Structure, vibrational assignment, normal coordinate analysis, and ab initio calculations of methylisocyanate☆

Abstract The infrared (3200-30 cm −1 ) and Raman (3200-10 cm −1 ) spectra of gaseous and solid methylisocyanate, CH 3 NCO, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained and qualitative depolarization ratios have been measured. The CNC bend has been observed in the far infrared and low frequency Raman spectra of the gas at approximately 172 cm −1 . An additional far infrared band at ≈50 cm −1 has tentatively been assigned as the methyl torsional mode, although it could be due to the Δν = 1, Δ l = ± 1 transitions of the CNC bending mode. A complete assignment of the vibrational fundamentals is proposed. The structural parameters, force constants, and vibrational frequencies have been determined from ab initio Hartree—Fock gradient calculations using the 6-31G* basis set. Additionally, structural parameters have been obtained with the 6-311 + + G** basis set with electron correlation at the MP2 level which are compared to those obtained from the microwave data and electron diffraction study. These results are compared with the corresponding quantities obtained for similar molecules.