Single crystal samples of L-leucine, C 6 H 13 NO 2 , a fundamental aliphatic amino acid of the human body, have been studied by Raman spectroscopy at temperatures from 300 to 430 K over the spectral range from 50 to 3100 cm −1 . A tentative assignment of all bands is given. For high temperatures, several modifications on the Raman spectra were observed at about 353 K, giving evidence that the L-leucine crystal undergoes a structural phase transition. An interpretation for this phase transition in terms of group theory analysis is given.

We have studied the amino acid L-leucine (LEU) using inelastic neutron scattering, X-rays and neutron diffraction, calorimetry and Raman scattering as a function of temperature, focusing on the relationship between the local dynamics of the NH(3), CH(3), CH(2) and CO(2) moieties and the molecular structure of LEU. Calorimetric and diffraction data evidenced two novel phase transitions at about 150 K (T(1)) and 275 K (T(2)). The dynamical susceptibility function, obtained from the inelastic neutron scattering results, shows a re-distribution of the intensity of the vibrational bands that can be directly correlated with the phase transitions observed at T(1) and T(2), as well as with the already reported phase transition at T(3) = 353 K. Through the analysis of the Raman modes, the new structural arrangement observed below T(1) was related to conformational modifications of the CH and CH(3) groups, while the behavior of the N-H stretching vibration, ν(NH(3)), gave insight into the intermolecular N-H…O interactions. The observation of changes in the translational symmetry in the crystalline lattice, as well as anharmonic dynamics, allows for localized motions in LEU.

Raman spectra of a crystal of L-leucine, an essential amino acid, were obtained for pressures between 0 and 6 GPa. The results show anomalies at three pressure values, one between 0 and 0.46 GPa, another between 0.8 and 1.46 GPa, and a third at P ∼ 3.6 GPa. The first two anomalies are characterized by the disappearance of lattice modes (which can indicate occurrence of phase transitions), the appearance of several internal modes, or the splitting of modes of high wavenumbers. The changes of internal modes are related to CH and CH 3 unit motions as well as hydrogen bonds, as can be inferred from the behavior of bands associated with CO 2 − moieties. The third anomaly is a discrete change of the slopes of the wavenumber versus pressure plots for most modes observed. Further, decompression to ambient pressure generates the original Raman spectrum, showing that the pressure-induced anomalies undergone by L-leucine crystals are reversible.

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