Thermal isomerisation of protonated chlorobenzene studied in the gas phase using high-energy collision induced decomposition mass spectrometry

Abstract

The high-energy collision induced decomposition (CID) mass spectra of protonated chlorobenzene and chlorobenzene-d5 change significantly when the protonating reaction conditions in the ion source are changed. By monitoring relative peak heights as a function of gas pressure and temperature in the ion–molecule reactor, it is possible to distinguish changing abundances of up to four different protonated isomers. These are the para [C(4)], ortho [C(2)] and ipso [C(1)] ring-edge-protonated isomers and the Cl-protonated molecule (CL). It is therefore possible to elucidate the mechanism and energetics of their interconversion. Molecular orbital theory calculations place the proton affinity of the Cl atom site at ≈630 kJ mol^-1, >100 kJ mol^-1 below the most favoured site, which is at the C(4) ring carbon atom. Nevertheless, at low pressures and temperatures in CH₄, the Cl atom becomes protonated, the kinetics being consistent with proton transfer via a proton bound ion–molecule complex. At higher pressures, intramolecular migration carries the proton over to the ring. The tandem mass spectra then become independent of pressure but specific features vary dramatically with temperature. At high pressure, the shape of the curve showing the variation with temperature, and equivalent changes in the spectra of (C₆D₅Cl)H⁺, are consistent with a simple 3-step kinetic model. At the lowest temperatures, it appears that C(4) protonation is almost complete and that migration of the proton around the ring is ‘frozen’, but heating to temperatures >300 K leads to rapid migration and equilibration between the C(2) and C(4) isomers. Fitting the model to the data gives energies of 2.4±1.6 and 23.5±3.5 kJ mol^-1 for the C(2) and C(1) isomers relative to C(4), and 46±2 kJ mol^-1 for the barrier to migration of the proton around the ring. A primary kinetic isotope effect of >3 is found for the thermally induced migration of the proton around (C₆D₅Cl)H⁺. At 200 K there is no primary kinetic isotope effect in the CID channel leading to loss of H from (C₆D₅Cl)H⁺, but it rises to an unprecedented high value of >90 as the temperature of the ion increases to >450 K.