Gas phase hydrogen/deuterium exchange reactions of fluorophenyl anions

Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophe nyl anions (C6H4F-, o-C6H3F2- m-C6H3F2-, p-C6H3F2-) have been studied using the flowing afterglow-selected ion flow tube technique. The C6H4F- anion e xchanges all hydrogens for deuterium upon reaction with D2O. The difluoroph enyl anions o-, m-, and p-C6H3F2- exchange three, two, and one hydrogen, re spectively, with D2O, whereas they undergo one, two, and three H/D exchange s, respectively, with CH3OD. The structures of the anions and the isotope e xchange dynamics within the intermediate ion-dipole complexes are discussed using ab initio molecular orbital calculations. Calculated values for the proton affinities of the most stable anions are 385.2, 378.0, 371.9, and 37 8.2 kcal/mol for C6H4F-, o-C6H3F2-, m-C6H3F2-, and p-C6H3F2-, respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D excha nge results are explained by the energy differences of the intermediate DO- and CH3O- species within the ion-dipole complexes; CH3O- is mobile within the "hot" intermediate complex, whereas DO- is nearly "frozen" within the c omplex and cannot migrate across the barriers caused by the fluorine atoms or by the pi electrons. (T Am Soc Mass Spectrom 1999, 10, 840-847) (C) 1999 American Society for Mass Spectrometry.