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 <b>ap</b> and synclinal in the <b>sc</b> rotamer. The <b>sc</b> conformer tends to be slightly more stable than the <b>ap</b> form. The internal energy difference was determined to be <i>E</i>_ap – <i>E</i>_sc = 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 <b>sc</b>. The frequencies of these two modes were determined by relative intensity measurements. The dipole moment of this conformer was determined to be μ_a = 12.16(6), μ_b = 5.91(4), μ_c = 0 (preset), and μ_tot = 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 <b>ap</b>. 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