The infrared spectrum of CHD2NC and the ground state structure of methyl isocyanide

Analysis of the rovibration spectrum of CHD₂NC has enabled the rotation parameter (A₀B₀) to be determined. Knowledge of the microwave B₀ and C₀ constants enables A₀(CHD₂NC) to be determined, which, when used in conjunction with all other B₀ and C₀ constants for isotopic methyl isocyanides, allows the ground state structure to be defined within fairly narrow limits. The CH bond length is much shorter than that determined by the substitution technique (r_s structure). It is now in almost perfect agreement with the value predicted by the CH vibration frequency in CHD₂NC, and falls in the range of CH bond lengths exhibited by all other symmetric top methyl group molecules.