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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