Conformational Equilibrium in Glycine: Experimental Jet-Cooled Raman Spectrum

The first observation of a gas-phase spontaneous,Raman spectrum of an amino acid, glycin (Gly, H(2)NCH(2)COOH), is reported. A molecular-beam source was combined With a high-sensitivity Raman setup to record a low-frequency (below 500 cm(-1)) vibrational spectrum. Ab initio (MP2) and density. functional theory (DFT) (BLYP, B3LYP, and PBE0) calculations with the aug-cc-pVTZ-basis. set were used to support vibrational assignments. Jet-cooling of glycine molecules was observed by varying the laser-nozzle distance. The acquired, medium resolution spectra (4 cm(-1)) allow the different glycine conformations to be distinguished. The structures of the two most stable glycine conformers, previously observed by electron diffraction and microwave spectroscopy, have been confirmed. Evidence, is reported for a third conformation (gauche trans trans). which has not been previously, experimentally reported. Band polarization measurements were used to confirm the vibrational assignments.