Microwave spectrum, conformational properties, and dipole moment of cyclopropylmethyl isocyanide (C3H5CH2NC)

The microwave spectrum of cyclopropylmethyl isocyanide, C3H5CH2NC, has been investigated in the 25-75 GHz spectral range. The spectra of two conformers were assigned. The H-C-C-N chain of atoms is antiperiplanar in the conformer denoted ap and synclinal in the sc rotamer. The sc conformer tends to be slightly more stable than the ap form. The internal energy difference was determined to be Eap - Esc = 0.2(7) kJ/mol from relative intensity measurements. The spectra of the ground vibrational state and six vibrationally excited states belonging to two different normal vibrations were assigned for sc. The frequencies of these two modes were determined by relative intensity measurements. The dipole moment of this conformer was determined to be μa = 12.16(6), μb = 5.91(4), μc = 0 (preset), and μtot = 13.52(6) × 10(-30) C m [4.05 (2) debye]. The spectra of the ground and of two vibrationally excited states belonging to the torsion and lowest bending vibration were assigned for ap. The microwave work was supported by quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. Most, but not all, of the theoretical predictions are in good agreement with experiment.