Microwave, Raman, and infrared spectra, r0 structural parameters, conformational stability, and vibrational assignment of cyanocyclohexane

The FT-microwave spectrum of cyanocyclohexane, c-C6H11CN has been investigated from 11,000 to 21,000 MHz and 99 transitions for the chair-equatorial conformer and 76 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances (Å) C1–C8,9 = 1.536(3)[1.536(3)], C8,9–C14,15 = 1.532(3)[1.532(3)], C4–C14,15 = 1.544(3)[1.544(3)], C6N7 = 1.162(3)[1.161(3)] and angles in degrees: ∠C1C8,9C14,15 = 111.6(5)[110.8(5)], ∠C6C4C14,15 = 110.0(5)[110.8(5)] and τC1C8,9C14,15C4 = 55.6(10)[56.9(10)]. Variable temperature (−55 to −100 °C) studies of a xenon solution were carried out and by using two conformer pairs from which an enthalpy difference of 63 ± 9 cm−1 (0.75 ± 0.11 kJ/mol) was obtained with the more stable form the chair-axial conformer. At ambient temperature, the abundance of the chair-axial conformer is 58 ± 8%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 819 (axial)/842 (equatorial) cm−1, a standard enthalpy difference of 46 ± 16 cm−1 (0.54 ± 0.19 kJ/mol) was obtained again with the chair-axial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 120 cm−1 (1.44 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis sets gives the chair-equatorial conformer more stable by 180 cm−1 (2.16 kJ/mol). A complete vibrational assignment is given for the chair-axial conformer and most of the fundamentals for the chair-equatorial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.