Reports: AC646999-AC6: Induction in Chemistry: Introducing Electrostatic Bonds

Thomas F. Keyes , Boston University

The aim of this project is to show that classical polarization can describe some aspects of chemical bonding, including ligand-protein binding and hydrogen bonding in water, plus the associated spectroscopy, and to apply the resulting theory to problems relevant to the petroleum industry. Our thesis is that polarizable classical mechanics can describe a regime of bonding, which we named "electrostatic", lying intermediate between the textbook options of ionic and covalent.

 Work on ligand binding to biomolecules, emphasized in previous reports, was completed. For CO binding to myoglobin, the energy, the position of the minimum in the Fe-C potential, the Fe-C vibrational frequency, the linear geometry of the FeCO fragment and the bending energy, and the increase of the CO Stark tuning rate and IR intensity, are all reproduced by our simple classical model.

 The FeCO bond is indeed an electrostatic bond, dominated by the polarization energy, which can be 10x the Coulomb energy at typical chemical bond distances. Fe-C electrostatic bonding occurs despite a Coulomb repulsion. The amazingly comprehensive description obtained for FeCO bodes well for the ultimate intended applications, electrostatic binding of impurities in crude oil to metal atoms, e.g. cobalt, in biochemical scavengers. Our model is so simple that I hope to continue with undergraduate researchers. Graduate student Raeanne Napoleon was supported in prior periods, and this work will prepare her for a future in the biotech or pharmaceutical sectors, and advance my scientific career by facilitating a desired change into the area of biophysical chemistry.

 In the past year the focus shifted to water and solvation, with the grant providing partial support for Dr. Revati Kumar. Our POLIR water potential was shown to provide total and three-body energies in close agreement with high-level quantum calculations, and closer than those of existing polarizable potentials, for isomers of the hexamer cluster, even though POLIR was not optimized for the hexamer. The Raman spectrum of neat water was obtained, in excellent agreement with experiment, and a review of polarizable water potentials was presented.

 Polarizable water-ion potentials, compatible with POLIR, were constructed for the divalent cations Ca2+, Mg2+, and Cu2+. Ion-induced infrared spectral shifts in the OH stretch region obtained from classical simulations are in agreement with experiment. The water-ion binding energies are dominated by classical electrostatics, even though the copper case, with the strongest interaction due to the small size, might be commonly considered to involve an intermediate-strength covalent bond; the bond energy is 110 kcal/mol. Three-body energies of the ion with the first solvation shell are in agreement with ab initio calculations.

 Expanding the study to include hydrophobic solutes, it was further shown that known trends in spectral shifts vs. hydrophobicity could be predicted from the distribution of hydrogen bond angles in the first solvation shell. For weak to moderate solute-water interaction strengths, the distribution  maintains the two-peak form of neat water, but with changes in the amplitudes of the peaks. A smaller or larger fraction of distorted (larger angle peak) bonds indicates a red or blue shift, corresponding to hydrophobic and hydrophilic solutes, respectively. For the strongest interactions, e.g. small, doubly charged Mg2+ or Cu2+, the distribution is dominated by a third peak, absent in neat water, at such large angles that there can be no meaningful hydrogen bonds in the first shell. Now the spectrum is redshifted by direct interaction of the solute with the covalent OH bond; the polarization energy is an essential component of the redshift.

 In sum, polarizable, classical methods provide a comprehensive description of the spectral shifts induced by the most strongly interacting ions, and of the energetics of the solvation shell, including the octahedral coordination complex, [Cu(H2O)6]2+. Coordination complexes may be described by electrostatic bonding, as an alternative to quantum mechanics.

 Pairwise additive, non-polarizable water potentials treat polarization in an "effective" fashion, adding it to the strength of the pair interaction in a manner that corresponds to the ambient liquid. Thus, the most significant breakdowns of this approach occur when the environment deviates from that of the bulk liquid. The solvation shell is one example, but there are many others of relevance to the petroleum industry; the oil-water interface, water in nanopores, and water enclosing guest molecules in the case of gas hydrates. Methane hydrate ice frustrated one attempt to collect the oil from the Deepwater Horizon disaster, and we will apply our ideas about polarization to hydrates in future work.

 Revati Kumar was already well known in the area of aqueous systems when she joined the project, and this work will further enhance her prospects for an Assistant Professor position.

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