The behavior of NH3 torsional vibration of l-alanine, l-threonine and taurine crystals under high pressure: A Raman spectroscopic study

Raman spectra of l-alanine, l-threonine and taurine amino acids crystals under high-pressure conditions at room temperature were investigated in a diamond anvil cell in order to observe the behavior of the torsional mode of NH3 unit. It was observed for l-threonine and taurine crystals that increasing pressure produces an increase in the wave number of the band associated to the torsional vibration of ammonium group, while for l-alanine crystal a decrease in the wave number is observed. The anomalous behavior of l-alanine is explained in terms of the length and geometry of the hydrogen bonds within the crystalline structure: due to a short average length the effect of pressure is to deform the hydrogen bond, while for l-threonine and taurine, pressure approximates the N and O atoms in the N–H⋯O intermolecular bond.