrum A

%ΔIrms d-DEA Spectrum A A (MHz) B (MHz) C (Mz) DEA1 3.15 5929.405 1042.949 1009.515 DEA2 4.57 6000.181 1066.069 1035.144 DEA3 3.65 5923.364 1059.451 1025.903 DEA4 1.15 5730.972 1032.701 999.901 DEA5 2.57 5866.786 1041.274 1009.052 DEA6 1.74 5791.029 1034.481 1000.275 DEA7 3.12 5858.99 1057.462 1025.528

Fig. 31: Deuteration Sites on Conformer ec1 of Diethanolamine

Table 23: Possible deuterated diethanolamine molecules with deuterated label and relative deuteration site on Conformer ec1

Deuteration DEA1 H3 DEA2 H2 DEA3 H1 DEA4 H1, H2, H3 DEA5 H2, H3 DEA6 H1, H3 DEA7 H1, H2

Similar results are seen for d-DEA spectra B and C. Table 24 gives the possible

Conformer ec1 isotopes and their relative %ΔIrms compared to d-DEA Spectrum B. In addition, the ab initio calculated rotational constants of Conformer ec1 are depicted in the table. While d-DEA Spectrum B is known to be the result of deuteration of Conformer

ec1, the exact deuteration sites cannot be determined, due to the lack of information on the experimentally determined A rotational constant. The deuterated combination presented in DEA3 and DEA7 are the most likely candidates, as they have relatively low

%ΔIrms values with d-DEA Spectrum B; in addition, DEA3 and DEA7 also have very similar B and C rotational constants to the experimentally determined rotational constants for d-DEA Spectrum B. The deuterated combination presented in DEA5 is also a possibility, due to its low %ΔIrms with d-DEA Spectrum B.

Table 24: %ΔIrms and Rotational Constants of Possible Deuterated Diethanolamine Molecules with d-DEA Spectrum B

%ΔIrms d-DEA Spectrum B A (MHz) B (MHz) C (Mz) DEA1 2.20 5929.405 1042.949 1009.515 DEA2 2.65 6000.181 1066.069 1035.144 DEA3 1.82 5923.364 1059.451 1025.903 DEA4 1.97 5730.972 1032.701 999.901 DEA5 1.78 5866.786 1041.274 1009.052 DEA6 1.96 5791.029 1034.481 1000.275 DEA7 1.20 5858.99 1057.462 1025.528

Likewise, Table 25 below illustrates the possible Conformer ec1 isotopes and their relative %ΔIrms values compared to d-DEA Spectrum C. While d-DEA Spectrum C is known to be the result of deuteration to Conformer ec1, the exact deuteration sites are not known. By comparing the %ΔIrms values and the B and C rotational constants of the calculated ab initio results, the most likely deuteration sites that result in d-DEA

Spectrum C are DEA4 and DEA6. Both DEA4 and DEA6 have low %ΔIrms values with

d-DEA Spectrum C; in addition, both also have relatively similar B and C rotational constants with the experimental B and C rotational constants for d-DEA Spectrum C.

Table 25: %ΔIrms and Rotational Constants of Possible Deuterated Diethanolamine Molecules with d-DEA Spectrum C

%ΔIrms d-DEA Spectrum C A (MHz) B (MHz) C (Mz) DEA1 1.81 5929.405 1042.949 1009.515 DEA2 3.24 6000.181 1066.069 1035.144 DEA3 2.30 5923.364 1059.451 1025.903 DEA4 0.60 5730.972 1032.701 999.901 DEA5 1.21 5866.786 1041.274 1009.052 DEA6 0.68 5791.029 1034.481 1000.275 DEA7 1.81 5858.99 1057.462 1025.528

d-DEA Spectrum D is assigned to Conformer ec2. However, the exact deuteration site that results in this spectrum is not able to be determined. The possible deuterated sites of Conformer ec2 are shown in Figure 32. In addition, the labels for these deuteration sites are given in Table 26. The %ΔIrms values obtained from the theoretical moments of inertia compared to the experimental d-DEA Spectrum D moments of inertia are given in Table 27. Table 27 shows that there are numerous possibilities for the locations of the deuteration of Conformer ec2 of diethanolamine that result in the spectrum obtained. The most likely deuteration combinations belong to

DEA11 and DEA13. With no b-type transition, however, exact determination of the deuteration site that results in d-DEA Spectrum D is impossible.

Fig. 32: Deuteration Sites on Conformer ec2 of Diethanolamine

Table 26: Possible deuterated diethanolamine molecules with deuterated label and relative deuteration site on Conformer ec2

Deuteration DEA8 H3 DEA9 H2 DEA10 H1 DEA11 H1, H2, H3 DEA12 H2, H3 DEA13 H1, H3 DEA14 H1, H2

Table 27: %ΔIrms and Rotational Constants of Possible Deuterated Diethanolamine Molecules with d-DEA Spectrum D

%ΔIrms d-DEA D A (MHz) B (MHz) C (MHz) DEA8 3.82 6265.006 1077.196 1049.082 DEA9 3.79 6207.144 1105.136 1075.987 DEA10 3.35 6179.345 1100.918 1067.403 DEA11 1.75 5990.89 1065.991 1037.713 DEA12 2.76 6139.805 1073.74 1047.595 DEA13 2.60 6110.096 1069.612 1039.046 DEA14 2.27 6055.961 1097.031 1065.862

Therefore, the d-DEA spectra A, B and C are all known to belong to deuterated

Conformer ec1 of diethanolamine, and d-DEA Spectrum D belongs to deuterated

Conformer ec2. However, unless the A rotational constant was better determined experimentally (by obtaining a b-type rotational transition, for example), the exact deuteration site that results in these spectra is impossible to determine.

5. 2-aminophenol

5.1. Background Information

A subsequent goal of my research was to study the structural characteristics of the molecule 2-aminophenol. 2-aminophenol (2ap) is depicted below in Fig. 33. It is a poor biodegradable intermediate, used in the manufacturing of dyes and photographic emulsions.34 Since it is easily oxidized, it tends to remove oxygen from solutions.

Therefore, when 2-aminophenol is released from industrial waste waters into streams and rivers, it will deplete the capacity of these environments to sustain aquatic life.34

Fig. 33: Structure of 2-aminophenol

While the molecule 2-aminophenol does have some environmental implications like diethanolamine, the primary reason for studying 2-aminophenol was for its unique structure. 2-aminophenol contains intramolecular hydrogen bonding networks (quite like diethanolamine). Upon the addition of water to complex with 2-aminophenol, it is unknown whether or not 2-aminophenol will have a conformational change to allow for

formation of new hydrogen bond networks. The primary reason for study was to examine whether new intermolecular hydrogen bond networks from the water complex replace the intramolecular hydrogen bond of the monomer. Previously, the molecule 2- aminoethanol was studied in a similar fashion.35 Upon the addition of water to 2- aminoethanol, the conformational structure of 2-aminoethanol did change slightly (the

OCCN torsional angle increased from 57° to 75° and the O-N distance increased from

2.796 to 3.100 Å). A change was also noted for the complex of glycidol-water.36

Therefore, this study of 2-aminophenol lays the ground work for further study; it is unknown whether or not the ring structure will prevent any conformational changes, or like 2-aminoethanol and glycidol, the structure will change upon addition of water. In addition, due to the chromophore in 2-aminophenol, this investigation will allow for future comparisons with laser experiments.

5.2. Theoretical Modeling

2-aminophenol has only briefly been studied in the past. An earlier study examined five conformers of 2-aminophenol at various levels of theory that determined that the cis-structure of 2-aminophenol was the lowest energy conformer.37 An illustration of this conformer is shown in Fig. 34. These five conformers were studied at the RHF, MP2 and B3LYP levels, with the basis sets used being the 6-31G(d), the 6-

311++G(d) and the 6-31++G(d,p).37 The energies of the three lowest energy conformers: cis, trans and gauche (see Fig. 34, 35 and 36) were compared; the relative energies of the conformers are given in Table 28. The cis conformer contains a hydrogen bond between

the hydroxyl-H and the amine-N (see Fig. 34). The trans conformer contains a hydrogen bond between the hydroxyl-O and an amine-H, where both amine-H’s point away (see

Fig. 35). The gauche conformer also contains a hydrogen bond between the hydroxyl-O and an amine-H, however, the hydrogen-bonded amine-H is in the same plane as the ring

(see Fig 36). Table 28 shows how the ΔE between the cis and trans conformers greatly decrease in comparative levels and basis sets, while the trans conformer is actually predicted to be more stable than cis at the RHF/6-31++G(d,p) level.

Fig. 34: Lowest energy cis-conformer of 2-aminophenol determined by Soliman et al37

Fig. 35: The trans conformer determined from Soliman et al37

Fig. 36: The gauche conformer determined from Soliman et al37

Table 28: Relative energies for cis, trans and gauche conformers at various levels of theory37

ΔE cis ΔE trans ΔE gauche Theory (KJ/mol) (KJ/mol) (KJ/mol) RHF/6-31G(d) 0.00 8.68 10.43 MP2/6-31G(d) 0.00 7.06 10.13 B3LYP/6-31G(d) 0.00 3.78 7.23

RHF/6-311+G(d) 0.00 6.68 8.72 MP2/6-311+G(d) 0.00 2.81 6.41 B3LYP/6-311+G(d) 0.00 1.27 4.05

RHF/6-31++G(d,p) 1.21 0.00 2.12 MP2/6-31++G(d,p) 0.00 3.29 6.48 B3LYP/6-31++G(d,p) 0.00 0.43 2.97

Our current theoretical modeling was based off of this study. Three initial structures were studied; these three structures were labeled as Cis, Trans and Gauche, and correlate with the cis, trans and gauche structures of Soliman, respectively (see Fig.

34, 35 and 36). The structures were first studied using the RHF level at the aug-cc-pVDZ basis set. This aug-cc-pVDZ basis set is part of a correlation-consistent basis set, and is known as a Dunning basis set. Here, the cc-p refers to ‘correlation-consistent polarized’ and the ‘V’ indicates valence-only basis sets. The ‘D’ or ‘T’ refer to double and triple, respectively. Therefore, a cc-pVDZ refers to a Double-zeta, while a cc-pVTZ refers to a

Triple-zeta. Furthermore, aug refers to augmented versions of the basis sets; so a basis set of aug-cc-pVDZ refers to an augmented basis set with Double-zeta.6 Following convergence at this level and basis set, the optimized structures were then studied using

B3LYP/aug-cc-pVDZ level. Finally, ab initio calculations were performed at RHF/6-

311++G(d,p) level of theory for the three conformers.

The starting Gauche configuration was found to have converged into the Trans conformer by comparing structures and energies. As a result, only two unique conformers were identified: the Cis Conformer and the Trans Conformer, which are depicted in Fig. 37 and 38, respectively. These structures were generated based off of the

RHF/6-311++G(d,p) level of theory. The relative energies for the two conformers are given in Table 29. The Cis Conformer was determined to be the low energy conformer

(with a ΔE=0.0 KJ/mol), while the Trans Conformer is 4.26 KJ/mol higher in energy. As with diethanolamine, intramolecular hydrogen bonding is a large part of the 2- aminophenol structure. In the Cis Conformer, the hydroxyl-H forms a hydrogen bond with the amine-N; while in the Trans Conformer, the hydroxyl-O forms a hydrogen bond with an amine-H.

Fig. 37: Cis Conformer determined from RHF/6-311++G(d,p) level of theory

Fig. 38: Trans Conformer determined from RHF/6-311++G(d,p) level of theory

We began our search for spectral transitions using the RRFIT predictions for the rotational transitions of the Cis Conformer. The dipole moments and relative energies of the Cis and Trans Conformers are given in Table 29. Since the Cis Conformer was the low energy conformer, and it had a large μb, b-type transitions were searched for the Cis

Conformer.

Table 29: Principal dipole moments and relative energy for the Cis and Trans Conformer determined from RHF/6-311++G(d,p) level of theory

Conformer ΔE (KJ/mol) μa (D) μb (D) μc (D) Cis 0 -0.0314 -2.6350 0.0004 Trans 4.26 0.0213 0.7994 1.1063

5.3. Spectrum and Hamiltonian Fitting

One spectrum was found for 2-aminophenol. 2-aminophenol is a solid at room temperature; in order to find this spectrum on the microwave spectrometer, the valve was heated to 175°C. A total of 11 b-type lines were measured for this spectrum. The 11 b- type rotational transitions are presented in Table 30, illustrating the quantum numbers and relative frequencies for the rotational transitions. From these 11 b-type transitions, the rotational constants were determined. The three rotational constants with their relative uncertainties are listed in Table 31. As is evident, the uncertainties for these rotational constants are all relatively low. Unlike in diethanolamine (where only a-type transitions were found), b-type transitions provide information on all three rotational constants. As a result, the relative uncertainty for the A rotational constant is much lower than the A uncertainties seen for diethanolamine. Typically, 200-300-shot scans were

enough to see the rotational transitions above the background noise. Some example rotational transitions are given in Fig. 39 and 40. Like diethanolamine, the rotational transitions of 2-aminophenol are not perfect doublets; this is the result of the N-atom in

2-aminophenol, which has a spin equal to one. This spin results in hyperfine structure due to quadrupole interactions.

Table 30: Quantum numbers and relative frequencies for 11 b-type transitions of 2- aminophenol Spectrum

Quantum Number Frequency (MHz) Obs-Calc (MHz)

404-313 11928.42 -0.02 414-303 12252.50 0.00 322-211 14119.69 0.03 505-414 14725.89 -0.02 515-404 14829.20 0.06 423-312 16372.04 -0.12 514-423 16449.26 0.01 606-515 17449.28 -0.03 616-505 17479.08 0.00 524-413 18417.10 0.09 331-220 18433.51 0.01

Table 31: Rotational Constants of Experimental Spectrum with their Relative Uncertainties

Rotational Constant Value (MHz) A 3362.0992(50) B 2217.6501(62) C 1344.45177(159)

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Intensity (arbitrary Intensity (arbitrary unit)

14828.8 14829 14829.2 14829.4 14829.6

Frequency (MHz)

Fig. 39: 515-404 Rotational Transition of 2-aminophenol at 14829.20 MHz, at 500 shots

Intensity (arbitrary Intensity (arbitrary unit)

14725.4 14725.6 14725.8 14726 14726.2 14726.4 Frequency (MHz)

Fig. 40: 505-414 Rotational Transition of 2-aminophenol at 14725.89 MHz, at 300 shots

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5.4. Discussion of Structure

In order to determine what conformer resulted in the spectrum obtained, a number of different factors were examined. By looking at the relative energies, the Cis

Conformer (being the lowest energy conformer) is the most likely conformer to be seen.

A difference in energy of 4.26 KJ/mol is pretty significant; therefore, the Trans

Conformer is expected to have a much lower population. The dipole moments of the two conformers also provide information to the likely conformer that resulted in the spectrum obtained. Table 29 lists the dipole moments for the two conformers. The Cis Conformer has a relatively high μb, which would indicate strong b-type transitions, while the Trans

Conformer has a relatively high μc dipole moment, which would indicate strong c-type transitions. Like in diethanolamine, the %ΔIrms values were also determined for this spectrum. The experimental moments of inertia were compared against the calculated moments of interia for the Cis and Trans Conformers. The results of this calculation are presented in Table 32. The %ΔIrms values are nearly the same, and so this value does not provide much information on assigning the spectrum to one of the conformers. The

%ΔIrms values are nearly identical because the only change in the structure from the Cis

Conformer to the Trans Conformer is a slight shift in the hydrogens. Since hydrogens have such a small mass, the change in the moments of inertia between the Cis and Trans

Conformer is very slight. While the %ΔIrms value does not provide much useful information on assignment, by examining the relative energies and dipole moments, a conclusion can be drawn. With the Cis Conformer being the lowest energy conformer

102

and also having the relatively high μb dipole moment (with b-type transitions), the spectrum can be assigned to the Cis Conformer.

Table 32: Results of %ΔIrms for Cis and Trans Conformer when compared with Experimental Spectrum

Conformer %ΔIrms Cis 1.21 Trans 1.50 .

6. Conclusion

In summary, this research has illustrated the use of microwave spectroscopy to further study the molecules of diethanolamine and 2-aminophenol. Two spectra of the normal species of diethanolamine were obtained using microwave spectroscopy. From these spectra, the relative rotational constants were obtained, in which structural information was gathered. The conformers of diethanolamine that resulted in these two spectra were determined. In addition, isotopic studies of diethanolamine were also carried out. Diethanolamine was deuterated, and four unique spectra of deuterated- diethanolamine were obtained. Without any b-type transitions for diethanolamine, the exact deuteration site that results in the obtained spectra is impossible to determine.

However, the d-DEA spectra A, B and C were determined to all belong to the lowest energy conformer (Conformer ec1) of diethanolamine, while d-DEA Spectrum D belongs to a deuterated species of Conformer ec2. In order to more fully understand the isotopic spectra of diethanolamine, a b-type transition would have to be obtained. While working on this project, a b-type transition was searched for, but never found. With the relatively low μb dipole moment, it is extremely challenging to find a b-type transition.

In addition to working on the diethanolamine project, the molecule 2- aminophenol was also studied. A spectrum relating to the lowest energy conformer of 2- aminophenol was obtained. From this spectrum, the rotational constants were gathered, in which structural information was procured. Future studies of this project will involve

103

104

searching for a second conformer. It will undoubtedly be difficult to find another conformer of 2-aminophenol, since the second lowest conformer has a relative energy of

4.26 KJ/mol higher in energy compared to the lowest energy conformer (the conformer seen). Further work on 2-aminophenol will relate the isotopic substitutions of 2- aminophenol. Most likely a deuteration will be performed in order to gain a better understanding of the positions of the amine-H’s and the hydroxyl-H. Following a deuteration study, a 2-aminophenol-water complex will likely be studied in order to study the difference between the intramolecular hydrogen bonding network and the intermolecular hydrogen bonding network (similar to 2-aminoethanol-water and glycidol- water complex studies in the past).

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