Molecular Dynamics Simulations of Electron−Alkali Cation Pairs in Bulk Water

Coudert, François‐Xavier; Archirel, Pierre; Boutin, Anne

doi:10.1021/jp0542975

Cited by 17 publications

(52 citation statements)

References 35 publications

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“…This implies that aqueous solutions of alkali cations and hydrated electrons should exhibit a statistical distribution of species rather than specifically forming contact pairs. 12 This previous work is suggestive, but to the best of our knowledge, there has been no simulation work targeted at experimentally realizable alkali cation-electron tight-contact pairs, such as the alkali atoms in liquid ethers.…”

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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“…Boutin and co-workers accomplished this with a perturbation-theorybased method, 26 and they found that the PMF consists of a well that is ∼5 k B T deep with an energetic minimum located at an e − −Na + distance of ∼2 Å: in other words, they found that Na + forms a stable contact pair with the hydrated electron. 17 Boutin and co-workers then went on to examine how the absorption spectrum of the MQC-simulated hydrated electron varies as a function of distance from the Na + cation. They found that proximity to a sodium cation indeed led to a blueshift of the electron's calculated spectrum and that the magnitude of the spectral shift varied roughly inversely with the sodium−electron distance.…”

Section: ■ Introductionmentioning

confidence: 99%

“…14 Boutin and co-workers' MQC simulations, however, found that the hydrated electron's spectrum changed shape quite a bit when in a contact pair with a sodium cation. 15,17 Finally, when Boutin and co-workers attempted to mimic the concentration dependence of the electron's spectrum by varying the electron−cation distance, 17 the spectral behavior had a different dependence than that seen experimentally. 14 These discrepancies mean that something in the MQC simulations is not capturing the correct experimental behavior.…”

Section: ■ Introductionmentioning

confidence: 99%

How Water–Ion Interactions Control the Formation of Hydrated Electron:Sodium Cation Contact Pairs

2021

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Although solvated electrons are a perennial subject of interest, relatively little attention has been paid to the way they behave in aqueous electrolytes. Experimentally, it is known that the hydrated electron’s (e aq –) absorption spectrum shifts to the blue in the presence of salts, and the magnitude of the shift depends on the ion concentration and the identities of both the cation and anion. Does the blue-shift result from some type of dielectric effect from the bulk electrolyte, or are there specific interactions between the hydrated electron and ions in solution? Previous work has suggested that e aq – forms contact pairs with aqueous ions such as Na+, leading to the question of what controls the stability of such contact pairs and their possible connection to the observed spectroscopy. In this work, we use mixed quantum/classical simulations to examine the nature of Na+:e – contact pairs in water, using a novel method for quantum umbrella sampling to construct e aq ––ion potentials of mean force (PMF). We find that the nature of the contact pair PMF depends sensitively on the choice of the classical interactions used to describe the Na+–water interactions. When the ion–water interactions are slightly stronger, the corresponding cation:e – contact pairs form at longer distances and become free energetically less stable. We show that this is because there is a delicate balance between solvation of the cation, solvation of e aq – and the direct electronic interaction between the cation and the electron, so that small changes in this balance lead to large changes in the formation and stability of e ––ion contact pairs. In particular, strengthening the ion–water interactions helps to maintain a favorable local solvation environment around Na+, which in turn forces water molecules in the first solvation shell of the cation to be unfavorably oriented toward the electron in a contact pair; stronger solvation of the cation also reduces the electronic overlap of e aq – with Na+. We also find that the calculated spectra of different models of Na+:e – contact pairs do not shift monotonically with cation–electron distance, and that the calculated spectral shifts are about an order of magnitude larger than experiment, suggesting that isolated contact pairs are not the sole explanation for the blue-shift of the hydrated electron’s spectrum in the presence of electrolytes.

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“…We report here mixed quantum-classical simulations (QCMD), successfully used to better understand the solvated electron-cations interactions in bulk water, [17,18] applied to the study of the solvated electron in water confined in the pores of a siliceous faujasite. A schematic view of a faujasite supercage is represented in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

Confinement effect on the hydrated electron behaviour

Coudert

Boutin

2006

Chemical Physics Letters

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We report the first direct simulation of an excess hydrated electron confined in a zeolite nanopore by means of mixed quantum-classical molecular dynamics. The experimental dependence of the hydrated electron absorption spectrum maximum upon water loading in faujasites is reproduced. The diffusion of the confined hydrated electron is also studied and a prediction of the diffusion coefficient is provided.

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Molecular Dynamics Simulations of Electron−Alkali Cation Pairs in Bulk Water

Cited by 17 publications

References 35 publications

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

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

How Water–Ion Interactions Control the Formation of Hydrated Electron:Sodium Cation Contact Pairs

Confinement effect on the hydrated electron behaviour

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Molecular Dynamics Simulations of Electron−Alkali Cation Pairs in Bulk Water

Cited by 17 publications

References 35 publications

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

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

How Water–Ion Interactions Control the Formation of Hydrated Electron:Sodium Cation Contact Pairs

Confinement effect on the hydrated electron behaviour

Contact Info

Product

Resources

About

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

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