Dissociative Electron Capture of Hydrogen-Bonded Hydroxy Groups

While examining PFPE (perfluoropolyether)-based disk lubricants, extra lability was suspected for a hydrogen-bonded pair of hydroxy end-groups in the presence of a nucleophile (electron donor). Plausible reaction sequences upon electron capture of hydrogen-bonded methanol dimer, hydrogen-bonded ethanol dimer, and 1,2-propanediol were first examined by a molecular dynamics method based on the potential given by a semiempirical SCF molecular orbital calculation, and subsequently by matrix isolation ESR spectroscopy. In the latter experimental study, alcohol dimer anions were generated by co-trapping Na atoms and alcohol molecules in argon matrices and inducing electron transfer by photo-irradiation. In the cases of methanol and ethanol dimers, the molecular dynamics study predicted that on capture of an electron, the hydroxy hydrogen of the acceptor side would cleave as an atom, and then abstract a hydrogen atom from the nearest carbon of the proton donating alcohol. For 1,2-propanediol anion, the molecular dynamics study predicted C1-C2 bond scission followed by abstraction of the C2 hydroxy hydrogen by the C1 radical fragment. The matrix isolation ESR study revealed generation of radicals upon photo-irradiation when the alcohol concentration was sufficiently high to warrant the presence of dimers. The spectral patterns thus observed were totally consistent with the radical products predicted by the molecular dynamics calculations.