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

Abstract

The infrared and Raman spectra (3200–50cm−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°C) studies of the infrared spectra (3200–400cm−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±10cm−1 (1.21±0.11kJmol−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 (Å): CN=1.176 (3); CαN=1.432 (3); CαCβ,Cβ′=1.534 (3); CβCγ,Cγ′=1.542 (3); CγCγ′=1.554 (3) and angles (°):∠CαNC=177.8 (5); ∠CβCαN=110.4 (5); ∠CβCαCβ′=102.9 (5); ∠CαCβCγ=103.6 (5); ∠CβCγCγ′=105.9 (5). The results are discussed and compared to the corresponding properties of some related molecules.