Nuclear magnetic resonance and molecular orbital study of internal hydrogen bonding in salicyl alcohol. Principal site analysis based on the sidechain methylene and hydroxy proton long-range coupling constants

Abstract

The ¹H n.m.r. spectra of 2-hydroxybenzyl, 2-methoxybenzyl, 2-hydroxy-3-methoxy-, and 2,6-dimethoxy-benzyl alcohols are studied in various solvents. The long-range proton–proton spin-spin coupling constants, certain chemical shifts, and some STO-3G MO calculations are used to characterize the conformational behaviour of salicyl alcohol. The observed variation of the coupling constants is reproduced by a three-site model: salicyl alcohol and its 2-methoxy derivative seem to have three principal conformations: clsd1(= closed 1) with a hydrogen bond between the phenolic hydrogen and CH₂OH, open with a rather freely rotating CH₂OH sidechain, and clsd2 with a hydrogen bond between the CH₂OH hydroxy group and the phenolic oxygen atom. The free energy of the internal hydrogen bond in clsd2 is smaller than in clsd1. The presence of the clsd2 form is inferred by indirect methods, not from direct indicators like hydroxy stretching frequencies or proton chemical shifts. The hydrogen-bonded forms predominate in non-polar solvents. The hydrogen bonds are opened by nucleophilic solvents. For D₂O solutions a deuterium-bridged structure is suggested.