RAMAN AND INFRARED-SPECTRA, CONFORMATIONAL STABILITY, VIBRATIONAL ASSIGNMENT AND AB-INITIO CALCULATIONS OF METHYL CYANOFORMATE

The Raman (3100-10 cm(-1)) spectra of the liquid and solid and the inf rared (3100-400 cm(-1)) spectra of the gas and solid were recorded for three isotopic species of methyl cyanoformate, (CH3OCOCN)-C-12, (CH3O COCN)-C-13 and (CH3OCOCN)-O-18. 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 thre e 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 predic ted by ab initio calculations using the MP2/6-31G basis set, the infr ared intensity of the methyl torsion is very weak and, subsequently, t his fundamental is not observed in the far-infrared spectrum of the ga s. Therefore, an experimental barrier to internal rotation for the met hyl group could not be obtained from the vibrational data. However, th e 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 f or the s-trans conformer were also determined from ab initio calculati ons using the RHF/3-21G, RHF/6-31G and MP2/6-31G* basis sets. These r esults are compared with those obtained experimentally and with simila r quantities for some related molecules.