METHYL ISOCYANIDE: THE LOW FREQUENCY BANDS $\nu_{g}$ AND $\nu_{7}$, AND DETERMINATION OF THE ROTATIONAL CONSTANT $A_{0}$

Abstract

The weak perpendicular bands of the low frequency fundamentals $\nu_{g}$ and $\nu_{7}$ of $CH_{3}NC$ were measured in a long path cell with a high resolution Fourier transform spectrometer. Analysis of the $\nu_{g}$ band shows its origin to be $\nu_{o} = 267.3171 cm^{-1}, {\sim}4.57 cm^{-1}$ higher than previous estimates based on an unresolved peak of the bunched-up Q-branches seen in a low resolution scan. The $\nu_{7}$ band, at $\nu_{0} = 1130.6504 cm^{-1}$, is slightly affected by Fermi resonance with the combination $\nu_{4} + \nu_{8}$ whose band is observed just above $\nu_{7}$. On combining the observed $\Delta K = +1$ transitions of $\nu_{7}^{-} + \nu_{g}^{-}$ we reported recently, with the $\Delta K = -1$ transitions of ${\nu_{g}}^{-}$ and of the hot band ${\nu_{7}}^{-} + {\nu_{g}}^{-} - {\nu_{g}}^{-}$ accompanying $\nu_{7}$, we have been able to obtain accurate values for the ground state rotational constant $A_{0} = 5.246985(36) cm^{-1}$ and for ${D_{k}}^{0} = 8.375(12) \times 10^{-5} cm^{-1}$, which had never before been experimentally determined. Accurate spectroscopic constants will be reported for the $\nu_{g}, \nu_{7}$, and $\nu_{4} + \nu_{g}$ states.