Relation between cooperative effects in cyclic water, methanol/water, and methanol trimers and hydrogen bonds in methanol/water, ethanol/water, and dimethylether/water heterodimers

Abstract

Ab initio calculations at the MP2 level have been performed on water, methanol/water, ethanol/water, and dimethylether/water dimers and on water, methanol/water, and methanol cyclic trimers. Several properties of their hydrogen bonds have been investigated, such as interoxygen distances, O–H bond lengths, binding energies, electronic densities at hydrogen bond (HB) critical points and OH stretch vibrational frequencies. Results exhibit HB enhancements for dimers where the acceptor molecule corresponds to water (HDA dimers) as compared to dimers where the donor is water (HDD dimers). In particular, HB reinforcement depends on the number of alkyl groups bonded to the donor oxygen. For trimers, a comparison among their HB properties and those of dimers shows that HB reinforcements (as compared to isolated dimers) occurring in trimers correlate with HB reinforcements observed in (HDA dimers (as compared to (HDDs). In particular, HB properties of the cyclic water trimer are close to those of alcohol/water HDA dimers, and for the methanol cyclic trimer to that of the dimethylether/water HDA dimer. All of these results agree with an orbital interpretation of hydrogen bonding in terms of charge transfer from donor lone pairs to acceptor antibond σOH*, even if all of the HB properties in cyclic trimers may not be explained from this approach.