H-bonded open and cyclic dimers in the gas phase

Abstract

Two types of basic models of H-bonds are found in the gas phase: open dimers represented by ClH⋯O(CH 3) 2, and cyclic dimers of carboxylic acids. Open dimers are simpler and have given a collection of basic data of high interest, in particular when one is aiming at elaborating precise descriptions of the electronic structure of H-bonds. Cyclic dimers, although somewhat more complex, are shown to have greater potentialities, especially because they contain two interacting H-bonds. It is shown how the shape of the ν s (OH⋯O) IR band, which is most characteristic of H-bonds, much resembles that of open dimers, once Fermi resonances have been eliminated using a newly proposed peeling-off procedure. In contrast, intensities of these bands in carboxylic acid dimers display anomalies which are now thought to originate from the peculiar ring structure of these acids. It is suggested that these ring structures may be of importance for proton transfers occurring in biological reactions.