Hydrogen-bonded dimers with methanol as a proton donor: Ab initio MO studies

Abstract

The hydrogen-bonded open chain dimers CH 3OHCH 3CN(I), CH 3OHCH 3NC(II), CH 3OHH 2O(III), CH 3OHCH 3OH(IV) and CH 3OHCH 3F(V) are studied by the ab initio SCF-MO method. The geometries of both isolated molecules and dimers are optimized (STO-2G level). It is found that all the above hydrogen bonds have a nearly colinear moiety. The hydrogen bond stabilization energies Δ E are calculated by means of a double-zeta 4–31G basis set. The hydrogen bond energy components at equilibrium geometries of these complexes are calculated according to Morokuma's energy decomposing scheme. By inspection of the calculated results, one can see that all five complexes have electrostatic interaction (ES) greater than charge-transfer interaction (CT). Besides, ES(%) of complexes (I), (III) and (IV) are greater than that of (II) and (V), while CT(%) of (II) and (V) is greater than that of (I), (III) and (IV). The trend of CT(%) is (II) &>; (V) &>; (IV) &>; (I) &>; (III), which is the same as the increasing trend of the bond order of H⋯X and the decreasing trend of the bond order of proton donating OH bonds. This tendency indicates that charge transfer occurs mainly within the region OH⋯X.