How Does a Solvent Affect Chemical Bonds? Mixed Quantum/Classical Simulations with a Full CI Treatment of the Bonding Electrons

Understanding how a solvent affects the quantum mechanics and reactivity of the chemical bonds of dissolved solutes is of fundamental importance to chemistry. To explore condensed-phase effects on a simplemolecular solute, we have studied the six-dimensional two-electron wave function of the bonding electrons of the Na2 molecule in liquid argon via mixed quantum/classical simulation. We find that even though Ar is an apolar liquid, solvent interactions produce dipole moments on Na2 that can reach magnitudes over 1.4 D. These interactions also change the selection rules, induce significantmotional-narrowing, and cause a large (26 cm) blue shift of the dimer's vibrational spectrum relative to that in the gas phase. These effects cannot be captured via classical simulation, highlighting the importance of quantum many-body effects. SECTION Statistical Mechanics, Thermodynamics, Medium Effects U nderstanding how a solvent affects the quantum mechanics and reactivity of the chemical bonds of dissolved solutes is of fundamental importance to chemistry. The solvent's influence on chemical bonds arises from alteration of a solute's electronic structure due to both shortand longer-ranged interactions with hundreds of nearby solvent molecules. This solvent-induced alteration of the solute's electronic structure can alter the vibrational frequencies and/or dissociation energies of the solute's chemical bonds. Traditional molecular dynamics (MD) simulations based on classical pairwise additive force fields are unable to explore the detailed physics underlying the solvent's influence on chemical bonds because such simulations cannot account for changes in the solute's electronic structure with the environment. Classical models can be improved by adding many-body polarization terms; it has recently been shown that such terms are necessary to successfully reproduce the vibrational spectrum of liquid water and ice. It is unclear, however, whether classical polarizable models can universally describe the changes in the solute electronic structure that alter the nature of bond vibrations. This is particularly true in nonpolar systems where longer-ranged electrostatic interactions play a much smaller role than inherently quantum mechanical interactions such as Pauli repulsion. To capture these effects, the quantum mechanics of the solute valence electrons responsible for the chemical bonding and the interactions of these electrons with the surrounding solvent molecules must be accounted for at the Hamiltonian level. There has been some semiempirical work done to include solvent effects on the bonding electronic structure in simulations examining the photodissociation dynamics of I2 in rare gas liquids/matrixes 3,4 and I2 in rare gas and CO2 clusters, 5-7 but to the best of our knowledge, there has been no first-principles treatment of solvent-solute chemical bond interactions for diatomics in simple liquids. In this paper, we go beyond a semiempirical treatment of the solute's electronic structure and use first-principles quantum mechanics to calculate the influence of a solvent on the electrons in a chemical bond.We avoid the formidable task of solving Schr€ odinger's equation (SE) for all of the electrons on both the solvent and solute molecules in the simulation by adopting a mixed quantum/classical (MQC) approach, in which we solve the SE exactly for the valence electrons of a diatomic solute. Due to the relative simplicity of its electronic structure, we have chosen the sodium dimer (Na2) as our solute, andwe have investigated the chemical bond dynamics of Na2 dissolved in liquid argon. This leads to a computational problem that involves finding the electronic states of the two Na2 valence electrons in the presence of hundreds of classical Ar atoms and two Naþ core particles. We solve this problem using our recently developed two-electron Fourier grid (2EFG) method, which is outlined for this particular application in the Supporting Information (SI). We find that even though liquid Ar is an apolar solvent, its influence on the bonding electrons of Na2 is quite substantial; nonpolar interactions with Ar atoms that push the valence electrons off of the nuclear centers produce a large instantaneous dipole moment on the dimer that can reachmagnitudes of over 0.3 e-Å (1.4 D). The presence of the solvent-induced fluctuating dipole leads to both anew far-IRabsorption feature andanalterationof the selection rules for the dimer's infrared vibrational absorption. Received Date: October 5, 2009 Accepted Date: November 2, 2009