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

Abstract

a b s t r a c tVariable temperature (À55 to À145°C) studies of the infrared spectra (4000-50 cm À1 ) of allyl alcohol (3-hydroxypropene), CH 2 @CHCH 2 OH, 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 CAC bond) and the second one (t = trans, g = gauche, g 0 = gauche 0 ) is the relative position of the hydroxyl rotor, i.e. rotation around the CAO 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 À1 (1.62 ± 0.1 kJ/mol), the third most stable conformer Ct, 260 ± 46 cm À1 (3.11 ± 0.6 kJ/mol), and the fourth most stable conformer Gt 337 ± 75 cm À1 (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 0 . No evidence was obtained for the fifth conformer Gg 0 which is predicted by most of the ab initio calculations to be less stable by an enthalpy difference more than 500 cm À1 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(Å) C@C = 1.343(3)[1.340(5)], CAC = 1.499(3)[1.504(5)], CAO = 1.428(3)[1.419(5)] and angles( o ) \CCC = 122.8(5)[124.7(5)], \ CCO = 112.1(5)[114.9(5)] and \ 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.

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

“…Recently, we have been investigating the conformation stability and determined the structural parameters of several organoamines and alcohols . One of the first amines studies was ethylamine, which was determined to have the trans conformer more stable by 54 ± 4 cm –1 (0.65 ± 0.05 kJ/mol) than the gauche form.…”

1H NMR study of the hetero‐association of unsaturated alcohols with pyridine

“…Recent high‐level ab initio calculations establish that the most stable of the five possible conformers of allyl alcohol (2‐propen‐1‐ol), 1a , are the + ac,−sc and sp , +sc where the OH hydrogen is in the vicinity of the double bond. These are followed by the sp , ap , + ac , ap and + ac , +sc conformers, in that order . Again, for 2‐methylpropen‐1‐ol, according to ab initio calculations the + ac , −sc and sp , +sc conformers are the most stable .…”

“…A recent review drew attention to the importance of unconventional, weak hydrogen bonds, CH/ n , CH/π and XH/π (X = O, S, Se), in determining the conformations of organic compounds and bioconjugates . Early infrared (IR) spectroscopic studies of small unsaturated alcohols are confirmed by work which in many cases combines IR and/or microwave (MW) spectroscopy, and gas electron diffraction (GED) measurements and photoelectron spectroscopy, with ab initio calculations, to identify the preferred conformers. These studies of the conformational equilibria show them to be dominated by OH/π interactions, and the proximity of putative donors and acceptors is frequently taken as demonstrating that intramolecular hydrogen bonding (IHB) occurs.…”

1H NMR study of the hetero‐association of non‐symmetrical diols with pyridine; GIAO/DFT calculations on diols

“…The fact that the energies of the three lowest conformers above the g'GG't minimum differ by up to 1 kJ mol −1 means that the most important conformer populations, based on the relative Gibbs energies at 298 K, are significantly different from the published values. Variations in the MP2 values for allyl alcohol, for example, with different basis sets suggest that this is not necessarily due to the use of the more economical method 68. The leveling effect of solvation is much less marked than in the case of 1‐Me , except for some relatively high‐energy conformers, notably V, VII, VIII, and X.…”