Vibrational spectra and bonding in nitrocyclopropane

Raman spectra of nitrocyclopropane were recorded from 4000 to 30 cm −1 for the liquid and solid states. Infrared spectra were recorded from 4000 to 180 cm −1 for vapor phase and matrix isolated states. The low frequency torsional mode is located at 101 cm −1 in the liquid state (102 cm −1 in the solid state) and correlates well with the 84 cm −1 value for the vapor phase which is predicted on the basis of a torsional combination band in the infrared vapor phase spectrum. The calculated barrier to internal rotation for the vapor phase is 1645±50 cm −1 , 4.70±0.15 kcal mol −1 . A vibrational assignment is proposed for the first time and it supports structural arguments that there is significant pseudo‐conjugation between the cyclopropane ring orbitals and the substituent NO 2 group. Finally, a detailed temperature study reveals no appreciable spectral change; hence, nitrocyclopropane appears to exist as a single C s conformer over the range −173 to 100°C.