Site-specific addition of D2O to the (H2O)6− “hydrated electron” cluster: isomer interconversion and substitution at the double H-bond acceptor (AA) electron-binding site

Abstract

We report the results of an experimental study designed to establish whether, once formed, one of the isomer classes of the hydrated electron clusters, (H₂O)_n⁻, can interconvert with others when a water molecule is added by condensation. This is accomplished in an Ar-cluster mediated approach where a single intact D₂O molecule is collisionally incorporated into argon-solvated water hexamer anions, creating the isotopically labeled D₂O·(H₂O)₆⁻·Ar_n heptamer anion. Photoelectron and infrared predissociation spectroscopies are employed both to characterize the isomers generated in the condensation event and to track the position that the D₂O label adopts within these isomeric structures. Despite the fact that the water hexamer anion precursor clusters initially exist in the isomer I form, incorporation of D₂O produces mostly isomers I′ and II in the labeled heptamer, which bind the electron more (I′) or less (II) strongly than does the isomer I class. Isomers I and I′ are known to feature electron binding primarily onto a single water molecule that resides in an AA (A = H-bond acceptor) site in the network. Surprisingly, the D₂O molecule can displace this special electron-binding H₂O molecule such that the anionic cluster retains the high binding arrangement. In the more weakly binding isomer II clusters, the D₂O molecule fractionates preferentially to sites that give rise to the vibrational signature of isomer II.