The roles of electronic exchange and correlation in charge-transfer-to-solvent dynamics: Many-electron nonadiabatic mixed quantum/classical simulations of photoexcited sodium anions in the condensed phase

Abstract

The charge-transfer-to-solvent (CTTS) reactions of solvated atomic anions serve as ideal models for studying the dynamics of electron transfer: The fact that atomic anions have no internal degrees of freedom provides one of the most direct routes to understanding how the motions of solvent molecules influence charge transfer, and the relative simplicity of atomic electronic structure allows for direct contact between theory and experiment. To date, molecular dynamics simulations of the CTTS process have relied on a single-electron description of the atomic anion-only the electron involved in the charge transfer has been treated quantum mechanically, and the electronic structure of the atomic solute has been treated via pseudopotentials. In this paper, we examine the severity of approximating the electronic structure of CTTS anions with a one-electron model and address the role of electronic exchange and correlation in both CTTS electronic structure and dynamics. To do this, we perform many-electron mixed quantum/classical molecular dynamics simulations of the ground- and excited-state properties of the aqueous sodium anion (sodide). We treat both of the sodide valence electrons quantum mechanically and solve the Schrodinger equation using configuration interaction with singles and doubles (CISD), which provides an exact solution for two electrons. We find that our multielectron simulations give excellent general agreement with experimental results on the CTTS spectroscopy and dynamics of sodide in related solvents. We also compare the results of our multielectron simulations to those from one-electron simulations on the same system [C. J. Smallwood et al., J. Chem. Phys. 119, 11263 (2003)] and find substantial differences in the equilibrium CTTS properties and the nonadiabatic relaxation dynamics of one- and two-electron aqueous sodide. For example, the one-electron model substantially underpredicts the size of sodide, which in turn results in a dramatically different solvation structure around the ion. The one-electron model also misses the existence of an entire manifold of bound CTTS excited states and predicts an absorption spectrum that is blueshifted from that in the two-electron model by over 2 eV. Even the use of a quantum mechanics/molecular mechanics (QM/MM)-like approach, where we calculated the electronic structure with our CISD method using solvent configurations generated from the one-electron simulations, still produced an absorption spectrum that was shifted approximately 1 eV to the blue. In addition, we find that the two-electron model sodide anion is very polarizable: The instantaneous dipole induced by local fluctuating electric fields in the solvent reaches values over 14 D. This large polarizability is driven by an unusual solvation motif in which the solvent pushes the valence electron density far enough to expose the sodium cation core, a situation that cannot be captured by one-electron models that employ a neutral atomic core. Following excitation to one of the bound CTTS excited states,...

Real-time observation of the charge transfer to solvent dynamics

“…Femtosecond X-ray absorption spectroscopy at the I L-edges detected the birth of the neutral halogen, but the time resolution was not sufficient to capture the early dynamics 7 . Of particular relevance here are the studies by Ruhman et al 11,12 and Schwartz et al 13,14,[22][23][24][25] on the CTTS states of sodide (Na À ) in tetrahydrofuran (THF) and ether solvents, which enabled them to probe at high-time resolution both the signal of the ejected electron and the nascent neutral sodium atom. They found that electrons photodetached from the parent ion appear with their equilibrium spectrum after 500 fs with no further spectral evolution, nor a pump wavelength dependence.…”

Old acid, new chemistry. Negative metal anions generated from alkali metal oxalates and others

“…These characteristically blue solutions are now fabricated to control the amount of metal ions, including metal anions. Such solutions have been extensively studied for use in organic synthesis and elucidation of charge-transfer to solvent dynamics (CTTS) [2][3][4][5][6][7][8][9]. In the gas phase, a beam of positively charged alkali cations created from metal vapor can undergo double electron capture to generate a low density negative ion beam [10].…”

First principles multielectron mixed quantum/classical simulations in the condensed phase. I. An efficient Fourier-grid method for solving the many-electron problem

“…However, unlike electronic structure methods based on Gaussian basis functions, our 2EFG method can easily capture electronic wave functions that are localized away from atomic cores, such as the CTTS states of atomic anions, 11 as also demonstrated in detail in Paper II. We also demonstrated that the 2EFG method can handle molecular systems by calculating the PECs of the sodium dimer to a similar level of accuracy as a full CI calculation that used Gaussian basis functions.…”

“…For example, Alavi used PIB states to study the properties of two interacting electrons in a box. 11 As shown in the Supporting Information, 31 our 2EFG method is related to our previous real-space CI method. In addition, we previously developed a real-space CI method 10 and used it to study the hydrated dielectron [35][36][37] and aqueous sodide.…”

“…Although we expect our CI-based method to work well in the dissociation limit, does the piece-wise constant approximation in the electron-electron integrals give smooth PECs? 11 The classical interaction between the two Na + cores was taken to be the Coulombic interaction of two point charges ͑in other words, we neglected short-range repulsion between the cores at the nearequilibrium internuclear separations of interest͒. Unlike the harmonium system, we have no exact solutions of the electronic states of Na 2 with which to compare; however, we can compare our calculated PECs to experimental results 57,58 and to high-level quantum chemical calculations.…”