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 reactionpotential 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.
Gómez, Harry; Meloni, Giovanni; Madrid, James; Neumark, Daniel M. Anion Photoelectron Spectroscopy of Solvated Transition State Precursors. Journal of Chemical Physics. 7/8/2003, Vol. 119 Issue 2, p872.