Gas phase hydrogen/deuterium exchange reactions of fluorophenyl anions

Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophenyl anions (C 6 H 4 F −, o -C 6 H 3 F 2 −, m -C 6 H 3 F 2 −, p -C 6 H 3 F 2 − ) have been studied using the flowing afterglow-selected ion flow tube technique. The C 6 H 4 F − anion exchanges all hydrogens for deuterium upon reaction with D 2 O. The difluorophenyl anions o -, m -, and p -C 6 H 3 F 2 − exchange three, two, and one hydrogen, respectively, with D 2 O, whereas they undergo one, two, and three H/D exchanges, respectively, with CH 3 OD. The structures of the anions and the isotope exchange 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 378.2 kcal/mol for C 6 H 4 F −, o -C 6 H 3 F 2 −, m -C 6 H 3 F 2 −, and p -C 6 H 3 F 2 −, respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D exchange results are explained by the energy differences of the intermediate DO − and CH 3 O − species within the ion-dipole complexes; CH 3 O − is mobile within the “hot” intermediate complex, whereas DO − is nearly “frozen” within the complex and cannot migrate across the barriers caused by the fluorine atoms or by the π electrons.