How the shape of an H-bonded network controls water activation
Collaboration with Mark Johnson (Yale), Ken Jordan (Pitt), Anne McCoy (Ohio State), Al Viggiano (AFRL)

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We typically view water as a "spectator" species, serving as a fluctuating dipole and/or polarizable solvent medium. The manner in which ions can chemically activate water, however, requires significant study, particularly in the context of solar and biological energy conversion. In this work, we demonstrated the manner in which the cation of nitric oxide (NO⁺) can perform such a role in atmospheric chemistry.

The n=3–to–n=4 transition is the critical reaction point in NO⁺(H₂O)_n clusters. At this stage, the inert, electrostatically bound clusters are chemically converted to nitrous acid and protonated water complexes:

The source of these protonated water clusters in the ionosphere was once a mystery. While the role of NO⁺ in their formation has since been known, many details concerning the mechanism remained ambiguous.

Isomer-selective action spectroscopy was able to resolve the vibrational spectrum of the n=3 clusters at low temperatures. Identification of these isomers required a close collaboration between experiment and theory. While the structures are relatively easy to characterize, strongly anharmonic vibrational spectra (particularly of shared protons) are difficult to produce theoretically, and harmonic calculations were qualitatively incorrect in this regime. Yet these vibrational signatures were the crucial connection to the experimental spectra. Utilizing anharmonic spectra (vibrational perturbation theory and vibrational SCF/PT2) in combination with high-level ab initio calculations, we were able to definitively assign the observed experimental spectrum.


In short, three isomers were observed, with varying degrees of charge migration and proton sharing. Thus, the degree of activation of the O-H bond in water was critically dependent on the H-bond structure of the cluster network. Specific configurations were able to preferentially stabilize the charge- and proton-transferred product state. Accordingly, the red-shifted proton bands lit up the observed spectrum. The n=3 species (see fig above) was perilously perched prior to reaction; addition of a water further stabilizes the product state and effects HONO formation.

Note that the eigenvalue-based methods employed in this study were able to accurately reproduce the band positions in the vibrational spectra and assign the relevant isomers. Such "stick" spectra, however, ignore the information present in line shapes; recent developments in ab initio molecular dynamics have enabled us to reproduce the broad line shape, as well.

Reference:
"How the shape of an H-bonded network controls proton-coulped water activation in HONO formation"
R. A. Relph, T. L. Guasco, B. M. Elliott, M. Z. Kamrath, A. B. McCoy, R. P. Steele, D. P. Schofield, K. D. Jordan, A. A. Viggiano, E. E. Ferguson, M. A. Johnson.
Science 327 308–312 (2010).