Intra- vs. intermolecular hydrogen bonding: dimers of alpha-hydroxyesters with methanol

Abstract

Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an α-hydroxyester. Disruption of the intramolecular OH⋯OC contact in favour of a cooperative OH⋯OH⋯OC sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl α-hydroxyisobutyrate in seeded supersonic jet expansions. Comparison of the OH stretching modes with quantum-chemical harmonic frequency calculations and ¹⁸O labelling of methanol unambiguously prove the insertion of methanol into the intramolecular hydrogen bond. This is in marked contrast to UV/IR hole burning studies of the homologous system methyl lactate: (±)-2-naphthyl-1-ethanol, where only addition complexes were found and the intramolecular hydrogen bond was conserved. This switch in hydrogen bond pattern from aliphatic to aromatic heterodimers is thought to reflect not only a kinetic propensity but also a thermodynamic preference for addition complexes when dispersion forces become more important in aromatic systems.