Anion photoelectron spectroscopy of solvated transition state precursors

Photoelectron (PE) spectra have been collected for the clustered bihalide anions XHX-.(M) (X=Br, I) and BrHI-.(M), where M=H2O, HBr, and HI, in order to probe the effects of strongly solvating species on the PE spectra of transition state precursor anions. The PE spectra of the XHX-.(H2O) ions show similar vibrational progressions as the spectra of the bare BrHBr- and IHI- anions, indicating that photodetachment of the bare and hydrated ions accesses similar XHX transition state geometries on the X+HX reaction potential energy surfaces. These results are consistent with electronic structure calculations that predict a double hydrogen-bonded XHX-.(H2O) structure in which the symmetry of the strong XHX- hydrogen bond is largely preserved. In contrast, PE spectra of BrHBr-(HBr)(1,2) and IHI-(HI)(1,2) indicate that the addition of a single HBr or HI disrupts the symmetric XHX- bond, resulting in structures of the form X-.(HX)(n), and altering the geometry of the Franck-Condon region accessed on the neutral potential energy surfaces. Similarly, PE spectra of BrHI-.(HI) and BrHI-.(HBr) suggest anion structures of the form I-.(HBr)HI and I-.(HBr)(2), respectively. (C) 2003 American Institute of Physics.