Study of the nature of improper blue-shifting hydrogen bonding and standard hydrogen bonding in the X3CH...OH2 and XH...OH2 complexes (X = F, Cl, Br, I): A correlated Ab initio study.

Weak hydrogen bonding was studied in the XH...OH2 and X3CH...OH2 complexes (X = F, Cl, Br, I) using the correlated MP2 ab initio method with relativistic Stuttgart/Dresden pseudopotentials and basis set (SDD). The accuracy of the method was tested for selected nonrelativistic complexes by performing MP2 calculations with all-electron basis sets (6-311G** and TZVPP). The characteristics of bonding in the hydrogen halide complexes correspond to the standard H-bonding (an elongation of the X-H bond and red shift of its stretch frequency), whereas those in the X3CH...OH2 complexes (X = F, Cl) are typical of improper blue-shifting H-bonding (a contraction of the CH bond and blue shift of the respective stretch frequency). A natural bond orbital analysis revealed some important differences between both classes of complexes: a) the electron density transfer (EDT) in the former complexes is considerably larger than that in the latter complexes: b) the EDT in the former complexes is almost completely directed to the sigma*-antibonding orbital of the XH bond, which causes a weakening of this bond, its elongation, and a concomitant decrease of the XH stretch frequency. In the latter complexes, only a small portion of the EDT goes to the sigma*-antibonding orbital of the CH bond of the proton donor and a larger part is transferred to the remote (nonparticipating) part of the proton donor. As a consequence, the structural reorganization of the proton donor occurred, leading to the contraction of the C-H bond. The fact that a small red shift of the C-H stretch frequency was found in bromoform-water and iodoform-water complexes was explained by the competition of both the above-mentioned mechanisms with dominating passage of electron density to the sigma*-antibonding orbital of the C-H bond. For an explanation of all the geometric features of both types of complexes, it is however necessary to consider both charge transfer and electrostatic effects. The electrostatic effects fail sometimes to interpret the geometry changes in the proton donor.