Mail:
Dept. of Chemistry
Ohio State University
100 W. 18th Ave.
Columbus, OH 43210
Office:
412 CBEC
Email:
herbert@
chemistry.ohio-state.edu
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.