A scaled quantum mechanical force field and vibrational analysis for the gamma glycine crystal polymorph: Hydrogen bond stretching modes observed

Abstract

A scaled quantum mechanical force field has been obtained from periodic solid state density functional theory calculations on the glycine crystal γ polymorph unit cell at the DNP/GGA/PBE level. The assignments here differ in some respects from those obtained earlier, primarily where a broad weak band observed in both Raman and IR spectra at 285 cm −1, previously assigned to an overtone of a lattice mode, is assigned here as a mix of Hydrogen bond stretching and CC torsion. The standard deviation of the error between experimentally observed and calculated frequencies, for calculations performed on both the optimized and neutron diffraction crystal structures, is 6 cm −1 for a total of 118 vibrational modes.