Sarcosine (N-methylglycine) has been studied by matrix-isolation FT-IR spectroscopy and molecular orbital calculations undertaken at the DFT/B3LYP and MP2 levels of theory with the 6-311++G(d, p) and 6-31++G(d, p) basis set, respectively. Eleven different

A. GÓMEZ-ZAVAGLIA, R. FAUSTO.

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

Royal Society of Chemistry

Lugar: London; Año: 2003 vol. 5 p. 3154 - 3161

1463-9076

Neutral forms of glycine and their N-methylated derivatives, sarcosine (N-methylglycine) and N,Ndimethylglycine were, for the first time, observed in the solid state pure compounds. The substances were sublimated under high vacuum, quickly deposited onto a cold CsI substrate at 9 K and examined using FTIR spectroscopy within the temperature range 9–300 K. For all the compounds studied, the spectra obtained at 9 K after deposition revealed the presence of both the neutral and zwitterionic amino acid forms. The identification of the neutral forms and their vibrational signatures could be unequivocally established by taking into consideration the available data for the matrix-isolated neutral monomeric and aggregated amino acids as well as solid state data for acid and alkaline salts of glycine. Upon temperature increase, neutral forms convert nonreversibly to the zwitterionic forms, as noticed by the systematic increase in the relative intensities of the infrared bands ascribable to the latter species. Observation of aggregates of neutral amino acids in the solid phase, together with the fact that the amount of the neutral forms observed reduces with increasing both the temperatures of the cold substrate and of the vapour prior to deposition, enabled to conclude that the observed (neutral form)!zwitterion conversion occurs in the solid state, to the therein most stable zwitterionic species, and that intermolecular interactions in this phase should play an important role in the mechanism of proton transfer.