Conformational stability of cyclobutanol from temperature dependent infrared spectra of xenon solutions, r0 structural parameters, ab initio calculations and vibrational assignment

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000–400 cm−1) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000–100 cm−1) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq = equatorial or Ax = axial) and the second one (t = trans, g = gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring CO bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200 ± 50 cm−1 (2.39 ± 0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-t > Eq-g > Ax-g > Ax-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650 cm−1 than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t conformer 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 Eq-t conformer are: the distances C1C4 = 1.547(5) Å, C4C6 = 1.552(5) Å, CO = 1.416(5) Å and angles ∠C6C4C1 = 86.6(5)°, ∠C4C1C5 = 88.9(5)° and ∠C6C5C1C4 = 22.8(5)°. The results are discussed and compared to the corresponding properties of some similar molecules.