Rotational Isomers, Intramolecular Hydrogen Bond, and IR Spectra of o-Vinylphenol

Abstract

Absorption bands of OH stretching vibrations in IR spectra of o-vinylphenol (o-VP) in the weakly polar solvents CCl4 and n-hexane were studied. Several rotamers of the free OH group were observed for o-VP in n-hexane. The fraction of o-VP rotamers with an O-H…�a intramolecular hydrogen bond (IHB) was less than 20% according to experimental estimates for CCl4 solutions and calculations in the gas phase and cyclohexane. The theoretical effective enthalpy of the o-VP IHB was estimated for rotamer A (-�˚ H = 0.20 kcal/mol). basicity of the �a -bond and the geometric cono guration of the functional groups involved in forming the H-bond. The in? uence of substituents on the IHB strength between the OH group and the double-bond �a -electrons was studied by IR spectroscopy using nine o-VPs with variously substituted vinyl groups as examples (4). It was found that two bands were observed as a rule in the high-frequency region of IR spectra of the o-VPs. These were �� (OH)f in the range 3609-3612 cm -1 and �� (OH)b at 3524-3557 cm -1 . The quantity �˚�� (OH) varied in the range 55-85 cm -1 . The intensity ratio of these bands depended strongly on the position and number of alkyl groups in the vinyl substituent. The compound o-VP itself was studied experimentally (4, 5). The �� (OH)f band in IR spectra of its CCl4 solutions was observed at 3611 cm -1 ; �� (OH)b, 3557 with �˚�� (OH) = 54 cm -1 . IR spectra of the o-phenols were examined only in the range 3700-3500 cm -1 . Any absorption bands in the range 3500-3200 cm -1 were not reported (3-5). The enthalpy of the IHB in o-VP was not determined previously although an estimate of this parameter for its homolog o-isopropenylphenol was -�˚ H = 0.76 ± 0.11 kcal/mol (6). A conformational analysis of structurally ? uxional o-VP was recently completed (7). An analysis of the surface potential energy (SPE) of o-VP that was constructed using the B3LYP/6-311G(d) method showed that o-VP existed in the gas phase and in CCl4 solution as a mixture of rotamers A, B, C, and D that were produced by internal rotation of the OH and vinyl groups and three optical isomers A1, B1, and C1 of these rotamers: