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

Abstract

We have studied the amino acidL-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 NH₃, CH₃, CH₂ and CO₂ moieties and the molecular structure of LEU. Calorimetric and diffraction data evidenced two novel phase transitions at about 150 K (T₁) and 275 K (T₂). 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 T₁ and T₂, as well as with the already reported phase transition at T₃ = 353 K. Through the analysis of the Raman modes, the new structural arrangement observed below T₁ was related to conformational modifications of the CH and CH₃ groups, while the behavior of the N–H stretching vibration, ν(NH₃), 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.