EFFECTS OF SOLVATION ON CHEMICAL BONDING - AN ELECTRON-FLOW ANALYSIS

Effects of nonspecific solvation on chemical bonding, described with a simple self-consistent reaction field model, are rigorously analyzed in terms of electron flow and electronegativity equalization between two molecular fragments A and B. In most (but not all) systems AB, the energy-lowering rise in the dipole moment that accompanies solvation is the result of an enhanced charge transfer between A and B, the enhancement stemming from both the increased electronegativity difference Delta(chi AB) and the decreased bond hardness kappa(AB). In systems, such as H . Cl, H . CN, and CH3 . CN, that ensue from interactions between charged closed-shell fragments (H++Cl-, H++CN-, CH3++CN-, etc.) the energy-stabilizing effect of solvation is a trade-off between the energy lowering due to the enhanced charge-transfer component of bonding and destabilization due to diminished covalent bonding. On the other hand, interactions between electrically neutral fragments.(NH3+SO3, etc.) produce systems, such as the zwitterion of sulfamic acid ((H3N)-H-+ . SO3-), in which charge-transfer and covalent components of bonding are strengthened in tandem by solvation. The aforementioned phenomena account for the experimentally observed solvation-induced changes in the A-B bonds, namely their lengthening (or even a complete dissociation) in the former systems and shortening in the latter ones. (C) 1995 American Institute of Physics.