The Conformation of 1-Alkyl-2-phenylpropan-1-ols Studied by Ab Initio MO Calculations. Relevance of the CH/(pi) and OH/(pi) Hydrogen Bonds

Ab initio MO calculations were carried out, at the MP2/6-311G(d,p)//MP2/6-31G(d) level, for the conformation of a series of diastereoisomers of 1-alkyl-2phenylpropan-1-ols CH3CH(C6H5)CH(R)OH and 2-phenylalkanes CH3CH(C6H5)CH2R (R = CH3, C2H5, i-C3H7, t-C4H9). A rotamer where R is gauche to C6H5 and anti to the benzylic methyl group has been found to be the most populated. The distances between the CH and OH hydrogens and the phenyl group in the interacted rotamers have been found to be short. The distribution of the possible rotamers depends on the presence or absence of the OH group and the configuration of the alcohol diastereoisomers. The results are discussed in the context of weak attractive molecular forces; the CH/(pi), OH/(pi), and CH/O hydrogen bonds, as well as unfavorable steric effects occurring between R and Me and between vicinal CH groups.