DATTATRAY K. SAWANT, JOSHUA J. KLAASSEN, JAMES R. DURIG, DEPARTMENT OF CHEMISTRY, UNIVERSITY OF MISSOURI-KANSAS CITY, MO 64110 USA.
\noindent The infrared and Raman spectra (3200 to 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 (-55 to -100oC) studies of the infrared spectra (3200 to 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 \pm 10 cm-1 (1.21 \pm 0.03 kJ mol-1) with the Ax conformer the more stable form. The percentage of the Eq conformer is estimated to be 38 \pm 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 heavy atom distances (Å): CN = 1.176 ; C\apha-NC= 1.432; C\apha-C ,C ' = 1.534; C -C , C ' = 1.542; C -C ' = 1.554 and angles (o):\angleC\apha-NC = 177.8; \angleC C\apha-NC = 110.4; \angleC C\aphaC '= 102.9; \angleC\aphaC C = 103.6; \angleC C C ' = 105.9. The results are discussed and compared to the corresponding properties of some related molecules.