Fully polarizable QM/MM calculations: An application to the nonbonded iodine-oxygen interaction in dimethyl-2-iodobenzoylphosphonate

The compound dimethyl-2-iodobenzoylyhosphonate is unusual in that it forms well-ordered crystals that clearly show short iodine-oxygen interactions in which both the iodine and the oxygen are in their normal oxidation states. These interactions were studied using a new hybrid quantum mechanical-mole cular mechanical approach that employs a polarizable molecular mechanics co mponent. The electric field at the molecular mechanics atoms was calculated from a distributed multipole expansion of the wave function; this induced dipoles on the molecular mechanics atoms. The electrostatic potential in a spherical shell around the induced dipoles was reproduced through induced c harges on the atomic center and those bonded to it using an analytical (rat her than numerical) procedure. The new atomic charges (induced charges plus permanent charges) were then able to interact with the quantum mechanical entity and polarize the wave function. The procedure was iterated to conver gence. The calculations show that the iodine atom becomes more positive in the crystal environment (modeled by a chain of three molecules of dimethyl- 2-iodobenzoylphosphonate). Thus, while the cooperative effects of the cryst al environment may not be the only feature stabilizing this unusual interac tion, they do play a significant role in reducing the otherwise unfavourabl e iodine-oxygen monopole-monopole interaction. (C) 2000 John Wiley & Sons, Inc.