STABILITIES OF COMPLEXES OF BR- WITH SUBSTITUTED BENZENES (SB) BASED ON DETERMINATIONS OF THE GAS-PHASE EQUILIBRIA BR- + SB = (BRSB)-

Equilibria involving some forty substituted benzenes (SB) and the bromide ion (SB + Br- = SBBr-) in the gas phase were determined with a pulsed electron, high-pressure mass spectrometer (PHPMS). The resulting -DELTA-G1o provide information on the stabilities of the SBBr- complexes. Previous work, involving gas-phase thermochemical data for X- (CH3O-, F-, Cl-, Br-, I-) and quantum chemical calculations, indicate that the most stable SB.X- complexes might have a variety of structures, depending on X- and the nature of the substituents. Thus X- may engage in hydrogen bonding to an aromatic hydrogen atom, or lead to a sigma-bonded (Meisenheimer) complex, or form a complex where X- is on an axis perpendicular to the benzene plane. A Taft substituent analysis of the delta-DELTA-G1o indicates that Br- and Cl- form aromatic C-H hydrogen-bonded complexes with all singly substituted benzenes. The field effects of the substituents provide the dominant contribution to the bonding in these complexes. Similar conclusions are reached also for the singly substituted nitrobenzenes and Br-. A clearcut analysis of the bonding to Br- when triply substituted benzenes with strongly electron withdrawing substituents like CF3, CN, and NO2 are present could not be obtained. In these cases all three bonding structures mentioned above may have similar stabilities.