STRUCTURAL, CONFORMATIONAL AND VIBRATIONAL STUDIES OF ISOCYANOCYCLOPENTANE FROM INFRARED, RAMAN SPECTRA AND AB INITIO CALCULATIONS

Author Institution: Department of Chemistry, University of Missouri-Kansas City, Mo 64110 UsaThe infrared and Raman spectra (3200 to 50 cm$^{-1}$) of the gas, liquid or solution, and solid have been recorded of isocyanocyclopentane, $_{c}$-C$_{5}$H$_{9}$NC. FT-microwave studies have also been carried out and 23 transitions were recorded for the envelope-axial (Ax) conformer. Variable temperature (-55 to -100$^irc$C) 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 ($\AA$): C$\equiv$N = 1.176 ; C$_{\apha}$-N$\equiv$C= 1.432; C$_{\apha}$-C$_{\beta}$,C$_{\beta}$$^{\prime}$ = 1.534; C$_{\beta}$-C$_{\gamma}$, C$_{\gamma}$$^{\prime}$ = 1.542; C$_{\gamma}$-C$_{\gamma}$$^{\prime}$ = 1.554 and angles ($^irc$):$\angle$C$_{\apha}$-N\equivC = 177.8; $\angle$C$_{\beta}$C$_{\apha}$-N\equivC = 110.4; $\angle$C$_{\beta}$C$_{\apha}$C$_{\beta}$$^{\prime}$= 102.9; $\angle$C$_{\apha}$C$_{\beta}$C$_{\gamma}$ = 103.6; $\angle$C$_{\beta}$C$_{\gamma}$C$_{\gamma}$$^{\prime}$ = 105.9. The results are discussed and compared to the corresponding properties of some related molecules