Hydrogen bonding in the gas phase. Infrared spectroscopic investigation of hydrogen fluoride alcohol–heterodimers

Abstract

For the first time, an infrared investigation of gas phase hydrogen bonded complexes formed by hdyrogen fluoride with alcohols has been made. Evidence has been given for the formation of 1 : 1 heterodimers ROH ⋯ HF and the vibration wavenumbers [graphic omitted](_F–H) have been estimated. The wavenumber displacements of the HF stretching vibration on H bond formation provide a measure of the electron donating abilities in the free alcohols. The electron donating power of the oxygen atom is increased by increasing methylation at the oxygen atom, as indicated by the constant increase of 75 cm^–1 min Δ[graphic omitted]_HF along the series H₂O ⋯ HF, CH₃OH ⋯ HF, (CH₃)₂O ⋯ HF. A similar but attenuated increase occurs on increasing methylation of the carbon atom in methanol as indicated by steps of about 25 cm^–1 in Δ[graphic omitted]_HF for complexes formed by CH₃OH, CH₃CH₂OH, (CH₃)₂CHOH and (CH₃)₃COH. On the other hand, the electron donating ability of the oxygen atom is decreased by the presence of the electron attracting groups CH₂Cl, CH₂F and CF₃. We confirm that where the vapour pressure of a component is a limitation on observing dimer formation in the gas phase, as is the case in most alcohols, it is advantageous to work at higher temperatures.