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

The FT-microwave spectrum of ethynylcyclohexane, c-C6H11CCH has been investigated from 11,000 to 21,000 MHz and 32 transitions for the chair-equatorial conformer and 22 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 equatorial[axial] conformer are: the distances (Å) C1C9,10 = 1.532(3)[1.537(3)], C9,10C15,16 = 1.544(3)[1.534(3)], C4C15,16 = 1.541(3)[1.545(3)], C6C7 = 1.210(3)[1.212(3)] and angles in degrees: ∠C1C9,10C15,16 = 111.0(5)[111.2(5)], ∠C6C4C15,16 = 110.7(5)[110.3(5)] and τC1C9,10C15,16C4 = 56.4(10)[55.3(10)]. Variable temperature (−60 to −100 °C) studies of a xenon solution were carried out and by using 10 conformer pairs an enthalpy difference of 124 ± 13 cm−1 (1.48 ± 0.16 kJ/mol) was obtained with the more stable form the chair-equatorial conformer. At ambient temperature, the abundance of the chair-equatorial conformer is 55 ± 3%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 822 (axial)/842 (equatorial) cm−1, a standard enthalpy difference of 172 ± 32 cm−1 (2.06 ± 0.38 kJ/mol) was obtained again with the chair-equatorial 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 135 cm−1 (1.62 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis set gives the chair-equatorial conformer more stable by 289 cm−1 (3.46 kJ/mol). A complete vibrational assignment is given for the chair-equatorial conformer and most of the fundamentals for the chair-axial 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.