Investigation of the 14N Quadrupole Hyperfine Structure and the Stark Effect of Methyl Isocyanate and Methyl Isothiocyanate by Microwave Fourier Transform Spectroscopy

We investigated the nitrogen-hyperfine structure (hfs) and Stark effect in the rotational spectra of methyl isocyanate, CH 3NCO, and methyl isothio­ cyanate, CH 3NCS, by microwave Fourier transform (MWFT) spectroscopy. The rotational spectrum of methyl isocyanate was first measured and analyzed by Curl et al. [1] and Lett and Flygare [2]. More recently Koput [3] improved the analysis by the model of the quasisymmetric top. Methyl isothio­ cyanate was investigated by Beard and Dailey [4], Siegel [5] and again Lett and Flygare [2]. Also this molecule is an example for a quasi-symmetric top and is presently under investigation by Koput [6 ]. As in both molecules the barriers to internal rotation are small, CH 3NCO; V3 = 83(15) cal/mol [2] and V f[= 59.20(29) cal/mol [6 ] for a linear NCO chain, = 58.92(31) cal/mol [6 ] for a bent NCO chain and CH3NCS: V3 = 304(50) cal/mol [2], we concentrated on the internal rotation ground state m = 0 and on the vibrational ground state to have a minimum influence of the low frequency CNCbending vibration. The values of V3 and Ff*7 differ by the model assumptions. The measurements by MWFT spectroscopy [7-9] are given in Tables 1 a and 1 b. Figures 1 and 2 give two examples of the recordings. The temperatures were around 2 5 °C, the pressures 0.3 mTorr. The