Scholarly article on topic 'In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures'

In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — W. Montgomery, Ph. Lerch, M.A. Sephton

Abstract Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.

Academic research paper on topic "In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures"

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Analytica Chimica Acta

journal homepage: www.elsevier.com/locate/aca

In-situ vibrational optical rotatory dispersion of molecular organic flWsMark crystals at high pressures

W. Montgomery^*, Ph. Lerchb, M.A. Sephtona

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

HIGHLIGHTS

• High pressure synchrotron source vibrational optical rotary dispersion is possible.

• Our method simultaneously collects high-quality polarized FTIR spectra.

• The high resolution of the synchrotron is essential for confident measurements.

GRAPHICAL ABSTRACT

L-alanine D-alanine

ARTICLE INFO

ABSTRACT

Article history:

Received 1 April 2014

Received in revised form 4 July 2014

Accepted 15 July 2014

Available online 19 July 2014

Keywords:

Chirality

Alanine

Vibrational optical rotatory dispersion Fourier transform infrared spectroscopy Diamond anvil cell

Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.

© 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/).

1. Introduction

The responses of molecular organic crystals at high pressures are relevant to a number of scientific fields, ranging from astrophysics [1] to pharmaceuticals [2-5]. Intense effort has been expended to understand the effects of high pressure on organic crystals, using techniques such as X-ray diffraction, Raman spectroscopy 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 rotatory dispersion; VORD, vibrational optical rotatory dispersion.

* Corresponding author. Tel.: +44 20 7594 5185. E-mail address: w.montgomery@imperial.ac.uk (W. Montgomery).

polymorphism, intermolecular motion of hydrogen bonds, breaking, distortion, symmetrization of hydrogen bonds and proton transfer [8-13]. The pressure-induced intermolecular bonding often involves hydrogen bonds enhanced by new geometric arrangements and increased intermolecular proximity. High pressure experiments offer a quantitative control over the proximity of atoms and the formation of intermolecular hydrogen bonds [9,13,14].

One aspect of high-pressure polymorphism of particular relevance to molecular organic crystals is absolute conformation, specifically referred to as chirality or handedness. Chirality results from the presence of asymmetric bonds arranged around carbon atoms. It plays a key role in biology, with practical effects in pharmacology, where one enantiomer may be beneficial while another is damaging [15].

http://dx.doi.org/10.1016/j.aca.2014.07.020

0003-2670/® 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/).

In the absence of direct observation of atomic arrangements using X-ray or neutron diffraction, chirality can be measured indirectly via sample interaction with polarized light, either linear or circular, over a range of wavelengths. There are a variety of possible combinations for valid measurements: common techniques include vibrational circular birefringence (circularly polarized light), and Raman optical activity (linearly polarized light) but these 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 molecular organic crystals under high pressure all have individual weaknesses when employed for the determination of molecular conformation. Firstly, classical vibrational spectroscopy, which is routinely performed at high pressures, does not distinguish between enantiomers. Secondly, non-optical techniques currently used for conformational analysis are not easily applied to the small volumes of organic material produced at high pressures, and in any case would offer only static pre- and post-experiment data.

Recent improvements in instrumental capability have lead to renewed interest in chiroptical techniques such as optical rotatory dispersion (ORD), which measures the response of a sample to linearly 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 Raman alternatives, although the effect was first observed in 1836 [17,18].

The immediate advantage of VORD is that one measurement can probe the vibrational structure (e.g., chemical bonds) and the conformation (chirality) simultaneously. The benefits of using a diamond anvil cell (DAC) when seeking to monitor any changes in chirality at high pressure include the possibility of continuous realtime monitoring, the wide window of infrared transparency of Type II diamonds, and the simultaneous visual observation of the sample.

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

one chiral carbon center, thereby providing interpretative simplicity. Crystalline L-alanine has been studied isothermally at pressures up to 15 GPa using X-ray diffraction (powder and single-crystal), neutron diffraction, Raman and optical microscopy [20-24]. Prior to our work, crystalline D-alanine had not been studied under static high pressures. Indeed, only two studies of D-isomers of amino acids (methionine and threonine) appear in the literature, and investigation of two enantiomers of the same amino acid exists only in the case of methionine albeit in two separate studies [2527]. Our approach of simultaneously examining the effects of high pressure in-situ on two optical isomers of the same organic molecule in solid state is therefore unprecedented.

2. Materials and methods

2.1. Materials

Single crystals as tabular cleavage fragments of L-alanine and D-alanine (typically ~0.1 mm x 0.05 mm x 0.01 mm) were taken from large grains (Fluka 5130 and 5140) and used in this study.

2.2. Instruments

2.2.1. Diamond anvil cell

To reach high pressures we used a membrane-type diamond anvil cell containing Type II diamonds with 0.5 mm culets (Fig. 1). A stainless steel gasket with a 0.25 mm diameter sample chamber and a pre-indented thickness of 0.1 mm depth was placed between the diamonds. Samples of the two enantiomers measuring approximately 0.1 mm x 0.05 mm x 0.01 mm were loaded on to a precompressed cesium iodide (CsI) window with good spatial separation. A small (0.020 mm) piece of ruby was also placed on the window before the diamond anvil cell was closed. Pressure was monitored throughout the experiments using the ruby fluorescence technique [28].

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

L-sample J Ub^ □ D-sample

aperture

Fig. 1. Experimental setup: schematic of the infrared optical rotatory dispersion high pressure experimental setup using synchrotron light. The inset (upper left) micrograph shows two samples loaded into the gasket hole of the diamond anvil cell, along with the ruby and CsI window. The main panel illustrates the optical path: polarized synchrotron light is modulated by the FTIR spectrometer, steered and focused by Cassegrain optics (optical elements not shown for clarity) into the sample chamber of the DAC, collected by further Cassegrain optics (not shown), steered to the motorized polarizer and focused (not shown) to the IR detector. The FTIR bench spectrometer delivers IR spectra. By combining the angular information, as described in the text, vibrational optical rotatory dispersion spectra are obtained.

2.2.2. Synchrotron-source VORD and FTIR spectroscopy

Transmission IR micro-spectrometry was performed using synchrotron source light (Fig. 1) at SOLEIL (France). At the SMIS beamline, experiments were carried out using a custom-made horizontal infrared microscope for large volume samples, equipped with two Schwarzchild objectives (47 mm working distance, NA 0.5) which produce a 22 mm (full width at half maximum) 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 a dominant bending magnet character [30]. To maximize the degree of linear polarization, a wire grid polarizer was inserted up-stream of the experiment (i.e., before the light reached the sample). A second wire grid polarizer was inserted between the detector and the IR collection objective. This polarizer could be freely rotated by 196°, and was controlled by a mechanical driver synchronized with IR data collection (Fig. 1).

2.3. Procedures

2.3.1. VORD and FTIR spectra collection

We 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) were measured 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 analysis

The angle of the polarized FTIR spectra collected relative to the crystallographic axes of the crystalline sample was assigned

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

At each pressure and polarizer position 9i, we measured the IR absorbance spectrum, Iv(Ui). The pressure-dependent VORD spectra were extracted from the data collected by modeling the phase shifts (') of cosine squared fits applied to the I vs. 9i data at each wavenumber (v) between 600-4000 cm-1:

IV(Uf) = A * cos 2(U¡ + ') + (offset - B * U¡)

where A is the amplitude, and the second term represents a background offset factor. In the model, B¡ are the polarizer angles and A, ', and the offset terms are the parameters which are solved for using a non-linear least squares fit based on the LevenbergMarquardt algorithm.

The background VORD spectrum was collected at each pressure point using the same process as for the samples Although the diamond anvils and the Csl window are isotropic across the wavenumber range and pressures used, the 'background is non-zero and variable with wavenumber due to the signal contributed by the Type II diamonds (fundamental at 1250 cm-1 and overtones at 1800-2200 cm-1), illustrated in Fig. 2. These values vary with pressure. Therefore, a careful determination of the background correction for each value of the experimental pressure is essential to obtain accurate VORD spectra. Therefore, the VORD for each pressure D' is:

D'(v) = 'sample(v) - 'background(v) (2)

The uncertainty of the fit was taken to reflect the error in our measurement of '(background) and '(sample). Specifically, for each wavenumber v, the error is the square root of the diagonal of the

Fig. 2. Background VORD: background spectra (taken through the diamonds and the CsI window) at 0,0.5,0.9,1.4,2.0,2.5 and 0.6 (descending) GPa. The peak at 1250 cm 1 is the fundamental vibration of diamond, and the noise centered at 2100 cm-1 is the overtones. Notice the changes with pressure.

covariance matrix of the fit of Iv(0,), i.e., the variances for each parameter.

3. Results and discussion

3.1. FTIR spectra

Polarized IR absorption spectra over the "fingerprint region" from 700-1350cm-1 of d- and L-alanine are shown in Fig. 3. This region of the IR spectrum mostly contains vibrations involving the chiral center, including stretching of the central C-C axis (850 cm-1, 918 cm-1), bending of the entire structure

(1014cm \ 1114cm and vibration of the methyl on the chiral carbon (1307 cm-1) [31].

The absence of substantial changes in these spectra with the application of pressure indicates that the molecules have not undergone any major atomic rearrangements such as crystalline phase transitions or chemical reactions. Small shifts in peak position are a typical response to pressure, as observed previously in a variety of organic molecules [7,32,33].

The spectra in Fig. 3 are commensurate with previous investigations using other techniques. In previous studies phase transitions were observed at low pressures (2-2.5 GPa), but it was shown that the structure remains in the orthorhombic space group

0J U C fO X!

O i/i -Q

D-alanine, a-axis

-JUxAJlA

--AaÍA^ JUjX^0

6 G Pa descending

2.5 GPa

2.0 GPa

1.4 GPa

0.5 GPa

800 1000 1200 D-alanine, c-axis

-ajwOUA

-aAAXXjiA"

-MajJ^/IA ^AajÍAA/V

descending

2.5 GPa

1.4 GPa

|.9 GPa

1.5 GPa

800 1000 1200

L-alanine, a-axis

UUuJLAÁ __ju_AJLaA

^ixjJuJ^0

6 GPa descending

2.5 GPa

2.0 GPa

1.4 GPa

0.9 GPa

.5 GPa

800 1000 1200

L-alanine, c-axis

juJIaM"

sjJiMj

6 GPa descending

2.5 GPa

2.0 GPa

0.9 GPa

axJUjX"

800 1000 1200

wavenumber, cm"

Fig. 3. Polarized FTIR: polarized FTIR spectra of l- and d-alanine at 0,0.5,0.9,1.4,2.0,2.5 and 0.6 GPa (a- and c-axis). For each sample the two spectra (aligned with the a- and c-axis of the crystal) are 90° apart, within the limit of measuring the polarizer rotation angle. Peak assignments in this region include stretching of the central C-C axis (850 cm-1, 918 cm-1), bending of the entire structure (1014 cm-1,1114 cm-1) and vibration of the methyl on the chiral carbon (1307 cm-1).

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

Differences in the a-axis and c-axis spectra of d- and L-alanine in Fig. 3 result from coarseness of the 22.5° step in polarizer angle and the relative angle of the two samples. In order to optimize data acquisition time, our polarizer rotation step was chosen to provide the best range of data for fitting the VORD spectra and not for precise identification of the crystallographic axes. No attempt was made during loading to align the crystallographic axes of the samples; as such the a-axis and c-axis spectra presented are not parallel between L- and D-alanine.

Further, the signal from the D-alanine is weak compared to that from L-alanine, most likely due to differing thickness of samples. The relatively thin D-alanine sample results in synchrotron noise at the same intensity as the sample signal, visible as broadening of the 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 to the 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 VORD spectra, it should be noted that not all peaks are due to vibrations involving the chiral atoms, necessitating knowledge of the vibrational peak assignments for correct interpretation. Achiral peaks will not show differences owing to changes in configuration [34].

The spectra correctly show opposite values for the enantiomers after identical processing. As with the FTIR spectra, there are no gross changes in the VORD spectra which would indicate a change in conformation. The data show that L- and D-alanine remain in

their ambient condition configurations up to 2.5 GPa. The error bars presented are the error bars from the cos2 fit and are miniscule compared to the value of the data - even with the relatively noisy data of D-alanine. The noise is carried over from the FTIR spectroscopy 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 are a specific subset of organic architecture with important scientific roles [35]. The effects of pressure on chiral molecules can be studied by use of a diamond anvil cell, synchrotron-source optical rotatory dispersion spectroscopy and micro-Fourier transform infrared spectroscopy to provide comparative insights into the fates of organic molecular enantiomers. Organic structures and high pressures are relevant to fields such as astrophysics, astrobiology, planetary science, geochemistry, forensic science and pharmaceuticals.

4. Conclusions

We have directly monitored the conformation of crystalline L- and D-alanine with the application of pressures up to 2.5 GPa using the diamond anvil cell and synchrotron-source VORD. The enantiomers of alanine are stable under these pressures, showing only the shifts in peak center typical of molecular organic crystals under pressure; crystalline identity is maintained. To ensure high-quality data, it is preferable to measure multiple enantiomers in the same loading of the diamond anvil cell and to collect background measurements at every pressure. This requirement of measuring three distinct spots (50-100 mm) within the sample chamber of the DAC (200 mm) makes the use of the synchrotron

Fig. 4. l- and d- alanine VORD: VORD spectra of l- and d- alanine at pressures of 0,0.5,0.9,1.4,2.0,2.5 and 0.6 GPa. d-alanine is in blue, and l-alanine is in red. Spurious peaks due to synchrotron background noise have been indicated with an *. Error bars are given as shaded regions. In most case they are on the order of the line thickness. The d-alanine signal has been multiplied by 3 in order to provide a 1:1 visual comparison between the two signals. The difference in signal is directly proportional to the difference in thickness of the two samples.

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

No distinct changes were observed in l- and d- alanine under the relatively low pressures produced. It is possible that at higher pressures, intermolecular bonds may play a stronger role and result in atomic movement about the chiral carbon. Having demonstrated that this measurement is feasible for the simplest chiral molecular organic crystal, alanine, applications for other chiral organic materials become evident. Through high pressure synchrotron source VORD, it becomes possible to directly determine the effects of proximity and intermolecular interaction on the conformation of organic molecules. Our technique opens a wide range of chiral organic compounds to study for molecular transformations at high pressures providing new analytical capability in many scientific fields.

Acknowledgements

We acknowledge SOLEIL for provision of synchrotron radiation facilities (Proposal ID "20120970") for measurements and we thank Dr. P. Dumas for assistance in using beamline SMIS and Dr. J.-P. Itie for access to the high-pressure facilities at beamline PSICHÉ. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 312284. We also acknowledge beamline X01DC at the Swiss Light Source (Paul Scherrer Institute) for experimental development and beamtime "20120314."

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