Microwave, structural, conformational, vibrational studies and ab initio calculations of isocyanocyclopentane

The infrared and Raman spectra (3200-50 cm(-1)) of the gas, liquid or solution, and solid have been recorded of isocyanocyclopentane, c-C5H9NC. FT-microwave studies have also been carried out and 23 transitions were recorded for the envelope-axial (Ax) conformer. Variable temperature (-65 to -100 degrees C) studies of the infrared spectra (3200-400 cm(-1)) dissolved in liquid xenon have been carried out. From these data, both the Ax and envelope-equatorial (Eq) conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 102 +/- 10 cm(-1) (1.21 +/- 0.11 kJ mol(-1)) with the Ax conformer the more stable form. The percentage of the Eq conformer is estimated to be 38 +/- 1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for both conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The structural parameter values for the Ax form are; for the heavy atom distances (angstrom): C N = 1.176 (3); C alpha-N = 1.432 (3); C-alpha-C-beta,C-beta', = 1.534 (3); C-beta-C-gamma,C-gamma = 1.542 (3); C gamma-C gamma(') = 1.554 (3) and angles (degrees):angle CO alpha-N C = 177.8 (5); angle C beta C alpha-N = 110.4 (5); angle C beta C alpha C beta' =102.9 (5); angle C alpha C beta C gamma = 103.6 (5); angle C beta C gamma C gamma' = 105.9 (5). The results are discussed and compared to the corresponding properties of some related molecules. (C) 2014 Elsevier B.V. All rights reserved.