Microwave Spectrum, Barrier to Internal Rotation, 14N Nuclear Quadrupole Interaction, and Normal-Coordinate Analysis in Methylisocyanate, Methylisothiocyanate, and Methylthiocyanate

Abstract

The microwave spectra of methylthiocyanate, methylisocyanate, and methylisothiocyanate have been re‐examined under high resolution. The rotational constants of methylthiocyanate are A = 15 703.0±2.0 MHz, B = 4155.59±0.1 MHz, and C = 3354.16±0.1 MHz. The barrier to internal rotation of the methyl group determined from the Q‐branch splittings in the v = 0 torsional state is 1600±80 cal/mole. Analysis of the ¹⁴N quadrupole splitting yields the following coupling constants: χ_aa = −3.13±0.05 MHz, χ_bb = 2.19±0.05 MHz, χ_cc = 0.94±0.05 MHz. The barrier to internal rotation in methylisocyanate is found to be 83±15 cal/mole from the splitting of the K = 0, m = ±3 lines, and χ_aa is 2.86±0.03 MHz. The corresponding parameters in methylisothiocyanate are found to be 304±50 cal/mole and 1.90±0.03 MHz. The microwave data are consistent with a CNC angle of 147°—148° in methylisothiocyanate as opposed to 140° in methylisocyanate and indicates C–N bond lengths in these compounds of 1.43–1.45 Å. The observed spectra of methylisocyanate and methylisothiocyanate cannot be fit by the usual semirigid internal‐rotation treatment. A reasonable fit of the K = 0 lines is obtained only if a semiempirical nonrigid correction factor of the form C_Tm² is used and, in addition, the methyl symmetry axis is tilted ∼3° toward the lone‐pair electrons on the nitrogen atom. Calculations based on a model which includes the effects of the CNC bending motion on the over‐all and internal rotation failed to improve the fit. It is concluded that an extensive vibration‐rotation‐interaction treatment is necessary in order to achieve a quantitative fit of the microwave data. A normal‐coordinate analysis of the three molecules studied was carried out using a valence force field. The present vibrational assignment in methylthiocyanate is substantiated, and the lowest‐frequency mode is found to contain a large contribution from the S–C≡N bend in addition to the CSC bend. The lowest‐frequency modes in methylisocyanate and methylisothiocyanate are likewise not pure CNC bends, and the CNC bending frequencies in both molecules must be lower than presently assigned.