Raman Spectroscopic Studies of Group 8 Metallocenes and Halogen-Bonded Cocrystals

Yann Desjardins-Langlais

Department of Chemistry Faculty of Science McGill University, Montreal

August 2014

A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Master of Science

List of Tables ...... iv List of Figures ...... v Abstract ...... vii Résumé ...... viii Acknowledgements ...... ix Chapter 1: Introduction ...... 1 Chapter 2: Methodology...... 3 2.1 Raman Spectroscopy ...... 3 2.2 Raman Instruments and Applications ...... 6 2.3 Symmetry and Factor Group Analysis...... 10 2.4 Diamond-anvil Cells ...... 12 2.5 Pressure Calibration Methods ...... 14 2.6 Raman Experimental Setup ...... 15 Chapter 3: Background Literature ...... 18 3.1 Group 8 Metallocenes ...... 18 3.1.1 Ferrocene ...... 20 3.1.2 Ruthenocene ...... 24 3.1.3 Osmocene ...... 26 3.2 Halogen-bonding in Cocrystals ...... 27 3.2.1 Cocrystal Properties ...... 27 3.2.2 Halogen Bonding ...... 28 3.2.3 Halogen-Bonded Cocrystals ...... 29 3.2.4 Raman Spectroscopy and Halogen Bonding ...... 30 Chapter 4: Raman Spectroscopic Studies of Group 8 Metallocenes ...... 33 5 4.1 Ferrocene, (η -C5H5)2Fe ...... 33 4.1.1 Raman Spectra Assignments ...... 33 4.1.2 Variable-temperature Raman Spectra ...... 35 4.1.3 Variable-pressure Raman Spectra ...... 39

5 4.2 Ruthenocene, (η -C5H5)2Ru ...... 44 4.2.1 Raman Spectra Assignments ...... 44 4.2.2 Variable-temperature Raman Spectra ...... 47 4.2.3 Variable-pressure Raman Spectra ...... 50 5 4.3 Osmocene, (η -C5H5)2Os ...... 54 4.3.1 Raman Spectra Assignments ...... 54 4.3.2 Variable-temperature Raman Spectra ...... 56 4.3.3 Variable-pressure Raman Spectra ...... 59 Chapter 5: Effect of Pressure on Halogen Bonding in Cocrystals ...... 63 5.1 Cocrystal Components ...... 63

5.2 (MDPPO)2•TFBB ...... 65

5.3 (MDPPO)2•TFIB ...... 71 Chapter 6: Conclusions ...... 77 References ...... 81 Appendix A Point Group Tables ...... 86 Appendix B Curve Fitting Using Wire 3.4...... 90

iii

List of Tables

4.1.1 Raman peak assignments of ferrocene ...... 34 4.1.2 Pressure shifting rates of ferrocene Raman peaks ...... 43 4.2.1 Raman peak assignments of ruthenocene ...... 46 4.2.2 Pressure shifting rates of ruthenocene Raman peaks ...... 53 4.3.1 Raman peak assignments of osmocene ...... 55 4.3.2 Pressure shifting rates of osmocene Raman peaks ...... 62 5.2.1 Raman peaks of MDPPO, TFBB and the 2:1 cocrystal ...... 66 5.2.2 Raman peaks and pressure shifting rates of TFBB cocrystal ...... 70 5.3.1 Raman peaks of MDPPO, TFIB and the 2:1 cocrystal ...... 71 5.3.2 Raman peaks and pressure shifting rates of TFIB cocrystal ...... 75

List of Figures

2.1.1 Differences between Rayleigh and Raman scattering ...... 4 2.1.2 Stokes and anti-Stokes intensities versus Rayleigh scattering ...... 6 2.2 Schematic representation of a Raman microscope ...... 9 2.4 Representation of a diamond-anvil cell with gasket ...... 13 2.5 Ruby fluorescence spectrum upon 514.5 nm excitation ...... 14 2.6.1 Raman spectrometer set-up ...... 16 2.6.2 Inside view of Raman spectrometer set-up ...... 17 3.2 Examples of bonding interactions between MDPPO and TFIB ...... 32 4.1.1 Ferrocene Raman spectra recorded with 514.5 and 785 nm lasers ...... 33 4.1.2 Ferrocene Raman spectra at various temperatures ...... 37 4.1.3 Ferrocene spectra of C-H stretching region at various temperatures ...... 37 4.1.4 Ferrocene Raman peak positions at different temperatures ...... 38 4.1.5 Ferrocene spectra at various pressures in 100-1265 cm-1 region ...... 39 4.1.6 Ferrocene spectra in the C-H stretching region at various pressures ...... 40 4.1.7 Ferrocene Raman peak positions at different pressures ...... 42 4.2.1 Ruthenocene Raman spectra recorded with 514.5 and 785 nm lasers ...... 45 4.2.2 Ruthenocene Raman spectra at various temperatures ...... 48 4.2.3 Ruthenocene C-H stretching region at various temperatures ...... 48 4.2.4 Ruthenocene Raman peak positions at different temperatures ...... 49 4.2.5 Ruthenocene spectra at various pressures in 100-1265 cm-1 region ...... 50 4.2.6 Ruthenocene spectra at various pressures in C-H stretching region ...... 51 4.2.7 Ruthenocene Raman peak positions at various pressures ...... 52 4.3.1 Osmocene Raman spectra recorded with 514.5 and 785 nm lasers ...... 54 4.3.2 Osmocene Raman spectra at various temperatures ...... 57 4.3.3 Osmocene spectra at various temperatures in the C-H stretching region ..57 4.3.4 Osmocene peak positions at various temperatures ...... 58 4.3.5 Osmocene spectra at various pressures in 100-1265 cm-1 region ...... 59 4.3.6 Osmocene spectra at various pressures in C-H stretching region ...... 60 v

4.3.7 Osmocene peak positions at various pressures ...... 61 5.1.1 Raman spectrum of MDPPO taken with 514.5 nm laser ...... 63 5.1.2 Raman spectrum of TFBB taken with 514.5 nm laser ...... 64 5.1.3 Raman spectrum of TFIB taken with 514.5 nm laser ...... 64

5.2.1 Raman spectrum of (MDPPO)2•TFBB cocrystals ...... 65 5.2.2 Pressure spectra of TFBB cocrystal in 100-1265 cm-1 region ...... 67 5.2.3 Pressure spectra of TFBB cocrystal in 1365-3500 cm-1 region ...... 68 5.2.4 Raman peak positions of TFBB cocrystal at various pressures ...... 69 5.3.1 Pressure spectra of TFIB cocrystal in 100-1265 cm-1 region ...... 72 5.3.2 Pressure spectra of TFIB cocrystal in 1365-3500 cm-1 region ...... 73 5.3.3 Raman peak positions of TFIB cocrystal at various pressures ...... 74

Abstract

Raman spectroscopy has been used on two different types of crystalline materials: group eight metallocenes and halogen-bonded cocrystals. Ferrocene, ruthenocene and osmocene were examined at various temperatures and pressures. A phase transition was detected at 164 K in ferrocene but none was detected in the case of ruthenocene and osmocene. For the high-pressure studies, ferrocene exhibited a phase transition at ~41 kbar, while ruthenocene did not. More interestingly, osmocene exhibited a phase transition at 33 kbar and a possible second transition at 38 kbar, which could not be confirmed definitively. Cocrystals of bromine- and iodine-containing tetrafluorobenzene compounds with methyldiphenylphosphine oxide were examined under various pressures to test the strength of the intermolecular halogen bonds. Both these complexes did not show any red-shifts in their Raman spectra as the pressure was increased, so it appears that the intermolecular interactions were weakened rather than increased. The iodine- containing compound did, however, show a structural change.

vii

Résumé

La spectroscopie Raman a été utilisée sur deux types de matériaux cristallins: les métallocènes du groupe huit et des cocristaux avec liaison halogène. Le ferrocène, le ruthénocène et l’osmocène ont été examinés à différentes températures et pressions. Une transition de phase a été détectée à 164 K dans le ferrocène mais aucunes transitions n’ont été aperçues pour le ruthénocène et l’osmocène. Pour les expériences de haute pression, le ferrocène a démontré une transition à ~41 kbar, tandis que le ruthénocène non. Plus intéressant, l’osmocène a présenté une transition de phase à 33 kbar et possiblement une deuxième à 38 kbar, qui n’a pas pu être confirmée définitivement. Des cocristaux de composés bromés et iodés de tétrafluorobenzène avec oxyde méthyldiphénylphosphine ont été examinées sous diverses pressions pour tester la force des liaisons intermoléculaires halogènes. Les deux complexes n’ont pas démontré un décalage vers le rouge dans leurs spectres Raman. Alors, il semble que les interactions ont été affaiblies plutôt qu’augmentées. Le composé iodé a, cependant, démontré un changement structurel.

viii

Acknowledgements

I especially would like to thank my advisor, Ian Butler, for his guidance and support during my research. I would also like to thank Jean-Philippe Guay for making the stainless steel gaskets. I would like to thank Christopher Nickels for the synthesis of the cocrystals. I would also like to thank Richard Rossi and Weihua Wang for their help with moving and troubleshooting the Raman instrument. I would also like to thank Joël Poisson for help with the instrument and his support.

Chapter 1

Introduction

Raman spectroscopy has proved to be a great asset to the field of crystallography.

Observing vibrational modes often allows easy differentiation between crystals, some of which have different forms that are known as polymorphs1-4. In addition, Raman spectroscopy allows the detection of phase transitions5-7.

This thesis will focus on two different applications of Raman spectroscopy with respect to crystalline materials. The first part will deal with some famous molecular crystals, while the second emphasizes intermolecular interactions in cocrystals, specifically halogen bonding interactions.

The molecular crystals selected for investigation in the first part of the thesis are the group eight metallocenes, which are sandwich-like structures with metal atoms located in the centre (explained in greater detail in Chapter 3). The metals belonging to group eight are iron, ruthenium and osmium. These three metallocenes are particularly interesting as they have high molecular symmetry. Moreover, they have numerous important industrial applications8-14. Although these types of compounds have been around since 195115, no high-pressure studies have been performed using Raman spectroscopy16-18. Much of the previous vibrational spectroscopic work has been focused on variable-temperature studies19-25. In this thesis, the effects of both temperature and pressure on the structures of these compounds will be investigated.

The cocrystals chosen involve tertiary phosphine oxides and halogen bond donors that are highly fluorinated aromatic compounds. No previous high-pressure studies on

these types of cocrystals have been reported in the literature. We will look into the effect of pressure on the halogen bonds specifically. For the sake of comparison, the halogen bonding in other types of cocrystals will also be described26.

In Chapter 2, the focus will be on Raman spectroscopy. Information will be provided to understand what is the Raman effect. Then the focus will change from the design of a Raman instrument to understand how the spectra are actually recorded. Next, an introduction to molecular symmetry and correlation splitting will be given to help the reader understand how crystalline structure affects vibrational spectra. Finally, some details will be presented on the materials used in this thesis for the variable-temperature and –pressure studies.

In Chapter 3, some background information is provided and separated into two sections. The first section is on metallocenes. Examples of the application of metallocenes are given and then the focus switches to the temperature and pressure studies reported in the literature. The second section explains the role of halogen bonding in cocrystals.

In Chapter 4, the vibrational data obtained on the metallocenes are provided, initially for the effects of temperature and pressure on ferrocene. Next, ruthenocene is investigated and then finally osmocene.

In Chapter 5, the results of the high-pressure Raman studies on the cocrystals are presented. The spectra of the individual components are described first, followed by a discussion of the data for the cocrystals themselves.

Chapter 2

Methodology

2.1 Raman Spectroscopy

The Raman effect was reportedly first discovered by C. V. Raman and K. S.

Krishnan in 1928 while looking at the light scattering from solutions27-28. Independently, two Russians, Landsberg and Mandelstam also published in 1928 the same phenomenon but instead of observing the effect in liquids, they were doing research with quartz crystals29. Contrary to Raman and Krishnan, Landsberg and Mandelstam also explained the effect that they had observed; an inelastic light scattering event had occurred30.

Shortly thereafter, C. V. Raman was awarded the 1930 Nobel Prize in Physics and the new phenomenon became known as the Raman effect. Although the effect was first discovered in 1928, the Raman effect was theorized a few years prior by some scientists such as Schrödinger31-32.

The Raman effect occurs when there is light scattering. Scattering is one of the ways in which light interacts with molecules in all types of matter. When light interacts with a molecule, the molecule will become excited. In the case of absorption, the molecule gets excited to a higher energy state, which corresponds to a molecular energy level. In the case of scattering, however, the state reached is a virtual one. This virtual state does not correspond to an actual energy level of the molecule, so it is therefore referred to as a virtual state. This state results from the coupling of the electric field of the incident light and the electrons of the molecule. This state is a very unstable energy level, which causes the interacting light to be immediately re-emitted. It is highly probably that the re-emission occurs at the same energy or frequency as the incident light.

This type of scattering is called Rayleigh scattering and also referred to as elastic light

scattering.

Raman scattering is inelastic light scattering. There is a transfer of energy to or

from the molecule. This transfer of energy occurs through the vibrational modes of the

molecule. The difference between Raman scattering and Rayleigh scattering is referred to

as the Raman shift. Since Raman shifts result from the vibrational modes of the

molecules, observing the shifts can be used to indirectly probe these vibrational energy

levels. Raman spectroscopy can therefore be used for vibrational studies. For an example

of how the Raman effect occurs in terms of energy levels, see Fig. 2.1.1.

Virtual Excited States

Infrared Absorption Rayleigh Scattering Stokes Scattering Anti-Stokes Scattering

Fig. 2.1.1 – Diagram showing the differences between infrared absorption, Rayleigh scattering and Raman scattering (Stokes and anti-Stokes)

Fig. 1: With inelastic light scattering, the light emitted can be at a higher or lower energy

Fig.level. 2: Emission at a lower and higher energy level is known as Stokes and anti-Stokes

scattering, respectively33. Stokes scattering Fig.is the 3: most probable type of Raman scattering

because at room temperature most molecules are initially in the ground state. For a

Stokes shift to occur, the electron can be in any state, although the transition from the ground state to the first excited vibrational state is most probable. Anti-Stokes on the other hand, cannot start from the ground state as the process goes from an excited state to a less excited or ground state.

The Boltzmann distribution represents the distribution of electrons into the ground state and excited state. Eq. 2.1 shows the probability of populating an energy state E while z refers to the partition function.

(2.1)

It is clear that at high energy levels, the population probability decreases rapidly when the temperature is very low. As the temperature is increased, however, the population of high energy states become more and more probable. At room temperature, the excited states are barely filled compared to the ground state. Anti-Stokes transitions therefore become highly improbable so, instead, Stokes scattering is used for typical

Raman spectroscopy. Fig. 2.1.2 shows an example with lines to illustrate the difference between intensities of the Stokes and anti-Stokes transitions. The transition has a net change of ν in the frequency of light, which corresponds to the vibrational transition.

Rayleigh Scattering

Stokes

Anti-Stokes

ν0 - ν ν0 ν0 + ν

Fig. 2.1.2 – Example of Stokes and anti-Stokes line intensities versus Rayleigh scattering.

Fig. 4:

Vibrational modes that are Raman active must undergo a change in the net

polarizability34. Infrared active modes occur when there is a net change in the dipole

moment35. Raman is a weak phenomenon. Inelastic light scattering is highly improbable

as only about one in 106 scattering events are inelastic36. In addition to low light levels,

Rayleigh scattering is a huge problem. The intensity of the elastic light scattering will

overpower the signal coming from the Raman scattering. Although Rayleigh scattering

can be blocked by filters, the problem of very low light levels still remains.

2.2 Raman Instruments and Applications

Raman instruments can come as spectrometers only or equipped with

microscopes. Some Raman instruments are made for easy detection, so microscopes are

not really needed. For more powerful instruments, however, microscopes are attached to

the Raman instrument and Raman imaging becomes possible. The ability to image and

then spatially locate certain compounds is extremely useful for biological applications.

To design a good Raman instrument, three things are needed: a bright monochromatic light source, an efficient dispersion system and a highly sensitive detector. There are also the optical systems for transmitting light to the sample and collecting the scattered light to consider. However, these optical systems typically do not change much when adapting a microscope for Raman analysis. Light collection is a major concern for large magnification lenses so the efficiency of the collection system will already have been taken care of.

Bright monochromatic light as a light source is essential. Lasers are a common example of light sources in Raman instruments. The increase in light level will increase the amount of scattering by the sample provided the sample does not degrade. Samples can melt, photobleach or burn when illuminated for too long and too intensely. The introduction of apertures in the system can tune the amount of light going to the sample.

After the light reaches the sample, the scattered light must be collected and then dispersed to separate the different wavelengths. The dispersion system typically includes components such as filters, prisms and gratings. Filters are used to block out Rayleigh scattering and then the light can be dispersed so that the Raman shifts can be observed.

Prisms and gratings are the main dispersion elements and can function well without a filter for the Rayleigh scattering. Filters are preferred in order to ensure that the detector is not saturated. You can also potentially scan for very small Raman shifts (100 cm-1 and less) which you would undoubtedly be unable to do without a filter.

Finally, the detector system plays a significant role in the functionality of the

Raman instrument. Photomultiplier tubes for a time were the most common choices as a detector for Raman32. The high sensitivity allowed much better Raman signals to be

observed. However, PMTs lack the ability to image and observe the Raman spectrum at various locations simultaneously. Charge-coupled devices became the replacement as

CCDs can be used for multichannel purposes37.

An example of a Raman microscope is shown in Fig. 2.2 and an explanation of the various components is given. The excitation laser is shone upon a beamsplitter which directs most of the light to the objective lens and then to the sample. A beamsplitter is used so that the scattered light can also pass through. Dichroic mirrors can also be used as the laser light can be reflected off the mirror while the Raman scattering would go through. A beamsplitter can be used with any laser wavelength which makes the system slightly more flexible.

After the light reaches the sample, the objective lens collects the scattered light.

Objective lenses are usually made with high numerical apertures to collect more light which will increase detection of Raman scattering32.

CCD Spectrometer

Focusing lens

Notch filter

Beamsplitter Lase r Objective lens Sample Fig. 2.2 – Schematic representation of a Raman microscope

Fig. 5: The light reaches a notch filter before being focused onto the detector. The notch filter is a filter that blocks out a small region of the light spectrum. The filter is used to block out the Rayleigh scattering so Raman scattering should go through easily.

Depending on how efficient these filters are, peaks at very low Raman shifts (<100 cm-1) can be detected. Another type of filter is an edge filter. Edge filters work by cutting off frequencies above a certain value, which allows photons with higher wavelengths to pass through. Notch filters have the advantage over edge filters as notch filters can be used to detect anti-Stokes shifts.

To make this simple Raman microscope more efficient, a pinhole-sized aperture is typically added after the beamsplitter. The lenses are changed such that the scattered light collected will be focused at the aperture only when the light had originated from a certain location in the sample. That specific location can then be clearly analyzed without seeing

light originating from other regions. The new microscope is called a confocal microscope. Many Raman instruments have confocal microscopes to increase resolution.

Raman instruments have been used in a wide variety of area such as forensic studies, biological applications, diagnostics, materials, pharmaceuticals and nanotechnology33,38-40. The wide variety of applications is partly due to the non-invasive and non-destructive nature of Raman spectroscopy. Also, Raman spectroscopy can be used easily on both solids and liquids increasing its usefulness.

2.3 Symmetry and Factor Group Analysis

Molecular symmetry has an enormous effect on Raman and infrared spectra. It is possible to use molecular symmetry to deduce the number of peaks that should appear in a typical vibrational spectrum. Molecular symmetry is the determining factor for the number of peaks appearing in the Raman and infrared spectra of liquids and gases.

However, solid crystals of a molecule may change the appearance of these spectra. Notably, a number of peaks can appear where there was previously a single peak.

A crystal has site symmetry and space group symmetry over and above the normal molecular symmetry. The site symmetry is the symmetry around a point in the crystal while the space group symmetry is the symmetry of the unit cell in the crystal. The site symmetry is typically very low such as Cs and Ci while the space group symmetry usually is much higher. Factor group analysis is important for explaining the behavior of crystals when analyzed by Raman and infrared spectroscopy.

Factor group analysis is used when a particular symmetry changes to another. The point of the analysis is to track each of the individual modes as they change. For example, the C2v point group has 4 irreducible representations: A1, A2, B1 and B2 (point group

tables are given in Appendix A). The C2 molecular point group only has the representations: A and B. If a molecule would go from a C2v point group to a C2, the A1 and A2 modes would become A while the B1 and B2 modes would become B. The easy way to see this is by looking at the point group tables, and taking all the symmetry operations that are in common. Note that one point group symmetry must be a subgroup of the other for this analysis to work.

The reverse scenario of a molecule with lower symmetry going to a higher symmetry is also possible. Imagine a C2 molecule that now has D2 symmetry. D2 symmetry has A, B1, B2 and B3 modes. By looking at the common symmetry operations, the A mode in C2 can become either A or B1 in D2 assuming the principal axis does not change. The B mode then becomes B2 or B3.

Note that if the principal axis changes, the axes indicated in the table must also change. The example given above where a C2 molecule becomes D2 is a good example.

There are three C2 rotation axes present so any one of these could be the principal axis.

The labeling of the modes depends on which one is the principal axis. In the case of C2, there was only one choice as a principal axis. If the principal axis of the D2 molecule is said to be the x axis of the C2 molecule, then the references to x, y and z also change.

Now suppose a molecule would go from a C2v symmetry to D2. In this case, since neither is a subgroup of the other, the transformations would go through two stages. The first transformation goes from C2v to C2 and the second C2 to D2. The principal axis is assumed to stay constant throughout for simplicity. The A1 and A2 modes of C2v become

A in C2 symmetry, which can be either A or B1 in D2 symmetry. Both A modes in C2v can be two different modes in D2 symmetry. A Raman spectrum of an original A1 mode will

now show the presence of the bands in D2 symmetry. The infrared inactive vibrational mode of A2 will also show the presence of a band as B1 in D2 symmetry is infrared active.

Although it is highly unlikely that a molecule with C2v symmetry becomes D2, it is an example of what could happen in a crystal. As the symmetry is lowered, an increase in the number of bands observed is possible for degenerate modes as they are split to non- degenerate modes. As the symmetry is increased from the site symmetry to the crystal symmetry, additional bands will most likely start to appear. In the example of the C2v to

D2 transition, each of the modes is doubled on going to D2.

2.4 Diamond-anvil Cells

A DAC or diamond-anvil cell is used for high-pressure studies. The sample is placed between two diamonds, which are squeezed together mechanically. The pressure increases as the diamonds come closer and closer together. To prevent scratching of the diamonds and to keep the sample fixed, a gasket is used which can be made from stainless steel. The gasket has a small hole punched through it so the sample can lie inside of the gasket. A fluid can be added to gasket to ensure hydrostatic pressures. The addition of a fluid is not necessary but it does ensure that pressures are constant throughout the sample.

Fig. 2.4 shows a diagram of a diamond-anvil cell with the gasket in between the two diamonds. The laser light would go through the diamond (either upper or lower) and would reach the sample in the hole within the gasket. Scattering can be collected above or below the two diamonds but many Raman microscopes such as the one used in this thesis work use backscattering. The lenses that shine light upon the sample will also be used to collect the light scattered.

Fig. 2.4 - Schematic representation of a diamond-anvil cell with a gasket. The laser light goes through the upper diamond to reach the sample located in the gasket hole. The drawback of using a DAC is that diamond itself has a strong peak at 1330 cm-

1. The necessary laser power to observe Raman scattering in the sample will cause the diamond peak to saturate the detector. To get around this problem, two spectra are recorded: one above the diamond line and another below. However, features near the diamond line are not observed so it is necessary that the compound of interest has spectral features in other regions.

Depending on the model of the DAC, different pressures can be reached. Some

DACs may reach up to 60 kbar, while others can go much higher. The pressures that can be reached depend on the size of the diamond. The smaller the diamonds, the higher the possible pressures. In addition, the mechanism on which the diamonds are brought together also has an impact on how much force it can give the diamonds which in turn influences the maximum pressure attainable.

2.5 Pressure Calibration Methods

The main method of calibrating the pressure is to use a small ruby chip. The fluorescence emitted from ruby can be used to deduce the pressure inside the cell. Ruby fluorescence shows a doublet appearing at approximately 694 nm (see Fig. 2.5 for sample spectrum). The second peak under normal conditions is located at 694.2 nm and it is the peak used for calibration. When the ruby detects an increase in pressure, the fluorescence becomes red-shifted.

Fig. 2.5 – Example ruby fluorescence spectrum using the 514.5 nm laser. The line indicates which peak maximum is used for calibration The difference in position between the ruby fluorescence versus the normal conditions can be translated into pressures using Eq. 2.5. The pressure in kbar is given as a function of the difference between the second peak and 694.2 nm.

(2.5)

Since this method of calibration observes fluorescence, the wavelength of the laser is extremely important as the wavelength must match an electronic transition of the ruby. In addition, the laser wavelength must be smaller than the fluorescence wavelength otherwise no fluorescence is observed.

2.6 Raman Experimental Setup

The Raman spectra in this thesis were obtained using a 785 nm diode laser (300 mW, 500 mW maximum) and a 514.5 nm argon ion laser (16.0 mW, 500 mW maximum intensity). The 514.5 nm laser was used mainly for the pressure work. Figs. 2.6.1 and

2.6.2 show pictures of the Raman spectrometer.

A microscope was used to focus and collect light. The objectives used were: 50x short working distance (for samples on glass slides and silicon wafer calibration), 20x long working distance (used for work involving the DAC) and a 50x long working distance (used with a thermal stage for temperature studies).

A CCD camera was used to collect the data. The calibration of the CCD was done by observing the 520 cm-1 Raman shift of a silicon wafer. The thermal stage, used for the temperature series presented later, is a Linkam THMS600 which can go from a temperature range of -196 to 600 °C. Temperatures above 300 °C require a water circulator to cool the stage. Temperatures below room temperature are achieved using liquid nitrogen.

The software used for the acquisition of the data is Wire 3.4. This software is also used for background correction and curve fitting to obtain peak locations. The temperature and pressure plots were made using MATLAB. Microsoft Excel was used to obtain the slope of the graphs.

Fig. 2.6.1 – The Raman spectrometer set-up with the 514.5 nm argon ion laser visible behind the spectrometer. The temperature controller is seen between the microscope and the computer. The DAC is shown under the 20x long working distance microscope.

Fig. 1.6.2 – View inside the spectrometer (top and bottom). Top shows the selection of filters (514.5 nm edge and notch filters and 785 nm notch), an aperture and a diffraction system (diffraction grating and prism). Bottom shows where the laser light comes in which goes through an aperture to control the light intensity. Mirrors then direct the light up towards the microscope.

Chapter 3

Background Literature on Metallocenes and Halogen-Bonded Cocrystals

3.1 Group 8 Metallocenes

Metallocenes are a group of compounds where a metal atom is sandwiched

5 between two cyclopentadienyl rings. The molecular formula is (η -C5H5)2M, where M refers to a metal atom. The so called group 8 metallocenes are compounds with group 8 metal atoms at their centre. These metallocenes are termed ferrocene, ruthenocene and osmocene. Ferrocene was the first metallocene discovered and was also the first ever sandwich compound created.

Ferrocene was discovered accidentally in 1951 when Pauson and Kealy wanted to couple two cyclopentadiene rings to form fulvalene15. Instead, they obtained a stable orange compound of unknown structure. These researchers thought that the iron atom was bonded to both rings by a single bond, as the structure would then have a resonance form which happens to be aromatic. The aromaticity would increase the stability or so they thought. Other researchers independently obtained ferrocene around the same time and imagined the same structure as Pauson and Kealy15. So, Pauson and Kealy were not

the first to prepare ferrocene, but they were the first to publish their results and consequently they are often credited with the discovery of ferrocene.

Later research by Fischer, Woodward and Wilkinson would reveal the sandwich nature of ferrocene15. Fischer deduced that ferrocene must be in an 18-electron complex as it would not react with carbon monoxide. An 18-electron configuration could only happen with a sandwich compound where all the -electrons of the cyclopentadienyl anions would be involved in bonding. Woodward and Wilkinson used an IR spectrum as evidence for their structure, which exhibited only a single C-H vibration, indicating that all C-H bonds in the molecule were equivalent.

After the discovery of ferrocene and its structure, the field of organometallics really took off with researchers synthesizing different sandwich compounds with different aromatic groups, different metals and additional ligands. It became almost a sort of race between Fischer and Wilkinson to synthesize and publish the most compounds they could15.

Additionally, the stability of ferrocene and an increased activity towards functionalization on the cyclopentadienyl rings compared to other aromatics made ferrocene a very interesting choice for a number of applications8. As Ornelas mentions

(2011), examples of applications can be sensors, catalysts, and electroactive materials but also as smoke suppressants13 and antiknock agents14.

An example where ferrocene is used as a sensor utilising its electronic properties would be for detecting glucose9-11. Glucose sensors work by having mediators linked to

electrodes and enzymes. These mediators can be ferrocene-based as ferrocenes are good electron carriers. The mediators are responsible for transferring electrons to the enzymes which will perform the redox reactions.

Another example is the use of ferrocene for catalysis. Ferrocene-based dendrimers have been created for the purpose of stabilizing palladium nanoparticles, which are later used for catalyzing hydrogenation12. The catalysts are also size selective as the palladium atom lies within a dendrimer so if a molecule needs to be reduced, it has to go inside the dendrimer which larger molecules would be unable to do.

The smoke suppressing properties of ferrocene derivatives as additives in PVC work in three ways: catalysis of dehydrochlorination, char formation and char oxidation13. By promoting char formation and later on char oxidation, less volatile gases are emitted which would reduce the amount of smoke produced. In addition, hydrogen chloride gas is toxic and helps in the formation of smoke so the removal of gas would also help in smoke suppression.

3.1.1 Ferrocene

The structure of ferrocene has long been under study. The more common belief is that ferrocene has molecular symmetry of D5d, which corresponds having the two cyclopentadienyl rings in a staggered conformation. However, previous research has shown by using X-ray and neutron diffraction that ferrocene does not have a staggered ring structure41-42. Instead, the rings may reside in between a staggered and an eclipsed structure. This possible conformation would have D5 molecular symmetry instead of D5d.

Ferrocene in solution, however, would most likely have D5d symmetry and many researchers have used this particular symmetry for vibrational assignments43-45.

A change in the molecular symmetry would result in a different space group as the space group P21/c requires centrosymmetric molecules. The symmetry of the crystal could come from a statistical nature. A large number of disordered ferrocene molecules with different ring orientations may average out to correspond to D5d symmetry, which would lead to the centrosymmetric crystals that are expected42.

Crystalline ferrocene with the P21/c space group exhibits some spectral changes from the solution form. Ferrocene goes from D5d molecular symmetry in a solution to site

46 5 symmetry Ci to a space group of C2h .

The site symmetry causes degenerate vibrational modes to break into two non- degenerate modes with the same symmetry – e.g., 1 E1g mode becomes 2 Ag modes. After

that, the higher symmetry of C2h compared to Ci site symmetry causes the non-degenerate modes to split between A and B modes. A non-degenerate mode such as A1g in D5d symmetry would become Ag in site symmetry Ci and then end up as Ag + Bg in C2h space symmetry. A degenerate mode such as E1u would split into 2 bands which both split further into 2 bands each resulting in 4 bands in total. The net result is that non- degenerate modes are doubled and degenerate modes are quadrupled.

However, it is highly unlikely that all these bands will spread enough so that every single mode will be observed. This so-called Davydov or correlation splitting will most likely be quite small compared to the width of the peaks observed, so individual

peaks will coalesce. Davydov splitting refers to the splitting of bands in crystals. The splitting occurs when there is more than one full molecule in the unit cell.

The effect of temperature on ferrocene has been the major focus of some spectroscopic work. It was discovered that ferrocene has a heat capacity transition at

163.9 K and a secondary transition at 169 K20-21,25. This phase transition is from an undercooled high temperature phase to a metastable low-temperature phase19,22-23,25. The ferrocene crystals go from monoclinic to triclinic as the temperature is lowered. The triclinic crystals of ferrocene are very ordered compared to the monoclinic ones20-21.

In addition to metastable phase, a stable low-temperature phase can be obtained by annealing the crystals for over a day at 190 K19. In this situation, the stable low- temperature phase adopts an orthorhombic structure. The presence of another phase and another crystal form makes ferrocene a complex molecule to analyse using variable- temperature work.

Furthermore, when ferrocene is cooled to near 100 K and below, it can start to disintegrate24-25. This disintegration can be quite violent as the crystals will break apart and the pieces scatter. It has been noted that disintegration only occurs for the triclinic phase and not for the orthorhombic phase25. It is believed that the disintegration occurs because of the presence of domains and that the crystallite pieces would contain a single domain24-25. As the temperature is lowered, the crystals are contracted more and more which adds strain to the system. Eventually, the energy must dissipate and this occurs by disintegration. Meanwhile, domain boundaries could contain the energy as the strain is increased24-25.

Although the effect of temperature on ferrocene has been studied extensively, not much work has been done using high pressure systems. There are only a few previous papers that have dealt with pressure16-18. Two of these were performed using infrared studies rather than Raman and the last paper was done using optical analysis.

The first paper was written by the Adams’ group (1980). These researchers embedded a few small crystals of ferrocene within a Nujol mull. IR spectra were collected in the 400-1800 cm-1 region and they also used far-IR to collect data for the

100-300 cm-1 region. Since the published spectra only went up to 1800 cm-1, the C-H stretches were not observed. These authors did observe a possible phase transition between 10.3 and 16.5 kbar.

The second paper by Roginski (1988) used a similar approach as Adams (1980).

Nujol was again used to obtain hydrostatic pressures but data were only recorded in the

750-1350 cm-1 region. The presence of a phase transition was noted and new peaks did emerge in the 800-920 cm-1 region at higher pressures. The presence of these peaks at higher pressures is more likely due to increases in intensity in the specific region. The increases in intensity would allow smaller splittings to appear and in fact, some of the peaks appear as shoulders at lower pressures.

The third paper by Duecker (1967) originally reported a ferrocene phase transition using an optical microscope and a diamond-anvil cell. This paper is listed third as the pressure was increased up to 20 kbar only. A phase transition was detected at 11.5 ± 0.5 kbar. It was pointed out that polycrystalline ferrocene can be used if there is enough time for equilibration.

3.1.2 Ruthenocene

Ruthenocene has a molecular symmetry of D5h, which means that the two cyclopentadienyl rings are eclipsed with respect to one another47-48. The eclipsed structure of the rings come from the fact that ruthenocene has a larger atom so the two rings are further apart. The interaction between the two rings is minimal so an eclipsed symmetry is more favourable as molecules can be packed much better49.

Contrary to ferrocene, ruthenocene forms orthorhombic crystals with a space group of Pnma47-49. The ruthenocene unit cell contains two full molecules, which are centrosymmetric with respect to one another.

Just like ferrocene, the spectrum of crystalline ruthenocene becomes much more complex compared to ruthenocene in solution. Ruthenocene goes from D5h molecular

16 50 symmetry to Cs site symmetry to D2h space group symmetry . There are a total of four molecules per unit cell which leads to a larger number of crystal vibrations per molecular vibration.

With ruthenocene, degenerate modes are split into A’ and A’’ while non- degenerate modes become either A’ or A’’. Becoming an A’ or A’’ mode depends on the symmetry around the horizontal plane in the site symmetry. However, which modes become A’ and A’’ depend on the frame of reference used as changing coordinate axes changes the result. The overall result is not affected by which frame of reference is used as there will be the same number of bands. After the site symmetry, the space symmetry of D2h causes each of the site modes to split into four modes. In other words, a non-

degenerate mode splits into four modes in the crystal while a degenerate mode splits into eight.

Ruthenocene, unlike ferrocene, does not undergo a phase transition as the temperature is lowered. This is evident when looking at previous papers by Bodenheimer

(1970) and Adams (1972). At liquid-nitrogen temperature and at room temperature, the peaks noted are the same barring minimal temperature shifts. There is the appearance of a few more peaks at lower temperature but this is the result of the overall sharper peaks and better resolved spectra, as there is less temperature broadening.

The only real work on pressure effects for ruthenocene was performed using IR17.

Just like ferrocene, ruthenocene was loaded into a diamond-anvil cell with Nujol to make the pressures hydrostatic. IR was used to observe ruthenocene in the region of 750-1350 cm-1. The authors recorded spectra for pressures up to 95 kbar. The most notable effects were the splitting of peaks into multiple peaks. The appearance of new peaks can be caused by differing pressure shifts and perhaps local breaks in symmetry. Local breaks in symmetry would then break the degeneracy of degenerate modes. For differing pressure shifts, multiple peaks very close together would appear as a single peak at atmospheric pressure but shifting at different rates would cause a spread of peaks to appear. Briefly, the observed changes with pressure do not appear to be due to the occurrence of an actual phase transition.

3.1.3 Osmocene

Compared to ferrocene and ruthenocene, very little analysis has been reported on osmocene. Some temperature work was performed50 although tracking the peak locations was not done. Additionally, no work on the effects of pressure on osmocene has been described in the literature.

Osmocene might not have been the focus of much research as it is less reactive than ferrocene. The larger more polarizable atom interacts better with the cyclopentadiene rings making them less reactive. Another reason why osmocene has not been analyzed very much is because the crystals are reported to be isomorphous to ruthenocene crystals. The molecular symmetry of osmocene is D5h with a space group of

Pnma51 just like ruthenocene. Consequently, the orthorhombic crystals of osmocene are expected to behave just like those of ruthenocene.

The temperature work that has been performed simply recorded the osmocene spectra at 77 K and at room temperature50. No real phase transitions seemed to occur. The numerous peaks observed at 77 K would widen as the temperature is increased which would give only a few peaks at room temperature. The researchers did not observe the appearance of new peaks nor the disappearance of others indicating the absence of a phase transition. Ruthenocene behaved the same way which further reinforces the isomorphous nature of osmocene.

Although there has been no prior work on the pressure analysis of osmocene, it may be expected to behave similarly to ruthenocene. The isomorphous nature of osmocene compared to ruthenocene may suggest that no phase transition will occur.

3.2 Halogen-bonding in Cocrystals

A cocrystal can be defined as a crystal with different components. The term component in this regard is purposely vague. The definition of a cocrystal is a subject open to debate52-53. Some would believe that only neutral molecules can form cocrystals, while others believe neutral and ionic molecules can be used52-53. The latter definition would include salts, solvates and hydrates.

3.2.1 Cocrystal Properties

Cocrystals have been used extensively in the field of pharmaceuticals52-54.

Regular solids could have very low solubility in water. This is problematic when an active pharmaceutical ingredient (API) does not dissolve well in water. The API will not be readily absorbed by the body and will be destroyed before it reaches the target area.

To increase the solubility, the API is turned into a cocrystal52,54.

Cocrystal properties cannot always be predicted52,54. A cocrystal that is added to a water solution may stay intact or become either a hydrate or salt. The cocrystal that stays intact will probably have a poor solubility due to its stability in water. A salt or hydrate would greatly improve the solubility of the cocrystal but the structure may not be stable in water. As structure often affects the function so stability in water is important.

Other than solubility studies, there has not been much work on thermal or chemical stability of cocrystals54. High-temperature studies of cocrystals can be useful to see if any phase transitions occur which may enhance structural stability. Verifying chemical stability can help to ensure that the desired product is not degraded throughout synthesis. Every crystal has the possibility of forming polymorphs. From understanding

how temperature or chemicals affect the compound of interest, it is possible that certain forms may be favoured over others during the synthesis.

3.2.2 Halogen Bonding

Halogen bonding is very similar to hydrogen bonding55. A halogen is bonded to a molecule that is highly electron withdrawing. The halogen will then have a positive dipole. The electropositive halogen will interact with highly electronegative atoms such as nitrogen and oxygen. The resulting intermolecular bond allows for the construction of cocrystals as the bond will direct the synthesis of the crystal.

Typically, the molecules that are good for halogen bonding are highly fluorinated56-57. As fluorine is the most electronegative atom, electron density will migrate to the fluorine atoms. The bond distance for the halogen bonding decreases as the number of fluorine atoms is increased57. Additionally, the bond angle was also reported to approach 180⁰, which is angle for the maximum interaction.

Part of this thesis will focus on cocrystals with the two halogen donors 1,4- dibromotetrafluorobenzene (TFBB) and 1,4-diiodotetrafluorobenzene (TFIB). The compound with iodine has already been shown to be useful in the formation of halogen- bonded cocrystals58-59. Bromine is quite similar to iodine so the bromine compound is expected to behave similarly.

3.2.3 Halogen-Bonded Cocrystals

Cocrystal synthesis is dictated by intermolecular bonds. The presence of certain functional groups combined with others can form the basis of the crystal. Examples of strong intermolecular bonds are two carboxylic acids facing one another such that the hydroxyl groups face the oxygen atoms of the carbonyls60. Two hydrogen bonds are formed and there are very strong interactions. Even if the intermolecular bonds are weak, they can affect the shape of the crystals61.

Halogen bonds can also be used to form desired crystal structures58-60. Although halogen bonds can be weaker than other intermolecular interactions, they have been used for cocrystal design58-60. To illustrate the differences in intermolecular strengths, a single hydrogen bond in acetic acid has a bond distance of 1.78 Ǻ 60. The halogen bond in a linear C-H···X bond would have a distance of 2.5 Ǻ while ··· interactions in benzene have a bond distance of 3.7 Ǻ 60.

If halogen bonds compete with other interactions such as hydrogen bonding, highly fluorinated complexes can be used to change the structure from having hydrogen bonding to halogen bonding62.

3.2.4 Raman Spectroscopy and Halogen Bonding

Raman spectroscopy is a powerful tool when it comes to the examination of halogen bonding. The formation of halogen bonds would weaken the C-X bond. It would also weaken the bonds involved with the halogen acceptor. So, small red-shifts are expected in the associated Raman peaks.

More interestingly, halogen bonding can be a tool to discover what happens in a crystal as the pressure is increased. As the pressure is increased, the volume becomes smaller thereby causing the molecules to come closer to one another resulting in an increase in the intermolecular interactions. As the intermolecular interactions are increased, the bonds, which are involved in halogen bonding for example, would weaken resulting in red-shifts of the Raman peaks.

Pressure-induced red-shifting has been reported in the literature for molecules with intramolecular bonds26,63. Research on molecules involved in hydrogen bonding has clearly shown the red-shift occurring in the O-H stretches63.

The effect of pressure on crystals with halogen bonding has also been

26 investigated, e.g., crystalline cyanuric chloride (NCCl)3 . A red-shift was observed for the C-Cl vibrations as well as for the out-of-plane ring vibrations. Cyanuric chloride halogen bonds involve the nitrogen atoms in the ring so a red-shift in at least one of the ring vibrational modes was expected. It was also noted that the bond distances involving the nitrogen and chlorine atoms were reduced as the pressure was increased.

Methyldiphenylphosphine oxide (MePh2P=O, MDPPO) has been shown to interact with halogen bond donors to create different structures62. This thesis will focus on cocrystals of MDPPO with the halogen donors mentioned earlier (TFIB and TFBB).

The two cocrystals synthesized are (MDPPO)2•TFIB and (MDPPO)2•TFBB. The crystal structures should exhibit a P=O···X halogen bonding motif. The C-X stretch and

P=O will be highly dependent on the halogen bonding interaction. Red-shifts of these vibrational modes are expected to occur as shown in previous halogen bonding studies26.

It is important to note that C-X··· interactions can also occur in cocrystals61-62.

If the P=O···X halogen bond is weakened, it is possible that a structural change can occur to favour C-X··· interactions instead. A high enough pressure could cause the lattice to rearrange to compensate for the increase in energy. An example of the C- X··· and

P=O···X interactions can be found in Fig. 3.2.1

Fig 3.2 – Examples of bonding interactions between two molecules of MDPPO and TFIB

Chapter 4

Raman Spectroscopic Studies of Group 8 Metallocenes

5 4.1 Ferrocene, (η -C5H5)2Fe

4.1.1 Raman Spectra Assignments

Representative Raman spectra of ferrocene are shown in Fig. 4.1.1. The spectra were taken with 514.5 and 785 nm lasers. The 514.5 nm laser spectrum was baseline corrected for a better comparison. The peak positions are given in Table 4.1.1 together with their corresponding symmetry and vibrational assignments. The molecular point

group for the assignments is assumed to be D5d. This symmetry was used since ferrocene in solution has this symmetry and most of the literature has kept with the same assignments even for crystal form43-44.

Raman Shift (cm-1) Fig. 4.1.2 – Raman spectra of ferrocene taken with the 514.5 and 785 nm lasers. The 514.5 nm spectrum was baseline corrected and both spectra were scaled to their maximum intensity.

Table 4.1.1 – Raman peak assignments of ferrocene (cm-1) taken using 514.5 and 785 nm lasers

Laser

514nm 785nm Vibrational assignment 309.3 305.5 A Ring-metal stretch 315.4 311.6 1g - 385.1 E Ring tilt 392.6 389.8 1g

- 595.2 E2g Ring bend ()

- 808.7 A1g C-H bend () 828.6 - E C-H bend () 842.8 1g

- 890.2 E2g Ring bend (||)

- 995.1 E1g C-H bend (||)

1060.7 1056.9 E2g C-H bend () 1098.2 1095.3 A Ring breathing 1104.9 1101.8 1g

1189.9 1188.0 E2g C-H bend (||)

- 1353.2 E2g C-C stretch

1411.6 1408.2 E1g C-C stretch

3087.2 3086.3 E2g C-H stretch

3101.2 3099.4 E1g C-H stretch

3109.3 3107.7 A1g C-H stretch

It is important to note that ferrocene has relatively poor scattering abilities compared to the other group 8 metallocenes. The iron atom is smaller than are ruthenium and osmium which means that iron has a smaller atomic polarizability. Since Raman scattering intensities depend on changes in polarizability, ferrocene will give lower peak intensities compared to those for ruthenocene or osmocene.

The 514.5 nm spectrum was quite noisy and a few additional features appeared using the 785 nm laser. For the temperature data, both lasers can be used, so the 785 nm laser was the better choice. The 514.5 nm laser had to be used for the subsequent high- pressure measurements because the fluorescence of ruby is the calibration method and 34

ruby fluoresces at 694 nm. The 514.5 nm laser would be better to see the C-H stretching region as the relative intensities were quite low for the NIR laser.

For the high-pressure analysis, the 514.5 nm spectrum proved to be good enough.

Although the spectrum was very noisy and was missing some peaks, many of the features that are missing are in the 800-900 cm-1 region. The diamond anvil cell has a broad peak that appears in this region which renders these peaks useless for analysis. Furthermore, the small peak at 1350 cm-1 that appears with the 785 nm laser and disappears with the

514 nm laser could not be used for pressure analysis. Assuming that the peak was even present with the 514 nm laser, the peak is situated too close to the diamond peak. The peak does not lie in the scanning regions used for pressure analysis which were 100-1265 cm-1 and 1365-3500 cm-1.

As stated in a previous chapter, ferrocene has two molecules in the unit cell.

Consequently, additional peaks are expected appear due to Davydov splitting. From the spectra using both lasers, the ring-metal stretch, a C-H perpendicular bending and the ring breathing modes showed the presence of one additional band. Although two-to-four peaks for a single vibrational mode is expected, it might not be possible to observe them all. Small Davydov splitting combined with the poorer scattering properties of ferrocene should give rise to fewer peaks than anticipated.

4.1.2 Variable-temperature Raman Spectra

As stated earlier, the temperature effect on ferrocene is a known phenomenon.

The crystal of ferrocene becomes triclinic upon rapid cooling. The crystals become more

21 ordered but stay in D5 molecular symmetry . The phase transition occurs at 164 K which is roughly -109 ⁰C. There is also a situation where by annealing the ferrocene crystals

below 242 K for a few days, a stable low-temperature phase is produced. The resulting crystals of ferrocene are orthorhombic rather than triclinic. The experiment performed in this thesis was focused on the former variable-temperature study to confirm that the phase transition occurs at 164 K.

Ferrocene crystals were mounted on a temperature-controlled stage and Raman spectra were recorded at various temperatures above and below the anticipated the phase transition point with a single point taken where the expected transition should be. It is important to note that the temperature was rapidly cooled and then increased starting from -190 °C or 83 K. The final temperature reached was only 353 K as ferrocene would melt too quickly while the laser was on above this temperature.

From Figs. 4.1.2 and 4.1.3, the difference between the metastable low temperature phase and the stable high-temperature phase is quite apparent. The C-H stretching region shows a multitude of peaks at low temperature, while after the transition point, only three peaks can be distinguished using curve fitting (see Fig. 4.1.3). This is a clear indication that a phase transition has indeed occurred.

Raman Shift (cm-1) Fig. 4.1.2 - Sample Raman spectra of ferrocene at various temperatures above and below the phase transition at 163K recorded with the 785 nm laser.

Raman Shift (cm-1) Fig. 4.1.3 - Ferrocene C-H stretching region in the Raman spectra at different temperatures recorded with the 785 nm laser.

Although the C-H stretching region in Fig. 4.1.3 was quite noisy and low in intensity, a number of peaks can be fitted to match the data. Fig. 4.1.4 tracks the position of the ferrocene peaks versus temperature in all the different regions to show that temperature affects the two phases differently which further supports the hypothesis that a phase transition has taken place.

Fig. 4.1.4 - Position of ferrocene Raman peaks recorded at different temperatures using the 785 nm laser

4.1.3 Variable-pressure Raman Spectra

Figs. 4.1.5 and 4.1.6 show the pressure-induced changes in the ferrocene spectra in two different regions (above and below the diamond line). The first region was recorded from 100 to 1265 cm-1 and the second region from 1365 to 3500 cm-1. The second region was zoomed in on the C-H stretch modes. This was intentional to get a better view at the C-H stretching region. Moreover, the C-C stretch at 1400 cm-1 has very low intensity and no major changes that occurred.

Raman Shift (cm-1) Fig. 4.1.5 - Ferrocene spectra at various pressures in the 100-1265 cm-1 region. The region plotted above is smaller than the scanned region for a better view of the peaks of interest. Each spectrum was scaled by the maximum intensity of that specific spectrum.

Raman Shift (cm-1)

Fig. 4.1.6 - C-H stretching region of ferrocene at various pressures. Each spectrum was scaled by the maximum intensity of that spectrum. All the ferrocene peaks were blue-shifted as the pressure was increased. The most notable change was the ring-metal stretch which split into two peaks. The splitting of the peaks comes from Davydov splitting as explained earlier which is increased as the pressure was increased. The two peaks also shifted at different rates. The ring-metal stretch is a non-degenerate mode so the new peak must come from the crystal symmetry splitting of the mode.

The next change occurred with the ring tilt at around 390 cm-1. It also went from a singlet to a doublet peak. The ring tilt unlike the ring-metal stretch is a degenerate mode under normal conditions so the doublet could come from the breaking of the degenerate

mode symmetry. The other possibility is from Davydov splitting so it is uncertain as to what is the cause. The perpendicular C-H bending modes from the ring at around 1060 cm-1 also undergo the same effect.

In the region from 1365 to 3500 cm-1, as stated earlier, only the C-H stretches were high enough in intensity to run an analysis on. Since the C-H stretches are composed of multiple peaks that are not baseline resolved, curve fitting must be done to obtain the position of the peaks. The number of curves that were fitted depended on how many peaks seemed to appear. For example, ferrocene at 2.5 kbar can be separated into three different peaks but at 45.0 kbar, four to five different peaks could easily be plotted.

The greater the number of curves fitted, the better the fit will be but the smallest number of curves that would represent well the data was used instead. This conservative approach is to ensure that the effect of pressure (in this case the splitting of degenerate modes and/or Davydov splitting) is not overestimated.

The appearance of the C-H stretching peaks varied markedly with increasing pressure. It became difficult to associate the individual fitted curves to the original stretches. Since the peaks shift at different rates, a peak starting at a lower Raman shift position could end up at a higher Raman shift position with respect to another peak. Since there are no clear ways of distinguishing the individual peaks, the peaks will be assumed to stay in their relative positions for simplicity (i.e. a peak starting at a lower wavenumber will stay at a lower wavenumber throughout). Once again, Davydov splitting is the most likely reason for the emergence of new peaks at higher pressures.

The spectra did not seem to indicate an obvious transformation occurring so the position of the peaks must be tracked versus pressure (see Fig. 4.1.7). Looking at this

figure, there is indication of a phase transition occurring at around 41 kbar. The peaks are blue-shifted until 41 kbar when the peaks start to red-shift slightly or simply plateau. The pressure shifts can be found in Table 4.1.2 for further confirmation.

Fig. 4.1.7 - Position of ferrocene Raman peaks at different pressures using the 514 nm laser The low-pressure region refers to below the phase transition and high-pressure region refers to above it in Table 4.1.2. A blue-shift of 1.1 cm-1/kbar for one of the ring- metal stretches in the low-pressure region becomes red-shifted by 0.06 cm-1/kbar in the

high-pressure region. Most of the peaks follow the same trend of a high blue-shift value below the transition point and a low red-shift above it. The only exception is the peak at

-1 1058.1 cm , which is a E2g C-H bending mode that has comparable values for both regions.

Table 4.1.2 - Pressure shifting rates of Raman peaks of crystalline ferrocene taken with 514 nm laser

dν/dP (cm-1/kbar) Peak (cm-1) Low Pressure High Pressure 308.6 0.67 -0.05 314.7 1.10 -0.06 392.4 1.03 -0.17 398.2* 0.72 -0.62 1058.1 0.26 0.15 1072.5* 0.53 0.13 1104.4 0.51 -0.04 1411.4** 0.15 -0.03 1412.4 0.20 -0.10 3089.4 0.78 -0.12 3124.9** 0.52 -0.12 3099.5 1.25 -0.37 3109.0 1.36 -0.07 Notes: * appears at 16.9 kbar and ** appears at 35.6 kbar. “Low Pressure” and “High Pressure” refer to below and above the phase transition at ~41 kbar.

In addition, ferrocene has previously shown to exhibit a phase transition at 11 kbar18. However, the data obtained has a large gap in that region so the phase transition cannot be confirmed, but some of the existing peaks did split into separate peaks after this supposed transition point. The splitting could either be a result of a phase transition or simply two overlapping bands that separate from one another because they shift at different rates.

Previous pressure experiments on ferrocene have not shown any phase transition other than the 11 kbar phase transition16-18. Some of these experiments used crystallites of ferrocene suspended in Nujol16-17 compared to our experiment where only crystalline ferrocene was used with no other pressurizing fluid. The other experiment only went up to pressures of 20 kbar18, well below this new phase transition. It is therefore plausible that the existence of the high pressure phase transition depends on the initial state of ferrocene within the DAC. The processes in which the solid compensates for the increases in pressure would be different resulting possibly in additional transitions.

5 4.2 Ruthenocene, (η -C5H5)2Ru

4.2.1 Raman Spectra Assignments

As stated earlier, ferrocene is a poor Raman scatterer compared to ruthenocene and osmocene. Ruthenocene has the larger atom with a greater polarizability so an increase of Raman scattering is expected. This increase is obvious when looking at Table

4.2.1 and Fig. 4.2.1. Contrary to ferrocene, ruthenocene has D5h molecular symmetry and the Raman peaks are assigned using this symmetry.

Raman Shift (cm-1) Fig. 4.2.1 - Raman spectra of ruthenocene recorded using a 514.5 nm and 785 nm laser

Upon reference to Table 4.2.1, there are only a few minor differences between the two lasers used. The 785 nm laser gave slightly more information than did the 514.5 nm laser. Potentially, both lasers could be used effectively to observe temperature effects on ruthenocene. For consistency with the ferrocene experiment, the 785 nm laser was used.

As noted earlier, ruthenocene has Pnma space group with four molecules within the unit cell of the crystal. The inclusion of additional molecules increases the number of vibrational modes expected (i.e., four for non-degenerate and eight for degenerate modes). These increases can be readily seen in Table 4.2.1 where non-degenerate modes have up to two separate peaks while degenerate modes could have up to four. Again, the number of peaks appearing is less than what is expected because of small Davydov splittings

Table 4.2.1 - Raman peak assignment (cm-1) of ruthenocene using 514.5 and 785 nm lasers

Laser

514nm 785nm Vibrational assignments

113.0 - E2' Ring-metal-ring bend 327.6 324.2 A ' Ring-metal stretch 334.8 331.5 1 398.0 395.6 E '' Ring tilt 405.3 402.3 1 - 589.3 E ' Ring bend () 602.9 598.5 2

818.2 814.4 A1' C-H bend () 832.5 829.2

841.2 838.2 E1'' C-H bend () - 860.6 896.1 893.3 E ' Ring bend (||) 907.2 904.6 2 993.8 990.3

1004.5 1001.0 E1'' C-H bend (||) 1010.8 1007.5 1049.3 1046.4 E ' C-H bend () 1064.0 1060.0 2 1092.6 1089.8 A ' Ring breathing 1099.4 1096.4 1 1184.8 1181.9 1191.0 1188.1 E ' C-H bend (||) 1201.7 1199.5 2 1206.4 1204.2 1359.6 1356.5 E ' C-C stretch 1363.7 1361.1 2

1408.7 1405.9 E1'' C-C stretch 3080.9 3078.9 E ' C-H stretch 3086.8 3084.7 2 3096.4 3095.3 E '' C-H stretch 3105.3 3103.2 1

3113.0 3111.1 A1' C-H stretch

4.2.2 Variable-temperature Raman Spectra

The literature has previously shown that ruthenocene has no phase transitions upon cooling or heating. To further confirm the literature, the effect of temperature on ruthenocene was undertaken using our own instruments. Sample spectra were taken at various temperature to make a visual comparison and these are shown in Figs. 4.2.2 and

4.2.3. The Raman peak positions of ruthenocene at various temperatures were recorded using the 514.5 nm laser. The positions are plotted in Fig. 4.2.4. The temperature range of the experiment was from 83 to 373 K.

The C-H stretching region goes through a few transformations as the temperature is increased. Many of the individual peaks started to bunch up together making it difficult to distinguish the exact number of peaks present. This transformation is nicely seen in Fig. 4.2.3 where the peak below 3080 cm-1 at 83 K shifts by a large amount with temperature while the other peaks seem to shift relatively minimally. This difference in shifts caused the peaks to start coalescing.

-1 Raman Shift (cm ) Fig. 4.2.2 - Raman spectra of ruthenocene at various temperatures recorded using the 785 nm laser

Raman Shift (cm-1) Fig. 4.2.3 - Ruthenocene Raman spectra at various temperatures in the C-H stretching region

Fig. 4.2.4 - Ruthenocene Raman peaks recorded at different temperatures using the 514.5 nm laser

It was observed through both methods that as the temperature is increased, a few of the modes converged to a single peak, e.g., the C-C stretches, the ring tilt and the C-H bending modes. These specific modes are all degenerate. There is also background fluorescence at the lower temperatures which may just be dependent on the spot chosen for analysis. The silver heating block is responsible for the background signal.

Overall, there are not any major changes. We can thus conclude that ruthenocene has not undergone any phase transition, as was indeed anticipated.

4.2.3 Variable-pressure Raman Spectra

The effect of pressure on ruthenocene was observed using the same technique as for ferrocene. The spectra of ruthenocene were recorded until a pressure of 42.1 kbar was reached. The same two spectral regions as for ferrocene were examined. From Figs. 4.2.5 and 4.2.6, ruthenocene undergoes some spectral changes just like ferrocene. New peaks appeared for various modes such as the ring-metal stretch, the ring tilt and the C-H stretching modes.

Raman Shift (cm-1)

Fig. 4.2.5 - Ruthenocene Raman spectra at various pressures in the 100-1265cm-1 region. The individual Raman spectra were all scaled to afford the maximum intensity.

Raman Shift (cm-1)

Fig. 4.2.6 - Ruthenocene Raman spectra at various pressures in the C-H stretching region. The spectra were all individually scaled by the maximum intensity.

In addition, the shifting of the individual peaks can be seen from in Fig. 4.2.7. The additional peaks that appear as the pressure is increased seem to come from different shifting rates. There are many overlapping peaks due to small Davydov splittings and the modes may respond to pressure differently. There seems to be a C-C stretch that simply appears at around 33 kbar. The peak may have appeared as a shoulder for the relatively low intensity C-C stretch at lower pressures.

Fig. 4.2.7 - Raman peaks of ruthenocene at various pressures using the 514.5 nm laser

Unlike ferrocene, there were no detectable phase transitions. The peaks of ruthenocene were red-shifted linearly and there were no breaks as was the case for ferrocene where a number of peaks reached a plateau or started to red-shift. The shifting rates (pressure dependencies) are given in Table 4.2.2. Although some new peaks did start to appear, it is possible that the spots chosen for analysis at higher pressures were 52

responsible for this effect. This situation would account for the appearance of low intensity peaks. Other peaks appeared such as one of the C-H stretching modes, which is flanked by peaks on both sides, would only be discernible once these two peaks had separated enough.

Table 4.2.2 - Ruthenocene pressure shifting rates of Raman peaks recorded with 514.5 nm laser

Peak (cm-1) dν/dP (cm-1/kbar) 131.2 1.04 332.2 0.77 365.5† 0.99 399.0 0.41 405.9 0.63 412.3* 0.94 1058.9† 0.32 1063.7 0.62 1092.9 0.60 1099.6 0.48 1409.2 0.14 1419.8† 0.29 3080.6 0.23 3086.6 0.77 3100.7 0.52 3105.4 0.88 3134.9⁺ 1.17 3113.1 1.21 Note: * appears at 9.7 kbar, ⁺ appears at 26.3 kbar and † appears at 32.8 kbar

Although no phase transition was observed, the pressure reached was lower in the case of ruthenocene than for ferrocene. It is possible that a phase transition does occur at a higher pressure than ferrocene (which is around 41 kbar) and/or that the phase transition is not apparent with the data obtained (i.e., too few points above the transition point). It is also possible there is a phase transition that occurs at pressure between 10 and 25 kbar as there is no data could be recorded between these pressures. No phase transition has been reported in the literature so we perhaps should not have expected one17. 53

5 4.3 Osmocene, (η -C5H5)2Os

4.3.1 Raman Spectra Assignments

Osmocene is isomorphous with ruthenocene so its Raman spectrum should be quite closely similar to that of ruthenocene. Typical spectra of osmocene using both lasers are shown in Fig. 4.3.1 and the peak assignments are given in Table 4.3.1. Once

again, the peaks were assigned using D5h molecular symmetry. As expected, the osmocene spectrum is similar to that of ruthenocene except for a few minor differences.

There is a multitude of peaks appearing that are not present in ruthenocene. These additional peaks are the result of multiple combination or difference peaks. There are two additional peaks listed in Table 4.3.1 that appear at 787.2 cm-1 in the 514.5 nm spectrum and at ~1123 cm-1 in both spectra. There is also a large number of combination and difference bands appearing in the 2700 to 2900 cm-1 region, but these have been omitted from the table at this time.

Raman Shift (cm -1)

Fig. 4.3.1 - Osmocene Raman spectra taken with 514.5 nm and 785 nm lasers 54

Table 4.2.1 - Osmocene Raman peak assignments (cm-1) using the 514.5 and 785 nm lasers

Laser

514nm 785nm Vibrational assignment

112.6 115.5 E2' Ring-metal-ring bend 352.8 352.8 A ' Ring-metal stretch 359.4 359.4 1 413.9 413.9 E '' Ring tilt 419.9 419.9 1

522.4 522.4 E1'* Ring tilt

598.0 598.0 E2' Ring bend () 787.2 - Combination / difference band

814.9 814.9 A1' C-H bend () 845.6 845.6

852.1 852.2 E1'' C-H bend () 876.5 876.5

910.3 910.3 E2' Ring bend (||) 988.8 988.8 E '' C-H bend (||) 1004.5 1004.5 1 1031.0 1031.3

1049.5 1049.5 E2' C-H bend () 1062.4 1062.4 1088.9 1089.0 A ' Ring breathing 1095.8 1095.8 1 1123.7 1123.8 Combination / difference band

1182.9 1182.9 1192.4 1192.3 E ' C-H bend (||) 1199.3 1199.3 2 1211.4 1211.4 1354.2 1354.2 E ' C-C stretch 1359.3 1359.3 2

1402.0 1402.0 E1'' C-C stretch 3058.9 3059.0 3079.6 3079.6 E ' C-H stretch 3085.6 3085.7 2 3089.2 3089.2 3094.3 3094.3 E '' C-H stretch 3105.8 3105.8 1

3114.8 3114.8 A1' C-H stretch Note: * refers to a mode that is Raman-inactive and IR-active peak 55

The combination or difference modes could come from a large number of possibilities, so they will not be speculated upon here. Instead, the focus will be on why they appear in osmocene rather than ruthenocene. Ruthenocene and osmocene should both show the presence of these combinations bands but osmocene is a much better

Raman scatterer. The increase in Raman scattering from the larger polarizable osmium atom would facilitate the occurrence of these transitions.

Another interesting feature is the presence of an E1' band. This is a ring tilt mode and it is infrared active but Raman inactive in D5h symmetry. The appearance of the band in the Raman spectra could be due to a break in the local molecular symmetry. The cyclopentadienyl rings have low rotation barriers so osmocene which is normally D5h could go to D5 symmetry. Only a minor rotation of one of the rings would lower the symmetry. The net result is that both E1' and E1'' become E1 within the new symmetry. In the case of D5 symmetry, E1 is both infrared and Raman active.

The presence of this band in the Raman spectrum of osmocene is due to the good

Raman scattering properties of osmocene. Although ruthenocene might exhibit the same type of local symmetry breaking, the lower scattered light levels of ruthenocene might not permit this phenomenon to be observed.

4.3.2 Variable-temperature Raman Spectra

The effect of temperature on osmocene crystals is expected to be similar to that for ruthenocene crystals since they are isomorphous, as mentioned previously. The osmocene crystals were rapidly cooled to 83 K and then heated up to 398 K with Raman spectra being recorded at various temperatures (see Figs. 4.3.2 and 4.3.3 for

representative spectra). The position of each peak was then plotted versus temperature and the results are shown in Fig. 4.3.4.

Raman Shift (cm-1) Fig. 4.3.2 - Osmocene Raman spectra at various temperatures recorded using the 785 nm laser

Raman Shift (cm-1) Fig. 4.3.3 - Osmocene Raman spectra of the C-H stretching region at various temperatures using the 785 nm laser

Fig. 4.3.4 - Raman peak positions of osmocene at various temperatures using the 785 nm laser

As can be seen from the above figures, no phase changes were observed. A few of the peaks disappeared as the temperature was increased due to the increase of the noise in the baseline and others were shifted close to other peaks of the same vibrational modes.

4.3.3 Variable-pressure Raman Spectra

The effect of pressure on osmocene was expected to be the same as in the case of ruthenocene. The pressure was increased up to 40.7 kbar and the same two regions were scanned. Figs. 4.3.4 and 4.3.5 illustrate the recorded spectra at various pressures for the two regions investigated. The most important features to note are once again the C-H stretches, the ring tilt and the ring-metal stretch. At higher pressures, other bands start appearing in the C-H stretching region The ring tilt region shows a doublet where one of the peaks collapses. An additional peak then appears. This is strong evidence of a phase transition.

Raman Shift (cm-1)

Fig. 4.3.5 - Osmocene Raman spectra in the 100-1265 cm-1 at various pressures using the 514.5 nm laser. The individual spectra were scaled for maximum intensity.

Raman Shift (cm-1) Fig. 3.3.6 - Osmocene Raman spectra in the C-H stretching region at various pressures using the 514.5 nm laser. The individual spectra were scaled for maximum intensity.

From the pressure plots (Fig. 4.3.6), it can be seen that there is a phase transition that occurs at ~33 kbar. Observing the C-H bending mode at 1062 cm-1, there could potentially be a second phase transition at ~38 kbar. There are insufficient data points available to claim for sure that there is a second phase transition.

Fig. 4.3.7 - Position of osmocene Raman peaks at various pressures using the 514.5 nm laser

The pressure shift values can be found in Table 4.3.2. There are a few pressure shifts that are unchanged in the low- and high-pressure regions such as the ring tilt peak at 413.1 cm-1 which shifts by 0.56 cm-1/kbar and then by 0.48 cm-1/kbar after the phase transition. This particular peak also splits into two as the other ring tilt peak disappears.

Some pressure shift values alone cannot indicate whether a phase transition has occurred 61

or not. Many of the C-H stretches show similar shifts above and below the transition point. There are other peaks however which strongly support a phase transition, mainly the C-H bending and ring breathing modes. Both these modes almost double their shift values after the phase transition.

Table 4.3.2 - Raman peak pressure shifting rates of osmocene recorded using 514.5 nm laser

dν/dP (cm-1/kbar) Peak (cm-1) Low Pressure High Pressure 352.3 0.36 0.52 359.0 0.64 0.76 385.0+ 0.10 0.53 413.1 0.56 0.48 419.6 0.74 - 426.6‡ - -0.02 1062.4 0.61 1.37 1095.4 0.46 0.85 1401.9 0.21 - 3079.0 0.12 0.50 3086.2 0.68 0.63 3094.5 0.60 0.44 3094.6† - 0.53 3105.9 0.84 0.76 3114.7 1.18 0.93 3129.7* 1.03 - 3133.4† - 1.73 Note: * appears at 19.8 kbar, + appears at 29.1 kbar, † appears at 34.2 kbar and ‡ appears at 35.7 kbar. “Low Pressure” refers to below the phase transition at around 33 kbar and “High Pressure” refers to above it.

Briefly, osmocene does undergo a phase transition at around 33 kbar which is supported by the pressure shift data and the observed spectra, particularly in the C-H stretching region. One of the C-H bending modes indicates the possibility of a second phase transition occurring but there is not enough evidence to conclude this definitively.

Chapter 5

Effect of Pressure on Halogen Bonding in Cocrystals

5.1 Cocrystal Components

Cocrystals of (MDPPO)2•TFBB and (MDPPO)2•TFIB were prepared. Typical Raman spectra of each individual component are given in Figs 5.1.1 to 5.1.3. Tables 5.2.1 and

5.3.1 provide the peak locations for each individual component in comparison with those of the two cocrystals. A Raman spectrum of the halogen acceptor MDPPO is shown in

Fig. 5.1.1.

Fig. 5.1.1 - Raman spectrum of MDPPO taken with the 514.5 nm laser A representative Raman spectrum of the halogen donor TFBB is shown in Fig. 5.1.2 and a spectrum of TFIB is given in Fig. 5.1.3.

Fig. 5.1.2 - Raman spectrum of TFBB taken with the 514.5 nm laser

Fig. 5.1.3 - Raman spectrum of TFIB taken with the 514.5 nm laser

5.2 (MDPPO)2•TFBB

A representative Raman spectrum of the cocrystal is shown in Fig. 5.2.1

Fig. 5.2.1 – Raman spectrum of (MDPPO)2•TFBB cocrystals. Intensities are scaled by the max intensity. The spectrum was recorded with the 514.5 nm laser. The differences between the Raman spectra of the cocrystal and the monomers are quite small. Table 5.2.1 shows the position of the peaks in the cocrystal, as well as their location in each of the individual components. The table also helps to identify peaks and from where they come from (i.e., which component of the cocrystal).

The C-Br stretching vibration is assumed to be 338.6 cm-1 in TFBB and 335.9 cm-1 in the 2:1 cocrystal. The assumption is based on research published by Bailey (1968) and Yadav (1983). Bailey (1968) showed that bromine and iodine give similar stretches in C6F5X compounds. So, TFBB and TFIB are expected to have similar stretching vibration for C-Br and C-I bonds. Yadav (1983) assigned a C-I stretch in 1,2- diiodotetrafluorobenzene at 342 cm-1. The 1,2 form should have a similar stretching mode 65

to the 1,4 form. Taking these two ideas together, we can assume that the bromine complex has a stretching vibration at around 342 cm-1.

Table 5.2.1 – Raman peaks (cm-1) of MDPPO, TFBB and the 2:1 cocrystal of MDPPO and TFBB

MDPPO TFBB 2:1 cocrystal 113.0 113.0

134.4 147.0

160.7

172.6

182.3 180.1

211.8 210.8

240.4

261.4 254.9

282.3 286.6

301.1 305.3

310.5

338.6 335.9

377.5

396.9 396.3

411.6 411.1

444.1

508.2 504.8

618.5 618.0

684.7 684.5

907.0

922.0

998.7 998.6

1030.9 1028.0

1109.0

1124.3

1148.3

1172.5 1164.1

1191.3 1182.0

1407.3

1578.3

1592.7 1592.9

1618.9 1618.3

2915.7 2913.5

2979.9 2978.0

3057.6 3059.4

The effect pressure has on the P=O bond in the 1150-1200 cm-1 spectral region and the C-Br bond can be observed using Raman spectroscopy. Pressures were recorded up to 50.8 kbar and sample spectra are shown in Fig. 5.2.2 in the 100-1265 cm-1 region.

-1 Fig. 5.2.2 - Raman spectra of (MDPPO)2•TFBB at various pressures in the 100-1265 cm region The effect of pressure can be seen in the 100-200 cm-1 region, where there are large shifts in the positions of some of the peaks. These peaks belong to both MDPPO and TFBB. The second notable region is where the peak at ~1020 cm-1 occurs; this peak splits into a doublet. The third and final significant region is for the P=O stretching modes at around 1170 cm-1, where one peak begins to increase in intensity while the other is unaffected by pressure.

The 1365-3500 cm-1 region is shown in Fig. 5.2.3. There is very little change that occurs in this region. It is clear from this region and the previous one that there were no phase transitions taking place. Two of the C-H stretches appear to possibly split into two peaks. The middle peak broadens considerably which may be due to splitting of two

peaks. Therefore, for simplicity, this peak will be assumed to stay as a single but very broad peak. The third peak seems to possibly show a shoulder on each side, so keeping track of the middle as a single peak is easier. Furthermore, the effect of pressure on the

C-H stretch modes is not relevant in the P=O···X interactions.

-1 Fig. 5.2.3 - Raman spectra of (MDPPO)2•TFBB at various pressures in the 1365-3500 cm region Fig. 5.2.4 indicates the position of the individual peaks as a function of pressure and it clearly shows that P=O stretches is increasing in wavenumber due to pressure. The increase in pressure is at a rate of 0.16 and 0.30 cm-1/kbar for the 1172 and 1185 cm-1 peaks respectively (these pressure dependences are given in Table 5.2.2.). Contrary to what is expected the P=O stretch shifts to higher wavenumbers rather than lower wavenumbers.

The C-Br stretch at 337.0 cm-1 shifts by 0.29 cm-1/kbar. This stretching mode was also red-shifted, although blue-shifting was expected.

At higher pressures, the volume is decreased and the greater intermolecular interaction would result in red-shifts63. However, halogen bonding is very directional so a change in the structure of the lattice due to pressure may also weaken the bonding.

Fig. 5.2.4 - Positions of Raman peaks of (MDPPO)2•TFBB cocrystals at various pressures

Table 5.2.2 – Raman peaks and their associated pressure shift values of (MDPPO)2•TFBB cocrystals Peak (cm-1) dν/dP (cm-1/kbar) 117.3 0.75 152.4 0.55 178.6+ 0.72 173.1* 0.94 211.3 0.20 257.0 0.39 296.7 0.43 337.0 0.29 397.1 0.13 411.4 0.17 443.7 0.23 505.8 0.22 618.1 0.10 685.6 0.27 999.5 0.26 1027.7* 0.21 1028.2 0.29 1109.1 0.29 1172.9 0.16 1185.1 0.30 1401.0 0.34 1579.6 0.32 1594.4 0.36 1619.3 0.50 2918.6 0.92 2982.0 1.01 3063.0 0.78 Note: * refers to peaks appearing at 20.5 kbar while the + peak appeared at 41.4 kbar.

It does not appear that any phase transitions or large structural changes have occurred in the case of the (MDPPO)2•TFBB cocrystal under pressure. The halogen bonding, however, does appear to have weakened slightly under pressure. The small distortions produced in the crystal lattice with increasing pressure seem to have a greater effect than do the greater intermolecular interactions resulting from a reduced volume.

5.3 (MDPPO)2•TFIB

Table 5.3.1 shows the Raman peaks of the (MDPPO)2•TFIB cocrystal at 2.5 kbar compared to the individual components (see Figs. 5.3.1 and 5.3.2 for the 2.5 kbar spectra).

Table 5.3.1 – Raman peaks (cm-1) of MDPPO, TFIB and the 2:1 cocrystal of MDPPO and TFIB

MDPPO TFIB 2:1 cocrystal* 113.0 110.8 134.4 140.2 144.3 146.8 151.5 154.6 159.3 159.9 182.3 180.6 240.4 243.6 261.4 257.5 282.3 288.4 301.1 301.2 307.4 310.3 334.4 328.6 377.5 379.4 396.3 404.3 402.1 411.6 410.4 442.2 441.1 501.1 498.8 618.5 616.4 684.7 682.9 998.8 997.0 1030.9 1026.2 1109.0 1172.5 1163.2 1191.3 1187.4 1385.0 1377.9 1578.3 1579.3 1576.1 1592.7 1591.4 1611.2 1610.5 2915.7 2912.7 2979.9 2976.7 3057.6 3057.7

Note: * The cocrystal peak values are those at 2.5 kbar as all spectra of the cocrystal were obtained using the DAC

The effect of pressure on the halogen bond was investigated so spectra at various pressures were taken. The maximum pressure obtained was 29.1 kbar. Sample spectra at various pressures in the 100-1265 cm-1 region are shown in Fig. 5.3.1.

Fig. 5.3.1 - Raman spectra of (MDPPO)2•TFIB at various pressures in the 100-1265 cm-1 region As the pressure is increased, the P=O stretching region undergoes a few changes.

The peaks at 1187 cm-1 and 1178 cm-1 both converge to a single peak while the peak at

1163 cm-1 disappears. In place of the 1163 cm-1 peak, a new peak appears at a higher wavenumber.

A structural change can be the cause for the addition of a new peak and the removal of another. If the angle of the C-I with respect to the P=O bonds changes, there should be a small change the in the Raman spectra in both of those regions. Although there is no observable change in the C-I bond according to the Raman spectra, a small splitting will be less noticeable than pressure broadening.

There are also some changes in the region below 200 cm-1. Peaks start appearing in the scanned region. These peaks may have been present at room pressure under 100 cm-1 and they shifted into the scanned region.

The peak at 334.4 cm-1 is assumed to be a C-I stretch. The assignment was made from work by Bailey (1968) and Yadav (1983). This peak exhibited a very slight broadening and a shift occurred. Under pressure, the C-I stretch appeared to blue-shift, similar to the C-Br stretch.

The second region of interest, 1365-3500 cm-1 (see Fig. 5.3.2), did not show many significant changes. The middle C-H stretching mode broadened as the pressure was increased leading to a possible peak split. The peak was assumed to stay as a single peak although it is quite possible it split just like the third peak. The third peak however clearly showed the emergence of a shoulder appearing so it was separated into two peaks with curve fitting.

-1 Fig. 5.3.2 - Raman spectra of (MDPPO)2•TFIB at various pressures in the 1365-3500 cm region

The positions of the Raman peaks at various pressures are given in Fig. 5.3.3. In the P=O stretching region, the peak that appears has a pressure dependence of 0.13 cm-

1/kbar while the peak disappearing was shifted by 0.20 cm-1/kbar. The C-I stretch shifted by 0.23 cm-1/kbar. All the pressure dependencies can be found in Table 5.3.2. None of the peaks red-shifted as in the bromine complex.

Fig. 5.3.3 – Raman peak positions of (MDPPO)2•TFIB cocrystal at various pressures

Table 5.3.2 – Raman peaks and their associated pressure shift values of (MDPPO)2•TFIB cocrystals

Peak (cm-1) dν/dP (cm-1/kbar) 110.8 0.66 140.2 0.36 159.9 0.27 195.3* 0.41 257.5 0.41 301.2 0.41 328.6 0.23 402.1 0.23 410.4 0.18 441.1 0.22 498.8 0.25 616.4 0.11 682.9 0.33 997.0 0.30 1026.2 0.10 1032.9* 0.29 1111.4* 0.36 1163.2 0.20 1172.2* 0.13 1177.8 0.52 1187.4 0.17 1377.9 0.28 1576.1 0.25 1591.4 0.35 1610.5 0.67 2912.7 1.07 2976.7 1.24 3064.3+ 0.61 3057.7 0.91 Note: * refers to peaks appearing at 18.3 kbar while + indicates the peak appearing at 24.1 kbar.

There is a possible structural change occurring in the (MDPPO)2•TFIB cocrystal, which causes a new peak to appear in the P=O stretching region while another one collapses. The start of the structural change started below 18.3 kbar and seemed to have finished above 25.5 kbar. A rearrangement of the crystal lattice would affect the regions

involved in intermolecular bonding while there may not be any observable difference for the rest of the vibrations.

Chapter 6

Conclusions

This thesis focused on two different applications of Raman spectroscopy in crystalline materials. The first application was on molecular crystals of the group eight metallocenes. The second application concerned the halogen bonding in phosphine oxide cocrystals.

For the first application, ferrocene, ruthenocene and osmocene were examined in turn. Initially, Raman spectra of ferrocene were recorded using for both 785 and 514.5 nm lasers. The observed peaks were then assigned to their corresponding vibrational modes. The assignments were made assuming a molecular symmetry of D5d for consistency with assignments of ferrocene in the liquid phase.

The effects of temperature were analyzed using the 785 nm laser. The temperature of ferrocene was reduced to 83 K and spectra were taken at various temperatures as the temperature was gradually increased. The temperature was increased until ferrocene melted when illuminated with the laser. Selected spectra at various temperatures are shown for qualitative comparison. Next, the position of each peak was tracked with the variation in temperature to understand what occurs upon cooling. A phase transition was detected at 164 K, which confirmed the results of previous experiments in the literature.

The pressure effects were observed with the use of a DAC and the 514.5 nm laser.

A phase transition at 11 kbar, as claimed in the literature, could not be confirmed definitively. The observed splittings of certain peaks, at pressures above 11 kbar, however, provided some support for a possible phase transition. More importantly, a new phase transition was detected at ~41 kbar when a red-shift of the Raman peaks occurred.

The same approach was taken with ruthenocene. Raman spectra of ruthenocene were recorded using both lasers and the observed peaks were tabulated. The assignments of the peaks were done in D5h molecular symmetry.

The variable-temperature studies also went from 83 K until ruthenocene melted under laser irradiation. Ruthenocene did not evidence any phase transitions as anticipated. For the high-pressure studies, the pressure was increased up to a maximum of

42 kbar. As in the variable-temperature study, no phase transitions were detected.

The Raman spectra of osmocene were also recorded using both lasers and the observed peaks were assigned using D5h molecular symmetry. Interestingly, certain combination bands and formally Raman-inactive modes were detected. The appearance of these additional peaks was attributed to the increased Raman scattering capability of osmocene as osmium is a large polarizable atom.

Osmocene is isomorphous with ruthenocene, so it would be expected to exhibit the same spectroscopic results. No phase transition occurred as the temperature was changed. However, unlike ruthenocene, a clear phase transition did take place upon changing the pressure. The phase transition occurred at 33 kbar. A second transition possibly occurred at 38 kbar, but there was not enough evidence to support this conclusion definitively.

For the second Raman spectroscopy application, halogen bonding in phosphine oxide cocrystals under pressure was investigated. Cocrystals with either bromine- or iodine-containing compounds were prepared. The bromine complex was placed in the

DAC and the spectra at various pressures were recorded up to 51 kbar. The same process was also applied to the iodine complex but the maximum pressure obtained was 29 kbar.

For the bromine complex, no red-shifts were observed with increasing pressure, which indicated that the halogen bonding interactions were not getting stronger. This is contrary to what was expected as a lower volume caused by an increased pressure should result in an increase in the intermolecular interactions such as halogen bonding. A weakening in the intermolecular bonds could occur as the lattice becomes distorted.

Halogen bonding is very directional so the distance is not the only factor to be considered. Furthermore, no structural changes or phase transitions were detected.

The iodine complex also did not show the presence of red-shifts under pressure similar to the bromine complex. It did, however, exhibit a small structural change as an additional P=O vibrational peak appeared, while another related peak collapsed. No changes in the halogen stretches were seen, which was attributed to the low peak intensities.

To improve the current research, high-pressure XRD measurements should be taken. The advantage of XRD over Raman spectroscopy is that a crystal structure can be generated. With the crystal structure information, structural changes and phase transitions would be directly observed. Although Raman spectroscopy can see these changes indirectly, smaller changes in the crystal structure may go unnoticed. Furthermore, XRD measurements would allow direct observation of the crystals and it would be possible to see if the crystals distort under pressure thereby causing a weakening in intermolecular interactions. In addition, the techniques of XRD and Raman spectroscopy can be combined and it is possible to switch from one analytical technique to the other. The advantage is that spectra of both XRD and Raman can be taken on the same spot. This

type of work would be possible using the Canadian Light Source in Saskatoon,

Saskatchewan.

References

1. Johnston, C. T.; Helsen, J.; Schoonheydt, R. A.; Bish, D. L.; Agnew, S. F. Am.

Mineral., 1998, 83, 75–84.

2. Hu, Y.; Liang, J. K.; Myerson, A. S; Taylor, L. S. Ind. Eng. Chem. Res., 2005, 44,

1233–1240.

3. Dračínský, M.; Procházková, E.; Kessler, J.; Šebestík, J.; Matějka, P.; Bouř, P. J.

Phys. Chem. B, 2013, 117 (24), 7297–7307.

4. Simone, E.; Saleemi, A. N.; Nagy, Z. K. Chem. Eng. Res. Des., 2014, 92 (4), 594–

611

5. Hagemann, H.; Bill, H. J. Phys. C., 1985, 18, 6441–6456.

6. Kohl, I.; Winkel, K.; Bauer, M.; Liedl, K. R.; Loerting, T.; Mayer, E. Angew.

Chem. Int. Ed. 2009, 48, 2690–2694.

7. Sekiya, T.; Ohta, S.; Kamei, S.; Hanakawa, M.; Kurita, S. J. Phys. Chem. Solids,

2001, 62, 717–721.

8. Ornelas, C. New J. Chem., 2011, 35, 1973–1985.

9. Geise, R. J.; Rao, S. Y.; Yacynych, A. M. Anal. Chim. Acta, 1993, 281 (3), 467–

473.

10. Iijima, S.; Mizutani, F.; Yabuki, S.; Tanaka, Y.; Asai, M.; Katsura, T.; Hosaka, S.;

Ibonai, M.; Anal. Chim. Acta., 1993, 281 (3), 483–487.

11. Hendry, S. P.; Cardosi, M. F.; Turner, A. P. F.; Neuse, E. W. Anal. Chim. Acta.,

1993, 281 (3), 453-459.

12. Ornelas, C.; Aranzaes, J.; Salmon, L.; Astruc, D. Chem. Eur. J., 2008, 14, 50–64.

13. Neuse, E. W.; Woodhouse, J. R.; Montaudot, G.; Puglisi, C. Appl. Organomet.

Chem., 1988, 2, 53–57.

14. Chao, T. S.; Ownston, E. H., Jr. To improve octane rating. U.S. Patent 4,104,036,

Aug 1, 1978.

15. Werner, H. Angew. Chem. Int. Ed., 2012, 51, 6052–6058.

16. Adams, D. M.; Williams, A. D. J. Phys. Chem. Solids, 1980, 41 (10), 1073–1078.

17. Roginski, R. T.; Sbapley, J. R.; Drickamer, H. G. J. Phys. Chem. 1988, 92, 4316–

4319.

18. Duecker, H. C.; Lippincott, E. R. J. Chem. Phys., 1967, 46, 3691–3692.

19. Sorai, M.; Murkawa, S.; Ogasahara, K.; Suga, H. Chem. Phys. Lett., 1980, 76 (3),

510–514.

20. Edwards, J. W.; Kington, G. L.; Mason, R. Trans. Faraday Soc., 1960, 56, 660–

669.

21. Ogasahara, K.; Sorai, M.; Suga, H. Mol. Cryst. Liq. Cryst., 1981, 71, 189–211.

22. Shiomi, Y.; Sorai, M. Chem. Phys. Lett., 1983, 95 (2), 167–170.

23. Brock, C. P.; Fu, Y. Acta Crystallogr. Sect. B, 1997, 53, 928–938.

24. Bodenheimer, J. S.; Low, W. Phys. Lett. A, 1971, 36 (4), 253–254.

25. Naruse, M.; Sorai, M.; Sakiyama, M. Mol. Cryst. Liq. Cryst., 1983, 101, 219–234.

26. Wang, K.; Duan, D.; Zhou, M.; Li, S.; Cui, T.; Liu, B.; Liu, J.; Zou, B.; Zou, G. J.

Phys. Chem. B., 2011, 115 (16), 4639–4644.

27. Raman, C. V.; Krishnan, K. S. Nature, 1928, 121, 501–502.

28. Raman, C. V. Indian J. Phys., 1928, 2, 387–398.

29. Landsber, G. S.; Mandelstam, L. I. Z. Phys. B Con. Mat., 1928, 50 (11), 769–780.

30. Devereaux, T. P.; Hackl, R. Rev. Mod. Phys., 2007, 79 (1), 175–233.

31. Schrödinger, E. Ann. Phys., 1926, 28 (6), 1049–1070.

32. Adar, F.; Delhaye, M.; DaSilva, E. J. Chem. Ed., 2007, 84 (1), 50–60.

33. Das, R. S.; Agrawal, Y. K. Vib. Spectrosc., 2011, 57 (2), 163–176.

34. Koningstein, J. A.; Mortensen, O. S. Phys. Rev., 1968, 168 (1), 75–78.

35. Atkins, P.; De Paula, J. Elements of Physical Chemistry, 6th ed. Oxford

University: Oxford, 2013, 490–491.

36. Kiefer, W.; Long, D. A. Non-Linear Raman Spectroscopy and Its Chemical

Applications, Reidel: Dordrecht, 1982, 99–100.

37. Zhao, J.; McCreery, R. L. Appl. Spectrosc., 1996, 50 (9), 1209–1214.

38. Downes, A.; Elfick, A. Sensors, 2010, 10 (3), 1871–1889.

39. Lin, C.-C.; Kuo, M.-T.; Chang, H.-C. J. Med. Biol. Eng., 2010, 30 (6), 343–354.

40. Lyon, L. A.; Keating, C. D.; Fox, A. P.; Baker, B. E.; He, L.; Nicewarner, S. R.;

Mulvaney, S. P.; Natan, M. J. Anal. Chem., 1998, 70 (12), 341–361.

41. Takusagawa, F.; Koetzl, T. F. Acta Crystallogr. Sect. B, 1979, 35, 1074–1081

42. Seller, P.; Dunitz, J. D. Acta Crystallogr. Sect. B, 1979, 35, 1068–1074.

43. Long, T. V., Jr.; Huege, F. R. Chem. Commun. (London), 1968, (20), 1239–1241.

44. Bodenheimer, J.; Loewenthal, E.; Low, W. Chem. Phys. Lett., 1969, 3 (9), 715–

716.

45. Chhor, K.; Sourisseau, C.; Lucazeau, G. J. Raman Spectrosc., 1981, 11 (3), 183–

198.

46. Bodenheimer, J. S.; Low, W. Spectrochim. Acta, Part A, 1973, 29 (9), 1733–

1743.

47. Bodenheimer, J. Chem. Phys. Lett., 1970, 6 (5), 519–520.

48. Adams, D. M.; Fernando, W. S. Dalton Trans., 1972, (22), 2507–2511

49. Hardgrove, G. L.; Templeton, D. H. Acta Crystallogr., 1970, 12 (1), 28–32

50. Kimel’fel’d, Y. M; Smirnova, E. M.; Aleksanyan, V. T. J. Mol. Struct., 1973, 19,

329–346.

51. Boeyens, J. C. A.; Levendis, D. C.; Bruce, M. I.; Williams, M. L. J. Crystallogr.

Spectrosc. Res., 1986, 16 (4), 519–524.

52. Schultheiss, N.; Newman, A. Cryst. Growth Des., 2009, 9 (6), 2950–2967.

53. Aitipamula, S.; Banerjee, R.; Bansal, A. K.; Biradha, K.; Cheney, M. L.;

Choudhury, A. R.; Desiraju, G. R.; Dikundwar, A. G.; Dubey, R.; Duggirala, N.;

Goud, N. R.; Jetti, R. R. K. R.; Karpinski, P.; Kaushik, P.; Kumar, D.; Kumar, V.;

Moulton, B.; Mukherjee, A; Mukherjee, G.; Myerson, A. S.; Puri, V.; Ramanan,

A.; Rajamannar, T.; Reddy, C. M.; Rodriguez-Hornedo, N.; Rogers, R. D.; Row,

T. N. G.; Sanphui, P.; Shan, N.; Shete, G.; Singh, A.; Sun, C. C.; Swift, J. A.;

Thaimattam, R.; Thakur, T. S.; Thaper, R. J.; Thomas, S. P.; Tothadi, S.; Vangala,

V. R.; Variankaval, N.; Vishweshwar, P.; Weyna, D. R.; Zaworotko, M. J. Cryst.

Growth Des., 2012, 12 (5), 2147–2152.

54. Khan, M.; Enkelmann, V.; Brunklaus, G. J. Am. Chem. Soc., 2010, 132 (14),

5254–5263.

55. Wilcken, R.; Zimmermann, M. O.; Lange, A.; Joerger, A. C.; Boeckler, F. M. J.

Med. Chem., 2013, 56 (4), 1363–1388.

56. Nagels, N.; Hauchecorne, D.; Herrebout, W. A. Molecules, 2013, 18 (6), 6829–

6851.

57. Präsang, C.; Whitwood, A. C.; Bruce, D. W. Cryst. Growth Des., 2009, 9 (12),

5319–5326.

58. Tothadi, S.; Desiraju, G. R. Chem. Commun. (Cambridge, U. K.), 2013, 49 (71),

7791–7793.

59. Gao, H. Y.; Shen, Q. J.; Zhao, X. R.; Yan, X. Q.; Pang, X.; Jin, W. J. J. Mater.

Chem., 2012, 22 (12), 5336–5343.

60. Dunitz, J. D.; Gavezotti, A. Cryst. Growth Des., 2012, 12 (12), 5873–5877.

61. Gonnade, R. G.; Shashidhar, M. S.; Bhadbhade, M. M. J. Indian Inst. Sci., 2007,

87 (2), 149–165.

62. Oh, S. Y.; Nickels, C. W.; Garcia, F.; Jones, W.; Friščić, T. CrystEngComm,

2012, 14 (19), 6110–6114.

63. Shimada, H.; Nibu, Y.; Shimada, R. J. Raman Spectrosc., 2008, 39 (1), 32–39.

64. Bailey, R. T.; Hasson, S. G. Spectrochim. Acta., Part A, 1968, 24 (12), 1891–

1898.

65. Yadav, R. A.; Singh, I. S.; Sala, O. J. Raman Spectrosc., 1983, 14 (5), 353–357.

Appendix A Point Group Tables

Cs E σh

2 2 2 A’ 1 1 x, y, Rz x , y , z , xy

A’’ 1 -1 z, Rx, Ry yz, xz

Ci E i

2 2 2 Ag 1 1 Rx, Ry, Rz x , y , z , xy, yz, xz

Au 1 -1 z, x, y

C2 E C2

A 1 1 z, R x2, y2, z2, xy z

B 1 -1 x, y, Rx, Ry yz, xz

C2v E C2 (z) σv (xz) σv (yz)

2 2 2 A1 1 1 1 1 z x , y , z

A2 1 1 -1 -1 Rz xy

B1 1 -1 1 -1 x, Ry xz

B2 1 -1 -1 1 y, Rx yz

C2h E C2 (z) i σh

2 2 2 Ag 1 1 1 1 Rz x , y , z , xy

Bg 1 -1 1 -1 Rx, Ry xz, yz

Au 1 1 -1 -1 z

Bu 1 -1 -1 1 x,y

D2 E C2 (z) C2 (y) C2 (x)

2 2 2 A1 1 1 1 1 x , y , z

B1 1 1 -1 -1 z, Rz xy

B2 1 -1 1 -1 y, Ry xz

B3 1 -1 -1 1 x, Rx yz

D2h E C2 (z) C2 (y) C2 (x) i σ (xy) σ (xz) σ (yz)

2 2 2 A1g 1 1 1 1 1 1 1 1 x , y , z

B1g 1 1 -1 -1 1 1 -1 -1 Rz xy

B2g 1 -1 1 -1 1 -1 1 -1 Ry xz

B3g 1 -1 -1 1 1 -1 -1 1 Rx yz

A1u 1 1 1 1 -1 -1 -1 -1

B1u 1 1 -1 -1 -1 -1 1 1 z

B2u 1 -1 1 -1 -1 1 -1 1 y

B3u 1 -1 -1 1 -1 1 1 -1 x

2 D5 E 2 C5 (z) 2 (C5) 5 C’2

2 2 2 A1 1 1 1 1 x + y , z

A2 1 1 1 -1 z, Rz

E1 2 2cos(2π/5) 2cos(4π/5) 0 (x,y),(Rx, Ry) (xz,yz)

E2 2 2cos(4π/5) 2cos(2π/5) 0 .

5 5 2 3 D5h E 2 C5 (z) 2 (C5) σh 2 S5 2 (S5) C’2 σv

x2+ y2, A’1 1 1 1 1 1 1 1 1 z2

A’2 1 1 1 -1 1 1 1 -1 Rz

E’1 2 2cos(2π/5) 2cos(4π/5) 0 2 2cos(2π/5) 2cos(4π/5) 0 (x,y)

(x2- y2, E’2 2 2cos(4π/5) 2cos(2π/5) 0 2 2cos(4π/5) 2cos(2π/5) 0 xy)

A’’1 1 1 1 1 -1 -1 -1 -1

A’’2 1 1 1 -1 -1 -1 -1 1 z

- - (Rx, E’’1 2 2cos(2π/5) 2cos(4π/5) 0 -2 0 (xz,yz) 2cos(2π/5) 2cos(4π/5) Ry)

- - E’’2 2 2cos(4π/5) 2cos(2π/5) 0 -2 0 2cos(4π/5) 2cos(2π/5)

2 5 3 5 D5d E 2 C5 (z) 2 (C5) i 2 (S10) 2 S10 C’2 σd

x2+ y2, A 1 1 1 1 1 1 1 1 1g z2

A2g 1 1 1 -1 1 1 1 -1 Rz

E1g 2 2cos(2π/5) 2cos(4π/5) 0 2 2cos(2π/5) 2cos(4π/5) 0 (Rx,Ry) (xz,yz)

(x2- y2, E 2 2cos(4π/5) 2cos(2π/5) 0 2 2cos(4π/5) 2cos(2π/5) 0 2g xy)

- A 1 1 1 1 -1 -1 -1 1u 1

- A 1 1 1 -1 -1 -1 1 z 2u 1

- - - E 2 2cos(2π/5) 2cos(4π/5) 0 0 (x, y) 1u 2 2cos(2π/5) 2cos(4π/5)

- - - E 2 2cos(4π/5) 2cos(2π/5) 0 0 2u 2 2cos(4π/5) 2cos(2π/5)

Appendix B Curve Fitting Using Wire 3.4

Step 1: Open the software and load a spectrum.

Step 2: Zoom into a specific region.

Step 3: Click “Curve fit” on the “Analysis” tab.

Step 4: Click somewhere on the region to add a curve to fit.

Step 5: If needed, drag the middle icon to move the position and height of the curve and the side icons to adjust the width of the curve.

Step 6: Add in additional curves and play with their height and width. The purpose is to match the appearance of the data

Step 7: After all curves are added, right click and select “Start fit”.

Step 8: The curve information is given and the curves should fit the data extremely well. Right click and select “Copy results” to transfer the information to excel easily. 92

(Example of a bad fit)

Step 9: Add additional peaks to improve the fit. Note that too many peaks may not accurately describe the data so additional curves should be added only when needed.