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

Abstract

It was recently predicted by simulations and confirmed by neutron diffraction experiments that the structure of liquid tetrahydrofuran (THF) contains cavities. The cavities can be quite large and have a net positive electrostatic potential, so they can serve as pre-existing traps for excess electrons created via photodetachment from various solutes. In this paper, we use electron photodetachment via charge-transfer-to-solvent (CTTS) excitation of sodide (Na(-)) to probe for the presence of pre-existing cavities in a series of ether solvents: THF, diethyl ether, 1,2-dimethoxyethane (DME), and diglyme (DG). We find that electrons photodetached from sodide appear after a time delay with their equilibrium spectrum in all of these solvents, suggesting that the entire series of ethers contains pre-existing solvent cavities. We then use the variation in electron recombination dynamics with CTTS excitation wavelength to probe the nature of the cavities in the different ethers. We find that the cavities that form the deepest electron traps turn on at about the same energy in all four ether solvents investigated, but that the density of cavities is lower in DG and DME than in THF. We also examine the dynamics of the neutral sodium species that remains following CTTS photodetachment of an electron from sodide. We find that the reaction of the initially created gas-phase-like Na atom to form a (Na(+),e(-)) tight-contact pair occurs at essentially the same rate in all four ether solvents, indicating that only local solvent motions and not bulk solvent rearrangements are what is responsible for driving the partial ejection of the remaining Na valence electron.

Real-time observation of the charge transfer to solvent dynamics

“…The remarkable (41 eV) energy dissipation shifting the emission from the ultraviolet to the VIS region in the first 60 fs is undoubtedly associated to close-range interactions with firstshell molecules readapting to the new charge distribution, as was already pointed out for I À in THF 24 ARTICLE response 31 . Thereafter, there is still significant charge on iodine, or else the fluorescence would be strongly reduced by lack of radiative recombination of the electron and the hole.…”

“…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.…”

Time-dependent fluorescence in nanoconfined solvents: Linear-response approximations and Gaussian statistics

“…Schwartz and co-workers carried out a series of studies in which they investigated the solvation of a neutral Na atom prepared in liquid THF using two approaches. [1][2][3][4][5][6][7] In one they generated the sodium atom by removing an electron from Na − while in the other they obtained it by adding an electron to Na + . If the system obeys linear response, the two experiments should display relaxation on the same time scale because the dynamics in both cases occur on different parts of the same underlying free energy surface.…”

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].…”