Vibrational spectroscopy and phonon‐related properties of monoclinic GABA, a non‐proteinogenic inhibitory neurotransmitter amino acid

Abstract The γ‐aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in vertebrate central nervous systems. Taking full account of the measured X‐ray diffractogram of its stable monoclinic polymorph, we have performed density functional theory (DFT) calculations to analyze/understand its infrared and Raman spectra considering its monoclinic polymorph, an isolated stabilized (120) plane, and a single GABA molecule. Besides, phonon dispersion/density of states, heat capacity, Debye temperature, and entropy/enthalpy/free energies were also obtained for the GABA monoclinic crystal. Vibrational signatures due to the GABA (120) planes are pointed out for the first time for several wavenumbers. The results obtained for the GABA monoclinic polymorph reinforce the need of dispersion‐corrected solid‐state calculations to describe the vibrational properties of molecular crystals instead of considering a single isolated molecule picture, even for wavenumbers larger than those usually associated with lattice modes.