The microwave spectrum of methyl isoselenocyanate: CH 3NCSe as a quasi-symmetric top

Abstract

The microwave spectrum of methyl isoselenocyanate, CH 3NCSe, has been measured in the region 18–40 GHz. The a-type J + 1 ← J rotational transitions ( J = 5 to 10) in the ground vibrational state of the molecule have been assigned for the three isotopic species with 82Se, 80Se, and 78Se. For the most abundant isotopic species, with 80Se, the rotational transitions in the excited states of the CNC bending mode v 10 = 1 and 2 have also been assigned. Contrary to the previous investigation [T. Sakaizumi et al., Bull. Chem. Soc. Japan 59, 1614–1616 (1986)], it has been shown that the observed spectrum is consistent with that of a quasi-symmetric top molecule. The rotation-vibration constants have been determined, showing an unusual variation with excitation of the CNC bending mode. The CNC bending potential function has been determined in preliminary calculations using a quasi-symmetric top model. The potential function is calculated to be very anharmonic, with an equilibrium CNC angle of about 162° and a barrier to linearity of the CNCSe skeleton of about 25 cm −1. From accidental resonances observed for the ground state transitions the barrier to internal rotation of the methyl group was estimated to be about 3 cm −1.