Knowledge Bank

A spectrum of the $\nu 4$ C-N stretching band of methyl isocyanide, $CH3NC$, has recently been obtained in high resolution and analyzed by He and $Bernheim.1$ Perturbations observed in the spectra were attributed to mixing with overtones of the $\nu 8$ CNC bending vibration. Additional information concerning the vibrational potential energy surface may be obtained from the spectra of isotopically substituted species. Consequently, isotopically pure methyl-$d3$-isocyanide, $CD3NC$, is synthesized and the high resolution, infrared diode laser spectrum of its $\nu 4$ band recorded. Analysis of the band is complicated by overlap with the $\nu 7$ band in this species. Additionally, a lack of ground state centrifugal distortion constants for the perdeuterated molecule makes it impossible to use combination differences in assigning the spectrum. Nevertheless, over 80 lines in the spectrum have been assigned and analyzed using the symmetric top Hamiltonian. The resulting spectroscopic constants will be presented.