Geometric isotope effect of various intermolecular and intramolecular C-H center dot center dot center dot O hydrogen bonds, using the multicomponent molecular orbital method

摘要

The geometric isotope effect (GIE) of sp-(acetylene-water), sp2-(ethylene-water), and sp3-(methane-water) hybridized intermolecular C-(HO)-O-... and C-(DO)-O-... hydrogen bonds has been analyzed at the HF/631++ G** level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the (HO)-O-... distance is shorter than (DO)-O-..., similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent ( R) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-(HO)-O-... hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-(HO)-O-... hydrogen bond shows the same tendency as that of intermolecular C-(HO)-O-.... Our study clearly demonstrates that C-(HO)-O-... hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.