The r0 structural parameters, vibrational spectra, ab initio calculations and barriers to internal rotation and linearity of methylisocyanate

The infrared (3500-300 cm−1) and/or Raman (3500-50 cm−1) spectra of gaseous, liquid and solid of methylisocyanate, CH3NCO, have been recorded. Fine structures of the nearly free internal rotation of the methyl rotor has been observed for the pseudodegenerate CH3 stretch and deformation from which the band centers and Coriolis coupling constants have been determined. Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d), 6-311+G(d,p), and 6-311+G(2d,2p) basis sets have been carried out. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and structural parameters have been determined and compared to the experimental quantities. Several differences are noted between the predicted and experimental values. By combining the three previously reported rotational constants for CH3NCO with the ab initio MP2/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The r0 values for the distance (Å) are: r(CN) = 1.447(3); r(NC) = 1.215(3); r(CO) = 1.166(3); r(CHa) = 1.089(2); r(CHs) = 1.093(2), and for the angles (degrees): ∠CNC=135.8(5); ∠NCO=172.6(5); ∠NCHa=108.6(5); ∠NCHs=110.8(5). Several of these parameters are significantly different from those previously reported from the latest microwave study. Predicted barriers to internal rotation and linearity as well as values of the centrifugal distortion constants are given and compared to experimental values when available. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results of some similar molecules.