Raman and infrared spectra, conformational stability, vibrational assignment and ab initio calculations of methyl cyanoformate

Abstract

AbstractThe Raman (3100–10 cm−1) spectra of the liquid and solid and the infrared (3100—400 cm− 1) spectra of the gas and solid were recorded for three isotopic species of methyl cyanoformate, 12CH3OCOCN, 13CH3OCOCN and CH318OCOCN. Additionally, the far‐infrared (370—60 cm− 1) spectrum of the gas of the normal species was recorded. Raman depolarization ratios were obtained for the liquid phases of all three isotopic species. These data were interpreted on the basis that the only stable conformation present at ambient temperature is the s‐trans rotamer where the methyl group is trans to the cyano group. As predicted by ab initio calculations using the MP2/6—31G* basis set, the infrared intensity of the methyl torsion is very weak and, subsequently, this fundamental is not observed in the far‐infrared spectrum of the gas. Therefore, an experimental barrier to internal rotation for the methyl group could not be obtained from the vibrational data. However, the theoretical barrier to internal rotation was calculated to be 424 cm− 1 (1.21 kcal mol− 1) using the RHF/6—31G* basis set. The structure, infrared intensities, Raman activities and vibrational wavenumbers for the s‐trans conformer were also determined from ab initio calculations using the RHF/3—21G, RHF/6—31G* and MP2/6—31G* basis sets. These results are compared with those obtained experimentally and with similar quantities for some related molecules.