Symmetry-induced kinetic isotope effects in the dissociation dynamics of CHCl3+ and CHCl4−

Dissociation processes of ionized freons: CHFCl2+ and CF2Cl2+ in the gas phase

The present study reveals the effects of symmetry on how the distribution and flow of energy play out on the decomposition of small halocarbons. Unimolecular decay of the freons CHFCl2 and CF2Cl2 when ionized has been investigated. Mass spectrometric results that encompass isotope effects (peak heights) and energy distribution in the exit channel (peak shapes) are interpreted by computational methods. Non-statistical processes of electronic predissociation and isolated state decay are shown to be directly associated with molecular symmetry.

Energy-dependent normal and unusually large inverse chlorine kinetic isotope effects of simple chlorohydrocarbons in collision-induced dissociation by gas chromatography-electron ionization-tandem mass spectrometry

Kinetic isotope effects (KIEs) occurring in mass spectrometry (MS) can provide in-depth insights into the fragmentation behaviors of compounds of interest in MS. Yet, the fundamentals of KIEs in collision-induced dissociation (CID) in tandem mass spectrometry (MS/MS) are unclear, and information about chlorine KIEs (Cl-KIEs) of organochlorines in MS is particularly scarce. This study investigated the Cl-KIEs of dichloromethane, trichloroethylene, and tetrachloroethylene during CID using gas chromatography-electron ionization triple-quadrupole MS/MS. Cl-KIEs were evaluated with MS signal intensities. All the organochlorines presented large inverse Cl-KIEs (<1, the departures of Cl-KIEs from 1 denote the magnitudes of Cl-KIEs), showing the largest magnitudes of 0.797, 0.910, and 0.892 at the highest collision energy (60 eV) for dichloromethane, trichloroethylene, and tetrachloroethylene, respectively. For dichloromethane, both intra-ion and inter-ion Cl-KIEs were studied, within the ranges of 0.820-1.020 and 0.797-1.016, respectively, showing both normal and inverse Cl-KIEs depending on collision energies. The observed Cl-KIEs generally declined from large normal to extremely large inverse values with increasing collision energies from 0 to 60 eV but were inferred to be independent of MS signal intensities. The Cl-KIEs are dominated by critical energies at low internal energies of precursor ions, resulting in normal Cl-KIEs; while at high internal energies, the Cl-KIEs are controlled by rotational barriers (or looseness/tightness of transition states), which lead to isotope-competitive reactions in dechlorination and thereby inverse Cl-KIEs. It is concluded that the Cl-KIEs may depend on critical energies, bond strengths, available internal energies, and transition state looseness/tightness. The findings of this study yield new insights into the fundamentals of Cl-KIEs of organochlorines during CID and may be conducive to elucidating the underlying mechanisms of KIEs in collision-induced and photo-induced reactions in the actual world.