P. Wipf 1 Chem 1140

Chem 1140; Spectroscopy

• UV-VIS • IR • NMR

UV-VIS Spectroscopy

The Absorption Laws

I 1 I0 I I2 Detector intensity of the transmitted incident beam light

I2 Overall transmittance: T = (what is actually measured) I1 I Internal transmittance: T i = (of interest to the spectroscopist) I0

Usually T0 = Ti Such differences as might exist can be minimized by using matched cells and setting T for the reference at 100%. P. Wipf 2 Chem 1140

The quantity I/I0 is independent of the intensity (I) of the source and proportional to the number of absorbing molecules

I0 . . Lambert-Beer Law: log = ! l c = A I

l = path length of the absorbing solution in [cm] c = concentration in moles/liter

Log I0/I = absorbance or optical density = A ! = molar extinction coefficient [1000 cm2 mol-1]

! i s a characteristic of a g iven compound, or more accurately, the light absorbing system of the compound, the so called chromophore. ! i s correlated to the size (Å) of the chromophore and of course wave-length dependent.

! = f (") ! ! f (c) (Approximation! An accurate determination of ! r equires determining A at various concentrations)

!: 10 - 105 (scales with extended #-systems) (dyes) P. Wipf 3 Chem 1140

UV-Vis spectra are usually plotted as A vs. ! plots:

A

hyperchromic shifts 2 hypsochromic bathochromic hypochromic

1 "#"$

n#"$

!max [nm] !

Though there are discrete levels of electronic excited states in a molecule, we do not observe absorption lines but broad peaks; the change in vibrational and rotational energy levels during absorption of light leads to peaks containing vibrational and rotational fine structure. Due to additional interaction with solvent molecules, this fine structure is blurred out, and a smooth curve is observed. (vapour phase: one can observe vibrational fine structure). P. Wipf 4 Chem 1140

Selection Rules

The irradiation of organic compounds may or may not give rise to excitation of electrons from one orbital to another orbital. There are transitions between orbitals that are quantum mechanically forbidden.

Two selection rules:

-!Spin-rule:!! The total spin S may not change during transition (S ! S; T ! T) -!Symmetry rule:! e—transitions between orbitals of identical symmetries !!!! are not allowed. !!!! (for ex. Even/uneven with regard to inversion).

n !* even even

! In reality, these quantum mechanical rules are not rigidly observed, however, due to molecular vibrations, the intensities (") of "forbidden" transitions are significantly reduced (and are usually of diagnostic importance).

n # $* band near 300 nm of ketones; " = 10-100 benzene 260 nm band with " = 100-1000. P. Wipf 5 Chem 1140

Chromophores

Definition:! Chromophore – light absorbing electron system of a compound

Rules:! - ! or n- orbitals that do not interact lead to a spectrum that is the ! sum of the individual absorptions of the isolated chromophores. - the longer the conjugated system, the longer the wavelength of the absorption maximum and the higher its intensity. - a bathochromic and hyperchromic effect is observed, when atoms with n-orbitals are directly attached to a chromophore (-OH, -OR, NH2, SH, SR, Hal…) = auxochromic groups.

! Isolated Chromophores

Of the once listed in table, only few are of practical significance (Vacuum-UV)

!# !# h& C C $% ! ! "max =~ 190nm P. Wipf 6 Chem 1140 P. Wipf 7 Chem 1140

! Conjugated Chromophores

!# !# h& C C $% ! ! "max =~ 190nm

!3" !" A A !2

! n

!1 H S

S H S=Auxochrome P. Wipf 8 Chem 1140

! Benzene and aromatic compounds

The UV spectra of benzenes are characterized by three major bands which have been given a variety of names. P. Wipf 9 Chem 1140

Only ! is allowed. B band is forbidden (loss of symmetry due to molecular vibrations; shows vibrational fine structure).

MO Diagram of Ferrocene - Fe FeCp2 2 Cp

! e1u ! a1g ! 4 p e2g ! a2u, e1u a2u e2g, e2u e2u

4 s UV Spectrum of Ferrocene a1g 300

250

e 200 4 p 1u a1g, e1g, e2g a1g 150 e2g

100 e , e

extinction [cm-1/M] 1g 1u

50 e1u

0

e1g -50 200 300 400 500 600 700 800 a , a a2u 1g 2u lambda [nm] a Ferrocene has a molar extinction coefficient of 96 M-1cm-1 at 442 nm 1g P. Wipf 10 Chem 1140

Nomenclature of Electronic Transitions; Symbols of Symmetry Classes

Symbols of symmetry classes:

A: sym. (according to a Cn operation) Examples: B: antisym. (according to a Cn operation) 1 1 A2 A1 1 1 E: 2-fold degenerate state B1u A1g 1 1 T: 3-fold degnerate state B2u A1g 1 1 E1u A1g

Indices:

g: sym. (according to an inversion operation) u: antisym. (according to an inversion operation)

1: sym. (according to a C2 axis that is orthogonal to a Cn axis) 2: antisym. (according to a C2 axis that is orthogonal to a Cn axis)

‘: sym. (according to a plane of symmetry "n that is orthogonal to a Cn axis) ‘: antisym. (according to a plane of symmetry "n that is orthogonal to a Cn axis)

Calculation of spectra:

! Bonus Problem: Calculate UV and IR Spectra of Ferrocene and Acetylferrocene, and Compare to Experimental Data; can you design a Ferrocene derivative that is green- colored? P. Wipf 11 Chem 1140

Infrared Spectroscopy

After considering ultraviolet and visible radiation (200-800 nm) which is energetic enough to affect the electronic levels in a molecule, we shall now consider radiation which has a longer wavelength: infrared radiation which extends beyond the visible into the microwave region and is capable of affecting both the vibrational and the rotational energy levels in molecules.

Range of commercial instruments:!! 2500 nm to 16’000 nm !! physical chemists:!! 25000 – 160’000 Å !! analytical chemists:!! 2.5 – 16 microns (µ) !! organic chemists:!! 4000 – 625 cm-1

wavenumber ! = 1/" = !/c "normalized frequency"

Use: simple, rapid, reliable means for functional group identification.

Vibrational modes

For a molecule comprised of N atoms, there are 3N-6 normal modes of vibration (3N-5 for linear molecules). To a good approximation, however, some of these molecular vibrations are associated with the vibrations of individual bonds or functional groups (localized vibrations) while others must be considered as vibrations of the whole molecule.

Localized vibrations are: Stretching modes:

coupled: simple H H H H C H C C

symmetric asymmetric P. Wipf 12 Chem 1140

Bending modes:

H H H H C C in plane deformations

scissoring (sym) rocking (asym)

H H H H C C out of plane deformations

wagging (sym) twisting (asym)

Transmission

O H C C C C other stretching N H C N C O C H X Y Z C N bending and stretching stretching combination bands N O stretching FINGERPRINT N H bending REGION

Absorbance 2500 1500

-1 4000 3000 2000 1000 cm

wave numbers P. Wipf 13 Chem 1140

Selection rules

1. In order to observe an absorption, the dipole moment of the excited vibrational state must differ from that of the ground state. Reason:!oscillating dipole interacts with oscillating electric vector of h!

O O observed

H C C H H C C H not observed

Analysis of IR - Spectra of Unknown Compounds

1. frequency, shape, intensity of an absorption band have all to be considered in the interpretation

2. all characteristic absorption frequencies of a functional group have to be considered (band can be missing or is caused by another function)

3. first the obvious absorptions should be identified: !!!!!!!!!!!!!!!!X-H, C=O, C=C, out of plane C-H

4. subsequently the strong absorptions in the fingerprint region should be analyzed

5. possible structures can now be proposed and have to be checked with reference spectra (or data from other spectroscopic techniques) P. Wipf 14 Chem 1140

An Introduction to NMR Spectroscopy

1H NMR

13C NMR

The types of information accessible via high resolution NMR include:

1. Functional group analysis (chemical shifts) 2. Bonding connectivity and orientation (J coupling), 3. Through space connectivity (Overhauser effect) 4. Molecular Conformations, DNA, peptide and enzyme sequence and structure. 5. Chemical dynamics (lineshapes, relaxation phenomena).

http://www.chem.ucla.edu/%7Ewebspectra/ P. Wipf 15 Chem 1140

Nuclear Spin:!The proton is a spinning charged particle and has also a magnetic moment.

1H: - nuclear spin quantum number B m = 1/2 - such a nucleus is is described as having a nuclear spin I of 1/2

m = +1/2 m = -1/2 lower higher energy energy

Because nuclear charge is the opposite of electron charge, a nucleus whose magnetic moment is parallel to the magnetic field has the lower energy.

The difference in energy is given by:!!" = h# B0/2$

# = magnetogyric ratio ( a constant, typical for a nucleus, which essentially reflects the strength of the nuclear magnet)

Bo = strength of the applied magnetic field

h = Planck’s constant (3.99 x 10-13 kJ s mol-1) P. Wipf 16 Chem 1140

Note that as the field strength increases, the difference in energy between any two spin states increases proportionally.

! nuclear spin quantum #

m =-1/2 Ei = -mh# Bo/2$ "! = -mµ Bo O N degenerate

m = +1/2

O Bo

[values of = 1/ were picked for m, so that the difference in energy between two neighboring states will always be an integer multiple of Bo (!h/2")].

The number of nuclei in the low energy state (N!) and the number in the high energy state (N") will differ by an amount determined by the Boltzmann distribution:

N"/N! = e(-#$/kT) k = 1.381 x 10-23JK-1

When a radio frequency (RF) signal is applied, this distribution is changed if the radio frequency matches #$.

#$ = h% = h& B0/2'

% = resonance frequency = & B0/2'

% i s therefore dependent upon both the applied field strength and the nature of the nucleus. P. Wipf 17 Chem 1140

1H: in a 2.35 T field (earth magnetic field = 0.00006 T) = 0.999984

~ 1 in 106 (! = 100 MHz)

- The difference in population of t he two states i s exceedingly small, in the order of few parts per million. (even smaller in 13C, because " is smaller). - Relatively low sensitivity of NMR compared to IR or UV - Large Bo needed to increase the population difference (usually given in MHz of 1H resonance frequency). P. Wipf 18 Chem 1140

SUMMARY

- Nuclear spin is a property characteristic of each isotope and is a function of Z and N.

- Each isotope with I ! 0 has a characteristic magnetogyric ratio (!) that determines the frequency of its precession in a magnetic field of strength B0 #B0 ! = 2"

It is this frequency that must be matched by the incident electromagnetic radiation for absorption to occur.

- When a collection of nuclei with I ! 0 is immersed in a strong magnetic field, the nuclei distribute themselves among 2I + 1 spin states, the relative population of which is determined by the Boltzmann distribution, usually being near unity

N# = e(-!E/kT) N"

- If two (or more) spin state populations become equal, the system is said to be saturated. P. Wipf 19 Chem 1140

Obtaining an NMR Spectrum

Magnet Source of RF radiation Detector + amplifier Plotter, sample

The magnet:

permanent electromagnet superconducting

cheap, stable, more expensive, expensive, fixed field stronger, variable stronger, variable 1.4T field field 18T (24T)

Strength of magnetic field shifts: lock necessary (= substance with strong, defined NMR signal) Older: reference internal, external CDCl3 TMS: 0.0 ppm singlett P. Wipf 20 Chem 1140

Once a stable field is established, the question remains as to whether that field is completely homogeneous throughout the region between the pole faces of the magnet.

N S

lines of magnetic flux not uniform

Sample

sample has to be placed near the center of the pole gap

!For 2.35 T, to achieve a precision of +/- 1 Hz (10 ppb at 100 MHz) the field must be homogeneous to the extent of +/- 2.35 x 10-8 T! Such a phenomenal uniformity, even at the center of the field , can be achieved only by means of two additional techniques:

Spinning of the sample ("averages" out small inhomogeneities)

Variation of the contour of the field by passing extremely small currents through shim coils wound around the magnet itself: Shimming (manually, automatically)

Paradox: Large sample in order to have as many nuclei as possible, small sample to increase uniformity of the field.

! narrow bore tubes P. Wipf 21 Chem 1140

The Pulsed Fourier Transform Technique

Further advances in S/N ratio improvement had to await the development of faster computer microprocessors: ~1970’s.

- RF radiation is supplied by a brief but powerful pulse of RF current through the transmitter coil. The spectral width of the pulse is chosen to cover absorption of all nuclei of interest.

The duration of the pulse (tp) determines RF the frequency range covered (Heisenberg's intensity

uncertainty principle: "# "t >_ h)

-1 > -1 SW ~ tp ; tp _ (4SW) 2SW

Frequency

!o

Optimum tp are obtained by trial and error and are usually in the order of 10 µs for ! = 90° for best S/N ratio.

The next step in the PFT process is to monitor the induced AC receiver signal. Digital data collection gives us the modulated free induction decay (function of Mxy). FID because the current intensity

decreases with time. This decay is the result of T2 (spin-spin) relaxation.

M P. Wipf 22 Chem 1140

the microprocessor samples the voltage in the receiver coil at a regular interval, called dwell time, td. Voltage td > (2SW)-1

td

t0 t0

Time

In a set of nuclei with different ! precession and T1/T2, the digital FID curve becomes very complex:

CH3

At this point it becomes necessary for the computer to recognize the patterns mathematically and extract the signal frequencies and relative intensities for each set of nuclei. This analysis is performed by a Fourier transfor- mation of the FID date.

" f( x) = ao + #{an cos2!nso x + bn sin 2!nsox} n= 1 where: The parent function is constructed by summing together a series of a0 = constant; sine waves. an = amplitude; x = period; so = fundamental frequency; xo = 1/so; and line width: uncertainty principle: n = order of harmonic $%$t > 1 1 %1/2 > T2* Nuclei that are slow to relax give sharp signals, nuclei that relax rapidly give broad signals (solids).

paramagnetic residues line broadening $%1 $%2 $%3 0

frequency P. Wipf 23 Chem 1140

- Wink, D. J. J. Chem. Ed., 1989, 66, 810. Spin-Lattice Relaxation Times in 1H NMR Spectroscopy. - Glasser, L. J. Chem. Ed., 1987, 64, A228. Fourier Transforms for Chemists. - King, R. W.; Williams, K. R. J. Chem. Ed., 1989, 66, A213, A243. The Fourier Transform in Chemistry.

Summary: A typical example of the generation of a PFT spectrum: P. Wipf 24 Chem 1140

!tp:! pulse time (µsec) !tacq: the length of the time a given FID signal is actually monitored ! !(resolution, the ability to distinguish two nearby signals, is inversely -1 !! prop. to tacq. R = (tacq) 3 sec ! 0.3 Hz

tw: delay time, to allow for equilibrium distribution

1 tw = 3T1 - tacq. (for H no waiting time)

+ dead time ( phasing necessary) phase correction

adding up, FT, spectrum P. Wipf 25 Chem 1140

Taking an NMR – Practical Consideration

- Use 5 mm tube filled with ~ 0.5 mL of solution containing 1-5 mg of sample (1H NMR).

- common deuterated solvents: CCl4, CDCl3, C6D6, DMSO-d6, D2O, CD3CN, CD2Cl2, d6-acetone, CD3OD (because of HCl formation, do not leave sample in CDCl3!)

- peak listings in ppm and/or Hz.

- paramagnetic metal ion ! broad peaks

Chemical shift

!H depends on Bo " therefore relative frequencies are reported:

! act. "! TMS 83.4Hz "6 ! = = 6 = 1.39x10 =1.39 ppm !o 60x10

1.39 ppm = ! = downfield from TMS. 2

0 ppm (!) 1

downfield upfield

deshielded shielded P. Wipf 26 Chem 1140

Integration

! Area under absorption peak ~ # of nuclei resonating at that !

But:!nuclei must relax to equilibrium between pulses, not generally true of 13C NMR! P. Wipf 27 Chem 1140

First-Order Spin-Spin-Coupling

1H - 1H

From previous discussions one could have gotten the impression that a typical 1H NMR spectrum exhibits just one signal for each set of equivalent 1H-nuclei and that the same thing is true for 13C spectra, as well as for spectra of any other isotope. However, there are many more lines in a spectrum, and while these extra lines do make a spectrum more complex, they also offer valuable structural information that complements the chemical shift data. P. Wipf 28 Chem 1140

CH3CH2OH:!!The spin states of two hydrogens (methylene group):

B0

M=-1 0 0 1 Total Magnetization

Three spin states with population ratio of 1:2:1

! For methyl hydrogens the net experienced field will depend on the magnetization of the neighboring methylene group!

! The methyl signal will be split into three lines with intensity ratio 1 : 2 : 1 (= spin-spin-coupling, homonuclear coupling because the coupling is between nuclei of the same isotope). ! Triplet.

Accordingly, for the methylene signal, the possible spin states of the methyl group determine its multiplicity (number of lines in the signal).

The spin states of three hydrogens:

M=3/2 M=-3/2 4 spin states with population ratio of 1 : 3 : 3 : 1

M=1/2 M=-1/2

Quartet P. Wipf 29 Chem 1140

Accordingly, a doublet is observed for hydrogens that are coupled to a methine (CH) proton.

The multiplicity of a given resonance = n+1 (n=# of neighboring equivalent nuclei). The relative intensities of the multiplet follow Pascal’s triangle. P. Wipf 30 Chem 1140

J J

J = spacing between lines.

The slight difference in energy between the resonances is the coupling constant J [Hz]. J’s are independent of instrumental parameters!

Consider a 3-spin system with/without equivalent nuclei:

Ha Hb Hc C C C

3 Ha will be a doublet with Jab

3 3 Hb will be a “doublet of doublets” with Jab and Jbc

3 Hc will be a doublet with Jbc P. Wipf 31 Chem 1140

Jbc

Jab Jab Jbc

Ha Hb Hc P. Wipf 32 Chem 1140

13C NMR

• 12C (98%) has I = 0 ! no NMR

• 13C (1.1.%) has I = 1/2 ! NMR

• (! = h" = #hBo/2$ % gyromagnetic ratio such that "obs & 1/4 that of 1H (300 MHz ' 75 !MHz)

• Observe typically 0 – 230 ppm (rel. to TMS)

Typically decouple the protons by saturating them with a second broadband RF puls (double resonance technique, second transmitter coil; “white noise” ! if the irradiating field is strong enough, not only will the 1H nuclei approach saturation, but virtually 1 1 all the H magnetization will be tipped into the x1y plane. Since the H nuclei are no longer aligned with (or against) the applied field (which is along the z axis) they can no longer augment or diminish the magnetic field experienced by the carbons. As a result, the coupling interaction disappears, and each 13C multiplet collapses to a singlet! (D coupling not effected!)

! causes all 13C resonances to be singlets ! affords Nuclear Overhauser Effect ! makes integration of 13C spectra unreliable P. Wipf 33 Chem 1140

Aromatics: not effected by ring current

128.5 ppm

substituted C’s are typically of lower intensity

++ - -

176.8 176.8 209.0 102.1 85.3 P. Wipf 34 Chem 1140

Carbonyls:

O O O O

H OH OEt 205.1 199.6 177.3 169.5 O RO OR Cl ~ 100 - 110 168.6 O O CH N 3 I H Heavy atom effect. 174.9 158.9