Flash photolysis of alkali metal anions in tetrahydrofuran and dimethoxyethane

Seddon, W. A.; Fletcher, John; Sopchyshyn, F.C.; Selkirk, E.B.

doi:10.1139/v79-286

Cited by 18 publications

(40 citation statements)

References 12 publications

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“…Our preliminary simulation studies of this system found that Na 0 clearly exists as a TCP in liquid THF, 18 in agreement with experimental interpretations. [1][2][3][4][13][14][15]19 In this work, we explore the molecular nature of the sodium TCP in significantly more detail. We find that the driving force for TCP formation lies in the creation of a tight solvation shell of ∼4 THF oxygens around the partially exposed Na + end of the TCP.…”

Section: Introductionmentioning

confidence: 99%

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

2010

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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.

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Section: Introductionmentioning

confidence: 99%

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

2010

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“…Since we could not independently determine the concentration of the sodide samples, we have assumed that the maximum extinction coefficient of the sodide CTTS transition was the same in all of the solvents as in THF. 47 For reference, the dashed gray curve in this figure shows the spectrum of sodide in THF taken from Gaussian-Lorentzian fit parameters in the literature. 47,67 The figure shows that the absorption spectra of sodide in THF, DME, and DG, all have essentially the same shape and an almost identical maximum wavelength.…”

Section: Background: the Spectroscopy Of Sodide And Its Photopromentioning

confidence: 99%

“…47 For reference, the dashed gray curve in this figure shows the spectrum of sodide in THF taken from Gaussian-Lorentzian fit parameters in the literature. 47,67 The figure shows that the absorption spectra of sodide in THF, DME, and DG, all have essentially the same shape and an almost identical maximum wavelength. The absorption spectrum of sodide in DEE and THF appear to be slightly broader than that in the other solvents, which we have argued previously is likely the result of either a small amount of potasside contamination or scattering from metal oxide particles in the DEE samples.…”

Section: Background: the Spectroscopy Of Sodide And Its Photopromentioning

confidence: 99%

“…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.…”

Section: Background: the Spectroscopy Of Sodide And Its Photopromentioning

confidence: 99%

“…Photoexcitation of the sodide CTTS transition in liquid THF is known to produce solvated electrons and neutral sodium atoms 47,48 …”

Section: Introductionmentioning

confidence: 99%

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

Larsen

Schwartz

2009

The Journal of Chemical Physics

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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.

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