Weak Hydrogen Bonds Make a Difference: Dimers of Jet-Cooled Halogenated Ethanols

Abstract

Abstract Hydrogen-bonded clusters of fluorinated and chlorinated ethanols exhibit rich isomerism in terms of monomer conformation, secondary contacts between the OH and CH groups and the halogen atoms, hydrogen bond topology, chirality recognition and acceptor lone electron pair choice. By expanding the six alcohols involving one to three fluorine or chlorine atoms at the methyl group in a supersonic slit jet expansion and by probing their monomer, dimer and trimer IR spectra between 800 and 4000 cm−1, this isomerism is unravelled in substantial detail. Argon relaxation experiments and complementary cluster Raman spectroscopy provide further information on the individual dimer conformations and on trimer assignments. Energy sequences, helicity- and topology-dependent OH red-shifts, differences between fluorine and chlorine, the influence of dispersion-like interactions and halogen number trends are uncovered and compared to systematic quantum-chemical calculations up to MP2/6–311+G* level. The experimental data provide rigorous reference values for an accurate and balanced quantum-mechanical description of weak hydrogen bond interactions to halogens in the presence of a strong hydrogen bond between oxygen atoms.