Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for cyanocyclopentane

The infrared spectra (3200-400 cm−1) of the gas, xenon solutions, and solid and the Raman spectrum (3200-100 cm−1) of liquid cyanocyclopentane, c-C5H9CN, have been recorded. From infrared spectra of xenon solutions at 10 different temperatures, the enthalpy difference between the more stable axial conformer and the equatorial form has been determined to be 109 ± 37 cm−1 (312 ± 106 cal/mol). The percentage of the equatorial conformer present at ambient temperatures is estimated to be 41 ± 7%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 182 ± 23 cm−1 (520 ± 66 cal/mol) whereas the energy difference obtained from the corresponding density functional theory calculations by the B3LYP method predict the equatorial conformer to be more stable by 164 ± 11 cm−1 (469 ± 31 cal/mol). By utilizing previously reported microwave determined rotational constants for both conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances C1-C2 = 1.542(5)[1.543(5)], C1-C3 = 1.542(5)[1.543(5)], C2-C4 = 1.542(5)[1.544(5)], C3-C5 = 1.542(5)[1.544(5)], C4-C5 = 1.559(5)[1.564(5)], C1-C6N8 = 1.467(5)[1.462(5)], C6N8 = 1.156(5)[1.159(5)] (Å) and angles in degrees ∠C3C1C2 = 102.0(5)[102.3(5)], ∠C1C2C4 = 103.6(5)[103.2(5)], ∠C1C3C5 = 103.6(5)[103.2(5)], ∠C2C4C5 = 105.7(5)[105.8(5)], ∠C3C5C4 = 105.7(5)[105.8(5)], ∠C6C1C2 = 111.5(5)[113.1(5)], ∠C6C1C3 = 111.5(5)[113.1(5)] (°) and τC1C2C4C5 = 25.8(5)[−26.1(5)], τC1C2C4C5 = 0.0(5)[0.0(5)] (°). Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.