Nature of Sodium Atoms/(Na⁺, e⁻) Contact Pairs in Liquid Tetrahydrofuran

Abstract

With no internal vibrational or rotational degrees of freedom, atomic solutes serve as the simplest possible probe of a condensed-phase environment's influence on solute electronic structure. Of the various atomic species that can be formed in solution, the quasi-one-electron alkali atoms in ether solvents have been the most widely studied experimentally, primarily due to the convenient location of their absorption spectra at visible wavelengths. The nature of solvated alkali atoms, however, remains controversial: the consensus view is that solvated alkali atoms exist as (Na + , e -) tight-contact pairs (TCPs), species in which the alkali valence electron is significantly displaced from the alkali nucleus and confined primarily by the first solvent shell. Thus, to shed light on the nature of alkali atoms in solution and to further our understanding of condensedphase effects on solutes' electronic structure, we have performed mixed quantum/classical molecular dynamics simulations of sodium atoms in liquid tetrahydrofuran (Na 0 /THF). Our interest in this particular system stems from recent pump-probe experiments in our group, which found that the rate at which this species is solvated depends on how it was created (Science 2008, 321, 1817); in other words, the solvation dynamics of this system do not obey linear response. Our simulations reproduce the experimental spectroscopy of this system and clearly indicate that neutral Na atoms exist as (Na + , e -) TCPs in solution. We find that the driving force for the displacement of sodium's valence electron is the formation of a tight solvation shell around the partially exposed Na + . On average, four THF oxygens coordinate the cation end of the TCP; however, we also observe fluctuations to other solvent coordination numbers. Furthermore, we find that species with different solvent coordination numbers have unique absorption spectra and that interconversion between species with different solvent coordination numbers requires surmounting a free energy barrier of several k B T. Taken together, our results suggest that the Na 0 /THF species with different solvent coordination numbers may be viewed as chemically distinct. Thus, we can explain the kinetics of Na TCP formation as being dictated by changes in the Na + solvent coordination number, and we can understand the dependence on initial conditions seen in the solvation dynamics of this system as resulting from the fact that the important solvent coordinate involves the motion of only a few molecules in the first solvation shell.

Searching for solvent cavities via electron photodetachment: The ultrafast charge-transfer-to-solvent dynamics of sodide in a series of ether solvents

“…The partial valence interaction of the electron with the sodium cation in the TCP gives rise to an absorption spectrum that peaks near 900 nm, part way between that of a gas-phase sodium atom and a solvated electron. 69 We note that somewhat surprisingly, there is no evidence in the literature for the presence of ͑Na + , e − ͒ TCPs at equilibrium in DEE; instead, pulse radiolysis of sodium cations in DEE produces a species with an absorption maximum at ϳ700 nm, suggestive of the formation of sodide. 73 However, previous ultrafast experiments 18,40 ͑and this study͒ suggest that ͑Na + , e − ͒ TCPs do form in liquid DEE on subnanosecond time scales.…”

“…͑2͒ has come from mixed quantum/classical molecular dynamics simulations, which show that the reaction of the sodium atom to produce the TCP and the subsequent solvation of the TCP in liquid THF is driven almost entirely by the motion of just four first-shell solvent molecules. 69 The simulations suggest that the observed chemical reaction to produce the ͑Na + , e − ͒ TCP from the neutral Na atom occurs when oxygen sites on the closest one or two solvent molecules move close enough to coordinate with the cation end of the TCP, displacing the sodium atom's valence electron. The simulations also indicate that the remainder of the TCP solvation that occurs on a ϳ5-10 ps time scale reflects the motion of oxygen sites on additional first-shell solvent molecules to create a tightly knit, four-site arrangement around the positive end of the TCP dipole.…”

“…68 The spectrum of the equilibrium neutral sodium species in liquid THF, also taken from pulse radiolysis experiments in the literature, 47 is shown as the purple dashed curve in this figure. This spectrum has been assigned on the basis of both quantum simulations 69 and experiments 14,44,47,[70][71][72] as arising from a ͑Na + , e − ͒ TCP, a species in which the electron is partially on the sodium cation and partially in the solvent. The partial valence interaction of the electron with the sodium cation in the TCP gives rise to an absorption spectrum that peaks near 900 nm, part way between that of a gas-phase sodium atom and a solvated electron.…”

“…This can be understood on the basis of simulations, which found that the density of cavity-supported electronic states increases with energy. 11,13 This suggests that excitation to higher-lying bound CTTS states increases the likelihood of coupling to cavity-supported disjoint states and thus decreases the number of electrons that remain close enough to their parent atoms to undergo rapid recombination. Thus, the excitation wavelength dependence of the geminate recombination dynamics of CTTS electrons photodetached from sodide provides a means to map out the density of the disjoint electronic states supported by the naturally occurring cavities in our series of ether solvents.…”

“…For example, it was recently predicted by molecular dynamics simulations 10,11 and verified by neutron diffraction experiments 12 that the solvent structure of liquid tetrahydrofuran ͑THF͒ is characterized by electropositive voids: the natural structure of liquid THF contains cavities that can be larger than an entire solvent molecule and the electrostatic potential in these cavities is favorable for solvating the excess electrons that are created via anion photodetachment in this solvent. Mixed quantum/ classical simulations have suggested that the high-lying excited states of anions dissolved in liquid THF are disjoint: 11,13 excited electrons can have density in multiple solvent cavities, including cavities far from the one that houses the ground-state anion.…”