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Charge penetration and the origin of large O–H vibrational red-shifts in hydrated-electron clusters, (H2O)n

J. M. Herbert and M. Head-Gordon
J. Am. Chem. Soc., 128, 13932–13939 (2006).

Abstract

The origin of O–H vibrational red-shifts observed experimentally in (H2O)n clusters is analyzed using electronic structure calculations, including natural bond orbital analysis. The red-shifts are shown to arise from significant charge transfer and strong donor-acceptor stabilization between the unpaired electron and O–H σ* orbitals on a nearby water molecule in a double hydrogen-bond-acceptor ("AA") configuration. The extent of e → σ* charge transfer is comparable to the n → σ* charge transfer in the most strongly hydrogen-bonded X(H2O) complexes (e.g., F, O, OH), even though the latter systems exhibit much larger vibrational red-shifts. In X(H2O), the proton affinity of X induces a low-energy XH…–OH diabatic state that becomes accessible in v = 1 of the shared-proton stretch, leading to substantial anharmonicity in this mode. In contrast, the H + OH(H2O)n–1 diabat of (H2O)n is not energetically accessible; thus, the O–H stretching modes of the AA water are reasonably harmonic, and their red-shifts are less dramatic. Only a small amount of charge penetrates beyond the AA water molecule, even upon vibrational excitation of these AA modes. Implications for modeling of the aqueous electron are discussed.

[DOI] [PDF]
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