Microwave spectrum of the quasilinear molecule, cyanogen isothiocyanate (NCNCS)

Abstract

The R-branch rotational spectrum of cyanogen isothiocyanate, NCNCS, produced by thermal isomerization of sulfur dicyanide, S(CN) 2, has been measured between 26.5 and 40.0 GHz. The molecule shows archetypal quasilinear dynamic behavior in that the rotational satellite spectroscopic patterns for the low bending vibrational states, which lie below the barrier to linearity, are characteristic of a bent asymmetric top species and as vibrational excitation increases and states lie closer to or above the barrier the patterns gradually correlate with linear characteristic patterns. The spectrum, which consists of four transitions ( J = 8 to 11), has been analyzed by treating the molecule as linear with one large amplitude, bending mode, ν 7; the observed rotational constants were expanded in terms of the expectation values of even powers of the large-amplitude coordinate and various empirical constants determined. The inclusion of a small term in the vibrational Hamiltonian which accounts for the variation of reduced mass with bending coordinate has been found to be important. Relative intensity measurements indicate a barrier to linearity of 308 ± 34 cm −1 and a vibrational spacing (in the bent limit) of 97 ± 11 cm −1. The dipole moment, as determined from the ν 7 = 3, l = 3 state, is 3.163(8) Debye.