Inverse Heavy-Atom Kinetic Isotope Effects in Chloroalkanes

Abstract

Intramolecular heavy-atom kinetic isotope effects (KIEs) are reported for reactions of some gas-phase ions relevant to atmospheric chemistry. The anionic complexes carbon tetrachloride/Cl-, chloroform/Cl-, and methylene chloride/Cl- (CHnCl5-n-; n = 0−2) were generated by various ionization methods and subjected to single-stage and tandem mass spectrometric analysis in a variety of instruments. Strongly pressure dependent, anomalous chloride abundances were observed in single-stage mass spectra under self-chemical ionization conditions for CHCl4- and CCl5- in a triple quadrupole and for CHCl4- in a quadrupole ion trap. Further examination of these complexes in triple quadrupole MS/MS experiments confirmed the presence of an unusually large inverse kinetic isotope effect. We suggest that the selective loss of 37Cl- is simply due to the difference in centrifugal barriers associated with the competitive reaction channels leading to the two chloride isotopes. This effect is greatly magnified near the threshold for dissociation, and an extreme value of an inverse kinetic isotope effect (0.026 ± 0.01) is displayed by the isotopomeric chloride adduct of chloroform, CH35Cl337Cl-, under single-collision conditions with xenon at 27 eV lab collision energy. The methylene chloride adduct, CH235Cl237Cl-, had a KIE close to unity (0.90 ± 0.05), and the carbon tetrachloride adduct displayed no apparent KIE, under these same conditions. Remarkably, in addition to the heavy-atom KIEs for chlorinated chloroform, (i) substantial H/D isotope effects are associated with chloride loss, (ii) the chlorine KIE values are strongly dependent on the particular isotopomer selected for examination, and (iii) the chlorine KIE for CHCl4- is not monotonic with collision energy. In sum, the results are indicative of a complex potential energy surface and of a mixture of dissociating structures or independent dissociation channels of the precursor.