Intra- vs. intermolecular hydrogen bonding: dimers of alpha-hydroxyesters with methanol.
Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an alpha-hydroxyester. Disruption of the intramolecular OH...O=C contact in favour of a cooperative OH...OH...O=C sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl alpha-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.
Borho
Nicole
N
Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, 37077, Göttingen, Germany.
Suhm
Martin A
MA
Le Barbu-Debus
Katia
K
Zehnacker
Anne
A
D003198:Computer Simulation; D019281:Dimerization; D006016:Glycolates / Q000737:chemistry*; D006860:Hydrogen Bonding; D006885:Hydroxybutyrates / Q000737:chemistry*; D007553:Isotope Labeling / Q000379:methods; D007773:Lactates / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D010103:Oxygen Isotopes; D011789:Quantum Theory; D012680:Sensitivity and Specificity; D017550:Spectroscopy, Fourier Transform Infrared / Q000295:instrumentation / Q000379:methods
D006016:Glycolates; D006885:Hydroxybutyrates; D007773:Lactates; D010103:Oxygen Isotopes; C516669:methyl alpha-hydroxyisobutyrate; C028444:methyl lactate; D000432:Methanol