46999-AC6
Induction in Chemistry: Introducing Electrostatic Bonds
I believe that polarization is far more important in chemistry then is generally realized. In short-ranged intermolecular and interatomic interactions, the polarization energy can be 10x the Coulomb energy. Thus, polarizable classical mechanics can describe a regime of bonding, which we named "electrostatic", lying intermediate between the textbook options of ionic and covalent.
The aim of this project is to apply our ideas about polarization to two problems of importance to the petroleum industry: 1. water, oil-water interfaces, water in pores, etc, where we regard hydrogen bonds as electrostatic bonds, and 2. the binding of impurities in crude oil to metal atoms in biochemical scavengers, which is also considered as electrostatic bonding. Graduate student Raeanne Napoleon was supported in the 2008 spring and summer semesters, and made a beginning in both areas.
1. Water
The foundation of our recently published ``POLIR'' water potential (P. Mankoo and T. Keyes, "POLIR: Polarizable, Flexible, Transferable Water Potential Optimized for IR Spectroscopy", J. Chem. Phys. 129, 034504 (2008)) is a careful treatment of short-ranged polarization. Since atoms are charge distributions, not points, a modified, damped electrostatic potential must be employed. POLIR is the first potential to yield good classical vibrational frequencies and absolute IR spectra in clusters, liquid, and ice. Optimizing the potential for IR spectroscopy was shown to be broadly beneficial: the spectrum is determined by the response of a molecule to an external field, and intermolecular interactions such as hydrogen bonds are determined by the response to fields from neighbor molecules.
Nonetheless, POLIR is parameterized in a somewhat haphazard fashion, and, collaborating closely with Dr. Christian Burnham, Raeanne set out to take the next steps. She has performed extensive quantal calculations of the dipole-hexadecapole moments of water in an electric field, for a sampling of physically relevant configurations.This involves writing a code that takes atomic multipoles from Gaussian and calculates molecular multipoles. The field-dependence builds in dipole and quadrupole polarizabilities. Fitting the results to a functional form gives an ab initio, highly accurate, electrostatic model.
We have not yet converted the electrostatic model to a full potential by adding intermolecular van der Waals terms. However, we have calculated IR spectra in liquid water and ice by taking configurations from MD with a standard potential, obtaining the electric fields consistently with the electrostatic model, and solving the Schrodinger equation for individual molecules in their local electric fields. Excellent agreement with experiment is found, in particular for the overtone region of ice which is significant in the search for planetary life, holding out the prospect of a comprehensive theory of condensed phase vibrational spectroscopy. More significantly for the project, the electrostatic model forms the foundation for our next polarizable potential. Version 2 of POLIR will be parameterized to reproduce Raeanne's calculated multipoles and multipole polarizabilities.
2. Ligand binding
Previously, we showed (P. Mankoo and T. Keyes, Classical Molecular Electrostatics: Recognition of Ligands in Proteins and the Vibrational Stark Effect, J. Phys. Chem. B 110, 25074 (2006)) that classical polarizable electrostatics agreed well with ab initio calculations for the binding and bending energies of ligands CO, NO, and OO to the heme iron in myoglobin. Raeanne has set up simulations of other heme proteins and cobalt-based (corrin ring) petroleum scavengers, e.g. vitamin B12, in CHARMM, with thousands of explicit waters, gaining essential familiarity with handling large systems.
Prior to considering binding of pollutants cyanide and hydrogen sulfide to cobalt, we further developed the theory. We have constructed new electrostatic models, reparameterized the original models, and implemented a superior short-ranged damping scheme due to Burnham. In addition to the binding and bending energy of Fe-CO, we are investigating the increase of the bond length from 1.128 to 1.17 Angstroms, and the 200 wavenumber redshift of the CO stretch, with classical polarizable electrostatics, providing a rather new perspective on what can be done without quantum mechanics. As with water, a correct description of the molecular response to an electric field is the key to both the spectroscopy and the energetics.
