Conformational study of sarcosine as probed by matrix-isolation FT-IR spectroscopy and molecular orbital calculations

Abstract

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 conformers were located in the potential energy surface (PES) of sarcosine, with the ASC conformer being the ground conformational state. This form is analogous to the glycine most stable conformer and is characterized by a NH⋯O intramolecular hydrogen bond; in this form, the carboxylic group assumes the cis configuration and the OCCN and LpNCC (where Lp is the nitrogen lone electron pair) dihedral angles are ca. 15 and −8°, respectively. The second most stable conformer (G′AT) exhibits a strong OH⋯N intramolecular hydrogen bond and a trans carboxylic group, being similar to the most stable form of N,N-dimethylglycine. These two forms were predicted to differ in energy by less than ca. 2 kJ mol−1 and represent ≈70% of the conformational population at room temperature. In consonance with the theoretical predictions, the FT-IR spectra of the matrix-isolated compound reveal the presence in the gaseous phase of the four conformers of sarcosine with experimentally accessible populations: besides conformers ASC and G′AT, forms GSC and AAC could also be identified experimentally. The assignment of the spectra of the different experimentally observed conformers of sarcosine was carried out.