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Abstract The hydrated electron, e–(aq), has attracted much attention as a central species in radiation chemistry. However, much less is known about e–(aq)at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e–(aq)at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion. The spectral maximum agrees with that for bulk e–(aq)and shows that the orbital density resides predominantly within the aqueous phase, in agreement with supporting calculations. In contrast, the chemistry of the interfacial hydrated electron differs from that in bulk water, with e–(aq)diffusing into the bulk and leaving the phenoxyl radical at the surface. Our work resolves long-standing questions about e–(aq)at the water/air interface and highlights its potential role in chemistry at the ubiquitous aqueous interface.
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Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface
Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions, including Cl−, Br−, I−, CN−, OCN−, SCN−, NO2−, NO3−, and ClOn− (n=1,2,3,4), several of which are archetypal examples of surface-active species. In all cases, we find that average anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br−(aq), I−(aq), ClO4−(aq), and SCN−(aq), and also for ClO−(aq), the charge-transfer (CT) energy is found to be larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl−(aq) ion has a slightly larger CT energy at the interface, but NO3−(aq) does not; these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to <20% of the total induction energy for each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions remains unclear. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities.
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- Award ID(s):
- 1955282
- PAR ID:
- 10355662
- Date Published:
- Journal Name:
- Molecules
- Volume:
- 26
- Issue:
- 21
- ISSN:
- 1420-3049
- Page Range / eLocation ID:
- 6719
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/molecules26216719
