Vibrational Spectroscopy 54 (2010) 107–111 Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec High-pressure Raman spectra of racemate dl-alanine crystals E.A. Belo a , J.A. Lima Jr. a,b , P.T.C. Freire a,∗ , F.E.A. Melo a , J. Mendes Filho a , H.N. Bordallo c , A. Polian b a Departamento de Física, Universidade Federal do Ceará, C.P. 6030, CEP 60455-760 Fortaleza, CE, Brazil b Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, Paris 6 et 7, CNRS UMR 7590, F-75015, Paris, France c Helmholtz-Zentrum Berlin für Materialien und Energie, Glienicker Straße, 100, D-14109 Berlin, Germany article info Article history: Received 20 January 2010 Received in revised form 17 March 2010 Accepted 29 March 2010 Available online 8 April 2010 Keywords: Organic crystals Phonons High pressure abstract Raman spectroscopy investigations of dl-alanine crystal under high pressures have been carried out up to 18.0 GPa. For instance, around 1.0 GPa and between 1.7 and 2.3 GPa changes in the Raman profile were observed and associated to conformational changes of the molecules in the unit cell or to a phase transition accompanied to slight conformational change of the molecule through CH and CH 3 groups. Moreover, between 6.0 and 7.3 GPa, the appearance of a new low energy lattice modes and to the splitting of a band assigned to the stretching vibration of the CCH 3 moiety were related to a second phase transition. Finally, changes in lattice modes, red shift of the band associated to CCH 3 stretching and increasing of line-width of the band associated to the wagging of CO 2 , between 11.6 and 13.2 GPa, are ascribed to a third phase transition. On release of pressure the original phase was obtained again. © 2010 Elsevier B.V. All rights reserved. 1. Introduction If gaining an understanding of the organic solid state is one of the big questions to be solved in chemistry, measuring dynamic prop- erties under pressure constitutes by far of the most powerful ways of getting insight on experimental details about the strength of atomic interactions [1]. On the experimental side, Raman scattering provides a suitable probe for studies of H-vibrational dynamics for given solids; and subtle volume changes that arise from changes in packing density due to the decrease in intermolecular distances in the H-bond network can be magnified by pressure. Pressure can be used to alter the spatial distribution of charges of a system, inducing polymorphism, changing the course of a reaction or the molecular assembly formed. Among molecular organic crystals, those of amino acids attract special attention – as biomimetics, as solid drugs, as materials for molecular electronics, as systems important for geo- and cosmo- chemistry. Amino acids constitute molecular systems where van der Waals interactions and H-bonds play important roles in the stability of the crystalline structure. Under these lines the search of high-pressure polymorphs of amino acids and the studies of the anisotropy of pressure-induced structural distortion not nec- essarily accompanied by a phase transition are of great interest [2–10]. For instance, alanine (C 2 H 4 (NH 2 )COOH) – the simplest amino acid with respect to the molecular structure as well as the most ∗ Corresponding author. Tel.: +55 85 3366 9906; fax: +55 85 3366 9450. E-mail address: tarso@fisica.ufc.br (P.T.C. Freire). widely used in protein construction – presents a very complex behavior when its solid state properties are investigated as a func- tion of temperature or pressure. Intriguingly, the unusual features in the lattice dynamics of crystal observed in the temperature range 170–220 K by diverse experimental techniques, such inelastic neu- tron scattering, dielectric permittivity, NMR and phonon-echo plus the stepwise negative thermal expansion observed along the c-axis [11–15] in hydrogenated l-alanine, are not followed by struc- tural instabilities as pointed by X-ray [16,17] and neutron [18,19] diffraction experiments. However, in partially or fully deuterated l-alanine clear evidence of a structural phase transition at low temperatures was obtained through Raman and neutron scatter- ing experiments, and ascribed to be consequence of change of hydrogen bond interactions [20,21]. On the other hand, when pres- sure is applied to hydrogenated l-alanine two structural phase transitions were observed, one from orthorhombic to tetragonal symmetry at ∼2.1 GPa [9,10] and another from tetragonal to mon- oclinic structure at ∼9 GPa [10]. Deuterated l-alanine also presents phase transitions under high-pressure conditions, but the pressure values where phase transitions are observed are lower than those of the hydrogenated samples, 1.5 and 4.4 GPa [22]. This difference is attributed to the different dimensions of hydrogen bonds in the unit cell of the hydrogenated and deuterated samples [22,23]. Up to date few works deal with the behavior of racemate amino acids, which contains two l- and two d-isomers of the amino acid in their unit cell, among them related to dl-alanine [24–26]. To the best of our knowledge high-pressure studies were performed in l and dl-serine, l- and dl-valine and l- and dl-cysteine only [27–29] (for a general review on structural and vibrational proper- ties of amino acids under pressure, see Ref. [30]). In such studies the 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.03.016