Infrared studies on glycine and its addition compounds

The infrared absorption spectra of glycine, mono- and di-glycine hydrochlorides and hydrobromides, their deuterated analogs and the ones containing ¹⁵N substituted glycine have been investigated from 3500 to 250 cm⁻¹ at room temperature as well as at liquid N₂ temperature. The spectra of the equilibrium mixtures containing the species (i) NH₃⁺CH₂COO⁻, (ii) NH₂⁺DCH₂COO⁻, (iii) NHD₂⁺CH₂COO⁻ and (iv) ND₃⁺CH₂COO have been discussed in terms of the vibrational modes of the groups NH₃⁺ (NH₂D⁺, NHD₂⁺, ND₃⁺), CH₂, COO⁻ and the skeleton CCN⁺ in the light of the effect of the coupling of vibrations on the absorption characteristics. The region 3200-1800 cm⁻¹ shows bands which seem to represent characteristic N⁺H⋯O/N⁺D⋯O bond properties and the Fermi resonance between N⁺H(N⁺D) stretch and the overtones/combinations (believed to be the cause of the complexity of this region) does not appear probable as revealed by our deuteration studies.

The spectra of monoglycine hydrochloride and hydrobromide indicate a dimer formation between two glycinium units (NH₃⁺CH₂COOH) through very strong hydrogen bonds (OH⋯O). The spectral data on diglycine hydrochloride and hydrobromide are consistent with the reported crystal structure.