Analytica Chimica Acta 842 (2014) 51–56

In-situ vibrational optical rotatory dispersion of molecular organiccrystals at high pressures

W. Montgomery a,*, Ph. Lerch b, M.A. Sephton a

aDepartment of Earth Science and Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UKb Swiss Light Source - Paul Scherrer Institute, CH 5232 Villigen-PSI, Switzerland

H I G H L I G H T S G R A P H I C A L A B S T R A C T

� High pressure synchrotron sourcevibrational optical rotary dispersionis possible.

� Our method simultaneously collectshigh-quality polarized FTIR spectra.

� The high resolution of the synchro-tron is essential for confident meas-urements.

A R T I C L E I N F O

Article history:Received 1 April 2014Received in revised form 4 July 2014Accepted 15 July 2014Available online 19 July 2014

Keywords:ChiralityAlanineVibrational optical rotatory dispersionFourier transform infrared spectroscopyDiamond anvil cell

A B S T R A C T

Organic structures respond to pressure with a variety of mechanisms including degradation,intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organicstructures are of particular interest because of the potential steric controls on the fate of pressurizedmolecules. Despite representing a range of opportunities, the simultaneous study of high pressures ondifferent forms of chiral structures is poorly explored. We have combined synchrotron-source vibrationaloptical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvilcell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid,alanine.ã 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Contents lists available at ScienceDirect

Analytica Chimica Acta

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1. Introduction

The responses of molecular organic crystals at high pressuresare relevant to a number of scientific fields, ranging fromastrophysics [1] to pharmaceuticals [2–5]. Intense effort has beenexpended to understand the effects of high pressure on organiccrystals, using techniques such as X-ray diffraction, Ramanspectroscopy and Fourier transform infrared spectroscopy (FTIR)[6–8]. High pressure applied to organic crystals can result in

Abbreviations: DAC, diamond anvil cell; GPa, gigapascal; ORD, optical rotatorydispersion; VORD, vibrational optical rotatory dispersion.* Corresponding author. Tel.: +44 20 7594 5185.E-mail address: w.montgomery@imperial.ac.uk (W. Montgomery).

http://dx.doi.org/10.1016/j.aca.2014.07.0200003-2670/ã 2014 The Authors. Published by Elsevier B.V. This is an open access article un

polymorphism, intermolecular motion of hydrogen bonds, break-ing, distortion, symmetrization of hydrogen bonds and protontransfer [8–13]. The pressure-induced intermolecular bondingoften involves hydrogen bonds enhanced by new geometricarrangements and increased intermolecular proximity. Highpressure experiments offer a quantitative control over theproximity of atoms and the formation of intermolecular hydrogenbonds [9,13,14].

One aspect of high-pressure polymorphism of particularrelevance to molecular organic crystals is absolute conformation,specifically referred to as chirality or handedness. Chirality resultsfrom the presence of asymmetric bonds arranged around carbonatoms. It plays a key role in biology, with practical effects inpharmacology, where one enantiomer may be beneficial whileanother is damaging [15].

der the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

52 W. Montgomery et al. / Analytica Chimica Acta 842 (2014) 51–56

In the absence of direct observation of atomic arrangementsusing X-ray or neutron diffraction, chirality can be measuredindirectly via sample interaction with polarized light, either linearor circular, over a range of wavelengths. There are a variety ofpossible combinations for valid measurements: common techni-ques include vibrational circular birefringence (circularly polarizedlight), and Raman optical activity (linearly polarized light) butthese have, to some extent, been superseded by chiral chro-matographic and X-ray and neutron diffraction techniques [16].

The techniques presently available for the study of molecularorganic crystals under high pressure all have individual weak-nesses when employed for the determination of molecularconformation. Firstly, classical vibrational spectroscopy, which isroutinely performed at high pressures, does not distinguishbetween enantiomers. Secondly, non-optical techniques currentlyused for conformational analysis are not easily applied to the smallvolumes of organic material produced at high pressures, and in anycase would offer only static pre- and post-experiment data.

Recent improvements in instrumental capability have lead torenewed interest in chiroptical techniques such as optical rotatorydispersion (ORD), which measures the response of a sample tolinearly polarized light rotated through a known series of angles.The first comprehensive modern measurements of vibrational ORD(VORD) used infrared active vibrations in place of visible or Ramanalternatives, although the effect was first observed in 1836 [17,18].

The immediate advantage of VORD is that one measurementcan probe the vibrational structure (e.g., chemical bonds) and theconformation (chirality) simultaneously. The benefits of using adiamond anvil cell (DAC) when seeking to monitor any changes inchirality at high pressure include the possibility of continuous real-time monitoring, the wide window of infrared transparency ofType II diamonds, and the simultaneous visual observation of thesample.

In order to directly observe the effect of pressure on chiralmolecules, we have combined the high-pressure capability of thediamond anvil cell with the benefits provided by a synchrotron-source FTIR spectroscopy beamline [19] to measure high-pressurevibrational optical rotatory dispersion for the first time. For theseunprecedented chiral experiments we chose to avoid complica-tions involving multiple stereocenters (i.e., chiral carbons) andperformed our measurements with L- and D-alanine. We chosealanine because it is the simplest chiral amino acid, containing only

Fig. 1. Experimental setup: schematic of the infrared optical rotatory dispersion high preshows two samples loaded into the gasket hole of the diamond anvil cell, along withsynchrotron light is modulated by the FTIR spectrometer, steered and focused by CassegDAC, collected by further Cassegrain optics (not shown), steered to the motorized polarizIR spectra. By combining the angular information, as described in the text, vibrational

one chiral carbon center, thereby providing interpretative simplic-ity. Crystalline L-alanine has been studied isothermally at pressuresup to 15 GPa using X-ray diffraction (powder and single-crystal),neutron diffraction, Raman and optical microscopy [20–24]. Priorto our work, crystalline D-alanine had not been studied under statichigh pressures. Indeed, only two studies of D-isomers of aminoacids (methionine and threonine) appear in the literature, andinvestigation of two enantiomers of the same amino acid existsonly in the case of methionine albeit in two separate studies [25–27]. Our approach of simultaneously examining the effects of highpressure in-situ on two optical isomers of the same organicmolecule in solid state is therefore unprecedented.

2. Materials and methods

2.1. Materials

Single crystals as tabular cleavage fragments of L-alanine andD-alanine (typically �0.1 mm � 0.05 mm � 0.01 mm) were takenfrom large grains (Fluka 5130 and 5140) and used in this study.

2.2. Instruments

2.2.1. Diamond anvil cellTo reach high pressures we used a membrane-type diamond

anvil cell containing Type II diamonds with 0.5 mm culets (Fig.1). Astainless steel gasket with a 0.25 mm diameter sample chamberand a pre-indented thickness of 0.1 mm depth was placed betweenthe diamonds. Samples of the two enantiomers measuringapproximately 0.1 mm � 0.05 mm � 0.01 mm were loaded on to aprecompressed cesium iodide (CsI) window with good spatialseparation. A small (0.020 mm) piece of ruby was also placed onthe window before the diamond anvil cell was closed. Pressure wasmonitored throughout the experiments using the ruby fluores-cence technique [28].

Since CsI and diamond are stable and optically isotropic up tofar higher pressures than those encountered in this study, theyhave no effect on the measurement of the VORD. CsI is IR-transparent over the range of wavelengths used, so it will not addnoise to the collected spectra. CsI acts as a pressure transmittingmedium and fills the sample chamber, allowing a thinner sampleto be used thereby guaranteeing sufficient light transmission.

ssure experimental setup using synchrotron light. The inset (upper left) micrograph the ruby and CsI window. The main panel illustrates the optical path: polarizedrain optics (optical elements not shown for clarity) into the sample chamber of theer and focused (not shown) to the IR detector. The FTIR bench spectrometer deliversoptical rotatory dispersion spectra are obtained.

W. Montgomery et al. / Analytica Chimica Acta 842 (2014) 51–56 53

2.2.2. Synchrotron-source VORD and FTIR spectroscopyTransmission IR micro-spectrometry was performed using

synchrotron source light (Fig. 1) at SOLEIL (France). At the SMISbeamline, experiments were carried out using a custom-madehorizontal infrared microscope for large volume samples,equipped with two Schwarzchild objectives (47 mm workingdistance, NA 0.5) which produce a 22 mm (full width at halfmaximum) IR spot. The system spectrometer is a Nexus 6700 [29].Resolution was 4 cm�1 and number of co-added scans were 25.

The IR synchrotron light delivered by the SMIS beamline has adominant bending magnet character [30]. To maximize the degreeof linear polarization, a wire grid polarizer was inserted up-streamof the experiment (i.e., before the light reached the sample). Asecond wire grid polarizer was inserted between the detector andthe IR collection objective. This polarizer could be freely rotated by196�, and was controlled by a mechanical driver synchronized withIR data collection (Fig. 1).

2.3. Procedures

2.3.1. VORD and FTIR spectra collectionWe rotated the polarizer and collected IR absorbance spectra at

22.5� steps. The entire polarization response of the background(CsI + diamond) and the sample (CsI + diamond + sample) weremeasured at each pressure point. Pressure points were 0, 0.5, 0.9,1.4, 2.0, 2.5 and 0.6 (decompression) GPa.

2.3.2. Spectral analysisThe angle of the polarized FTIR spectra collected relative to the

crystallographic axes of the crystalline sample was assigned

Fig. 2. Background VORD: background spectra (taken through the diamonds and the CsI wthe fundamental vibration of diamond, and the noise centered at 2100 cm�1 is the ove

through comparison of the lowest pressure point to previouslypublished work at ambient conditions.

At each pressure and polarizer position ui, we measured the IRabsorbance spectrum, Iv(ui). The pressure-dependent VORDspectra were extracted from the data collected by modeling thephase shifts (’) of cosine squared fits applied to the I vs. ui data ateach wavenumber (v) between 600–4000 cm�1:

IVðuiÞ ¼ A* cos 2ðui þ ’Þ þ ðoffset � B* uiÞ (1)

where A is the amplitude, and the second term represents abackground offset factor. In the model, ui are the polarizer anglesand A, ’, and the offset terms are the parameters which are solvedfor using a non-linear least squares fit based on the Levenberg–Marquardt algorithm.

The background VORD spectrum was collected at each pressurepoint using the same process as for the samples Although thediamond anvils and the CsI window are isotropic across thewavenumber range and pressures used, the ’background is non-zeroand variable with wavenumber due to the signal contributed by theType II diamonds (fundamental at 1250 cm�1 and overtones at1800–2200 cm�1), illustrated in Fig. 2. These values vary withpressure. Therefore, a careful determination of the backgroundcorrection for each value of the experimental pressure is essentialto obtain accurate VORD spectra. Therefore, the VORD for eachpressure D’ is:

D’ðvÞ ¼ ’sampleðvÞ � ’backgroundðvÞ (2)

The uncertainty of the fit was taken to reflect the error in ourmeasurement of ’(background) and ’(sample). Specifically, for eachwavenumber v, the error is the square root of the diagonal of the

indow) at 0, 0.5, 0.9, 1.4, 2.0, 2.5 and 0.6 (descending) GPa. The peak at 1250 cm�1 isrtones. Notice the changes with pressure.

54 W. Montgomery et al. / Analytica Chimica Acta 842 (2014) 51–56

covariance matrix of the fit of Iv(ui), i.e., the variances for eachparameter.

3. Results and discussion

3.1. FTIR spectra

Polarized IR absorption spectra over the “fingerprint region”from 700–1350 cm�1 of D- and L-alanine are shown in Fig. 3. Thisregion of the IR spectrum mostly contains vibrations involving thechiral center, including stretching of the central C–C axis(850 cm�1, 918 cm�1), bending of the entire structure

Fig. 3. Polarized FTIR: polarized FTIR spectra of L- and D-alanine at 0, 0.5, 0.9,1.4, 2.0, 2.5 anaxis of the crystal) are 90� apart, within the limit of measuring the polarizer rotation(850 cm�1, 918 cm�1), bending of the entire structure (1014 cm�1, 1114 cm�1) and vibra

(1014 cm�1, 1114 cm�1) and vibration of the methyl on the chiralcarbon (1307 cm�1) [31].

The absence of substantial changes in these spectra with theapplication of pressure indicates that the molecules have notundergone any major atomic rearrangements such as crystallinephase transitions or chemical reactions. Small shifts in peakposition are a typical response to pressure, as observed previouslyin a variety of organic molecules [7,32,33].

The spectra in Fig. 3 are commensurate with previousinvestigations using other techniques. In previous studies phasetransitions were observed at low pressures (2–2.5 GPa), but it wasshown that the structure remains in the orthorhombic space group

d 0.6 GPa (a- and c-axis). For each sample the two spectra (aligned with the a- and c- angle. Peak assignments in this region include stretching of the central C–C axistion of the methyl on the chiral carbon (1307 cm�1).

W. Montgomery et al. / Analytica Chimica Acta 842 (2014) 51–56 55

P212121, and an apparent change in symmetry was due to the a- andb-crystallographic axes reaching the same length, which haslimited effect on the FTIR spectrum [8,20–22].

Differences in the a-axis and c-axis spectra of D- and L-alanine inFig. 3 result from coarseness of the 22.5� step in polarizer angle andthe relative angle of the two samples. In order to optimize dataacquisition time, our polarizer rotation step was chosen to providethe best range of data for fitting the VORD spectra and not forprecise identification of the crystallographic axes. No attempt wasmade during loading to align the crystallographic axes of thesamples; as such the a-axis and c-axis spectra presented are notparallel between L- and D-alanine.

Further, the signal from the D-alanine is weak compared to thatfrom L-alanine, most likely due to differing thickness of samples.The relatively thin D-alanine sample results in synchrotron noise atthe same intensity as the sample signal, visible as broadening ofthe peak at 1307 cm�1, a spurious doublet appearing at 850 cm�1,and a shoulder at 1114 cm�1, and most notable when compared tothe L-alanine spectra.

3.2. VORD spectra

VORD spectra at pressures of 0, 0.5, 0.9, 1.4, 2.0, 2.5 and 0.6(recovery) GPa are presented in Fig. 4. When looking at VORDspectra, it should be noted that not all peaks are due to vibrationsinvolving the chiral atoms, necessitating knowledge of thevibrational peak assignments for correct interpretation. Achiralpeaks will not show differences owing to changes in configuration[34].

The spectra correctly show opposite values for the enantiomersafter identical processing. As with the FTIR spectra, there are nogross changes in the VORD spectra which would indicate a changein conformation. The data show that L- and D-alanine remain in

Fig. 4. L- and D- alanine VORD: VORD spectra of L- and D- alanine at pressures of 0, 0.5, 0.9due to synchrotron background noise have been indicated with an *. Error bars are givenalanine signal has been multiplied by 3 in order to provide a 1:1 visual comparison betweethickness of the two samples.

their ambient condition configurations up to 2.5 GPa. The errorbars presented are the error bars from the cos2 fit and are minisculecompared to the value of the data – even with the relatively noisydata of D-alanine. The noise is carried over from the FTIRspectroscopy and the “extra” peaks are indicated in Fig. 4.

3.3. Potential users

In response to pressure, organic structures can be degraded [5],transformed [1] and polymerized [6]. Chiral organic structures area specific subset of organic architecture with important scientificroles [35]. The effects of pressure on chiral molecules can bestudied by use of a diamond anvil cell, synchrotron-source opticalrotatory dispersion spectroscopy and micro-Fourier transforminfrared spectroscopy to provide comparative insights into thefates of organic molecular enantiomers. Organic structures andhigh pressures are relevant to fields such as astrophysics,astrobiology, planetary science, geochemistry, forensic scienceand pharmaceuticals.

4. Conclusions

We have directly monitored the conformation of crystallineL- and D-alanine with the application of pressures up to 2.5 GPausing the diamond anvil cell and synchrotron-source VORD. Theenantiomers of alanine are stable under these pressures, showingonly the shifts in peak center typical of molecular organic crystalsunder pressure; crystalline identity is maintained. To ensure high-quality data, it is preferable to measure multiple enantiomers inthe same loading of the diamond anvil cell and to collectbackground measurements at every pressure. This requirementof measuring three distinct spots (50–100 mm) within the samplechamber of the DAC (200 mm) makes the use of the synchrotron

, 1.4, 2.0, 2.5 and 0.6 GPa. D-alanine is in blue, and L-alanine is in red. Spurious peaks as shaded regions. In most case they are on the order of the line thickness. The D-n the two signals. The difference in signal is directly proportional to the difference in

56 W. Montgomery et al. / Analytica Chimica Acta 842 (2014) 51–56

essential. Synchrotron assisted FTIR allows focus of the beam tobelow the size of the sample while still delivering good signal,which allows confident sampling of the region of interest. Themeasurements are highly sensitive to the thickness of the sample.Our approach complements other techniques including X-ray andneutron diffraction.

No distinct changes were observed in L- and D- alanine underthe relatively low pressures produced. It is possible that at higherpressures, intermolecular bonds may play a stronger role andresult in atomic movement about the chiral carbon. Havingdemonstrated that this measurement is feasible for the simplestchiral molecular organic crystal, alanine, applications for otherchiral organic materials become evident. Through high pressuresynchrotron source VORD, it becomes possible to directlydetermine the effects of proximity and intermolecular interactionon the conformation of organic molecules. Our technique opens awide range of chiral organic compounds to study for moleculartransformations at high pressures providing new analyticalcapability in many scientific fields.

Acknowledgements

We acknowledge SOLEIL for provision of synchrotron radiationfacilities (Proposal ID “20120970”) for measurements and we thankDr. P. Dumas for assistance in using beamline SMIS and Dr. J.-P. Itie foraccess to the high-pressure facilities at beamline PSICHÉ. Theresearch leading to these results has received funding from theEuropean Community's Seventh Framework Programme(FP7/2007–2013) under grant agreement no. 312284. We alsoacknowledge beamline X01DC at the Swiss Light Source(Paul Scherrer Institute) for experimental development andbeamtime “20120314.”

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