A DETERMINATION OF PROTON-TRANSFER EQUILIBRIUM CONSTANTS IN BENZENE/HALOBENZENE MIXTURES AT VARIOUS TEMPERATURES USING A HIGH-PRESSURE ION SOURCE

A pulsed ion source has been designed and constructed for use with a Kratos MS 50 mass spectrometer in studies of ion-molecule reactions at total pressures from 1 to 5 torr at temperatures between 300 and 600 K. The source is demonstrated to be suited for the measurement of equilibrium constants as a function of temperature for proton-transfer reactions involving benzene and halogenated benxenes. The gas-phase bssicity order &HsF> C6HSEr, C6H6> CsHsCl is established at 334 K and shown to be quite sensitive to temperature. Extrapolation of experimental data leads to C6He> CeH&l> CeHsF above 850 K and to C6HsF> C6HsCl> C6H6 below 270 K. The standard entbalpy changes derived from van % Hoff plots indicate a proton affinity order CeHsF> CeHsCl> C6H6 which appears to be independent of temperature in the temperature range of the measurements. The standard entropy changes obtained from these plots provide insight into the site(s) of protonation of CeHsF and CeHsCl