Intramolecular Hydrogen Bonding and Conformations of α,α-Dialkyl- and α-Alkyl-o-methoxybenzyl Alcohols. II. Enthalpies of Hydrogen Bond Formation and Molecular Force Field Calculations on α-Alkyl-substituted Benzyl Alcohols

Abstract

Abstract Enthalpies of intramolecular hydrogen bond formation (ΔH) were determined for a series of α-mono- and α,α-dialkyl-o-methoxybenzyl alcohols and related compounds. The −ΔH values of the unsubstituted and α,α-dialkyl-o-methoxybenzyl alcohols are more than 2.5 kJ mol−1, while those for the corresponding α-monoalkyl derivatives are at most 1 kJ mol−1. This tendency is in accordance with the results on their ΔvOH and εb/εf values, showing the hydrogen bonded conformations of the α-monoalkyl derivatives are less favorable than those of the other alcohols in discussion. Molecular force field calculations on α-alkyl- and α,α-dialkylbenzyl alcohols were also carried out. The results shows that the hydroxyl/aryl coplanar conformation (ω=0°) is the most stable for the unsubstituted and α,α-dialkylbenzyl alcohols and the hydroxyl/aryl non-coplanar conformation (ω=30°) for α-alkylbenzyl alcohols. Since the OH···O distance might be farther and, hence, the overlap between the unshared electron pairs on the methoxyl oxygen and the anti-bonding OH orbital might be less in the non-coplanar conformation, α-monoalkyl-o-methoxybenzyl alcohols are less favorable than the corresponding α,α-dialkyl derivatives in the intramolecular hydrogen bond formation.