Conformational stability of allyl alcohol from temperature dependent infrared spectra of rare gas solutions, ab initio calculations, r0 structural parameters, and vibrational assignment

Variable temperature (-55 to - 145 degrees C) studies of the infrared spectra (4000-50 cm(-1)) of allyl alcohol (3-hydroxypropene), CH2=CHCH2OH, dissolved in liquid krypton and/or liquid xenon have been carried out. The infrared spectra of the gas and solid have also been recorded. From these data four of the five possible stable conformers have been identified and their relative stabilities determined. The order of the conformers stabilities has been obtained experimentally where the first indicator is for the relative position of the OH group (C = cis or G = gauche) to the double bond (rotation around the C-C bond) and the second one (t = trans, g = gauche, g' = gauche') is the relative position of the hydroxyl rotor, i.e. rotation around the C-O bond. The enthalpy differences have been determined between the most stable Gg conformer and the second most stable rotamer, Cg, to be 135 +/- 14 cm(-1) (1.62 +/- 0.1 kJ/mol), the third most stable conformer Ct, 260 +/- 46 cm(-1) (3.11 +/- 0.6 kJ/mol), and the fourth most stable conformer Gt 337 +/- 75 cm(-1) (4.03 +/- 0.9 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Gg > Cg > Ct > Gt > Gg. No evidence was obtained for the fifth conformer Gg' which is predicted by most of the ab initio calculations to be less stable by an enthalpy difference more than 500 cm-1 than the Gg form. The percentage of each conformer at ambient temperature is estimated to be Gg(54 +/- 2%), Cg(28 +/- 2%), Ct(8 +/- 2%) and Gt(11 +/- 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 ab initio MP2(full)/6-31G(d) 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 Gg and Cg conformers along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained for these two conformers. The determined heavy atom structural parameters for the Gg [Cg] conformer are: the distances(angstrom) C=C = 1.343(3)[1.340(5)], C-C = 1.499(3)[1.504(5)], C-O = 1.428(3)[1.419(5)] and angles(degrees) angle CCC = 122.8(5)[124.7(5)], angle CCO = 112.1(5)[114.9(5)1 and angle CCCO = 122.7(10)[5.63(5)]. Several of the CH distances are significantly different from the values previously reported from the earlier microwave data. The results are discussed and compared to the corresponding properties of some similar molecules. (C) 2009 Elsevier B.V. All rights reserved.