Structure―property relations in crystalline L-leucine obtained from calorimetry, X-rays, neutron and Raman scattering

• Published in: Physical chemistry chemical physics : PCCP 2011-04, Vol.13 (14), p.6576-6583
• Main Authors: FACANHA, Pedro F, XUESHE JIAO, FREIRE, Paulo T. C, LIMA, Jose A, DOS SANTOS, Adenilson O, HENRY, Paul F, YOKAICHIYA, Fabiano, KREMNER, Ewout, BORDALLO, Heloisa N
• Format: Article
• Language: English
• Subjects: Calorimetry; Chemistry; Crystal structure; Dynamics; Exact sciences and technology; Focusing; General and physical chemistry; Hydrogen Bonding; Leucine - chemistry; Models, Molecular; Molecular Conformation; Neutron Diffraction; Neutron scattering; Phase transformations; Phase Transition; Raman scattering; Spectrum Analysis, Raman; Structure-Activity Relationship; Temperature; Thermodynamics; X-Ray Diffraction; X-rays
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c0cp02278k

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.