Knowledge Bank

The $J0\to 1$ and $1\to 2$ a-type transitions of methyl isocyanate $(CH3NCO)$ have been observed. The spectrum is moderately complicated by internal rotation. Four lines in the 0-1 region and nineteen lines in the 1-2 region are easily observable at dry-ice temperature. The spectrum of the molecule have been calculated for range of CNC bond angles from $120\circ$ to $140\circ$ and a range of barriers from 0 to 1200 cal/mole. The best qualitative agreement with the observed spectrum corresponds to a CNC bond angle of approximately $140\circ$ and a surprisingly low barrier to internal rotation for the methyl group of about 20 cal./mole. Efforts to improve this qualitative agreement have to far been unsuccessful.