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

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 sigma* orbitals on a nearby water molecule in a double hydrogen-bond-acceptor (AA) configuration. The extent of e(-) -> sigma* charge transfer is comparable to the n -> sigma* charge transfer in the most strongly hydrogen-bonded X-(H2O) complexes (e. g., X 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 center dot center dot center dot-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.