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 ΔχAB and the decreased bond hardness κAB. In systems, such as H⋅Cl, H⋅CN, and CH3⋅CN, that ensue from interactions between charged closed-shell fragments (H++Cl−, H++CN−, CH+3+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⋅SO−3), 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.