r0 Structural parameters, conformational, vibrational studies and ab initio calculations of cyanocyclopentane

•Experimental enthalpy difference has been determined between the two conformers.•Vibrational assignments have been obtained for both conformers.•r0 structural parameters have been determined for both conformers.•Ab initio calculations have been performed for both stable conformers and the twist and planar forms.

The infrared and Raman spectra (3100–50 cm−1) of the gas, liquid or solution, and solid have been recorded of cyanocyclopentane, c-C5H9CN. Variable temperature (−60 to −100 °C) studies of the infrared spectra (3100–400 cm−1) of the sample dissolved in liquid xenon have been carried out. From these data, both the envelope-equatorial (Eq) and Ax conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 55 ± 12 cm−1 (0.66 ± 0.14 kJ/mol) with the Eq conformer the more stable form. The percentage of the Ax conformer is estimated to be 45 ± 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 r0 structural parameter values for the Eq[Ax] form are; for the heavy atom distances (Å): CN = 1.160 [1.160] (3); Cα–C = 1.463 [1.463] (3); Cα–Cβ, Cβ′ = 1.543 [1.545] (3); Cβ–Cγ, Cγ′ = 1.540 [1.541] (3); Cγ–Cγ′ = 1.552 [1.553] (3) and angles (°): ∠Cα–CN = 179.0 [178.9] (5); ∠CβCα–C = 113.1 [110.1] (5); ∠CβCαCβ′ = 103.0 [102.1] (5); ∠CαCβCγ = 104.1 [104.8] (5); ∠CβCγCγ′ = 106.3 [106.0] (5). The results are discussed and compared to the corresponding properties of some related molecules.

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