Energetics of Selected Gas Phase Ion-Molecule Reactions

The energetics of the gas phase negative ion-molecule association reactions M + X^{-} = McdotX^ {-}, where M are substituted benzenes, quinones and ethylenes and X^{-} are the halide ions (F^{-} , Cl^{-}, Br ^{-} and I^{ -}), were determined by equilibrium measurements with the pulsed electron high pressure mass spectrometer (PHPMS). Evaluation of the equilibrium constants for the halide association (XA) reactions leads directly to absolute bond free energy determinations, -DeltaG _sp{XA}{circ}, in McdotX^{- }. Under conditions where halide association equilibria could not be measured directly, relative bond free energies, -DeltaDeltaG _sp{XA}{circ}, were obtained by measurements of the transfer equilibria McdotX^{-} + M^' = M + cdotM^'cdot X^{-}. Combining -DeltaDeltaG_sp {XA}{circ} values with directly determined -DeltaG _sp{XA}{circ} values leads to further absolute bond free energy determinations. The hydrogen bond free energies in the singly substituted phenol complexes YPhOHcdotBr ^{-}, combined with previous data for X^{-} = Cl ^{-} and I^{ -} from this laboratory, are used to examine the substituent effects on hydrogen bonding. The dominant contribution to YPhOHcdotBr ^{-} stabilization, where the extent of proton transfer from YPhOH to Br^{ -} is small, is due to the field effects of the substituents with pi delocalization playing only a small part. Thus, the correlation with the acidity of YPhOH, where pi delocalization is important, is not very close. Substituent effect analysis of experimentally determined bond free energies and quantum mechanical calculations are used to gain structural information on the complexes McdotX^{-} where M does not possess substituents with protic hydrogens. The results indicate the complexes M cdotX^{-} have a variety of structures, depending on X^ {-} and the nature of the substituents. The temperature dependence of the equilibrium constants for the gas phase reactions HO^ - + HOH = HO^-cdotHOH and CH_3O^- + HOCH_3 = CH_3O ^-cdotHOCH_3 was measured with the PHPMS. The enthalpy and entropy changes were obtained from van't Hoff plots of the equilibrium constants. The enthalpy changes were found to be in good agreement with experimentally determined values reported by Meot-Ner and Sieck and recent theoretical results. The agreement for the entropy changes is found not to be as good. The electron affinities of 20 cyclic diones, mostly substituted maleic and phthalic anhydrides and maleimides and phthalimides, were determined with the PHPMS by measuring gas phase electron transfer equilibria A^ {-} + B = A + B^{ -} involving these compounds and reference compounds whose electron affinities were determined previously. The effects of substituents on electron affinities are similar to those observed previously for other groups of organic compounds.