Ion−molecule reactions between thiiranium ion 11 (m/z 213) and cyclohexene and cis-cyclooctene resulted in the formation of addition products 17a and 17b (m/z 295 and m/z 323, respectively) via an electrophilic addition pathway. Associative π-ligand exchange involving direct transfer of the PhS + moiety, which has been observed for analogous seleniranium ions in the gas phase, did not occur despite previous solution experiments suggesting it as a valid pathway. DFT calculations at the M06-2X/def2-TZVP level of theory showed high barriers for the exchange reaction, while the addition pathway was more plausible. Further support for this pathway was provided with Hammett plots showing the rate of reaction to increase as the benzylic position of thiiranium ion derivatives became more electrophilic (ρ = +1.69; R 2 = 0.974). The more reactive isomeric sulfonium ion 22 was discounted as being responsible for the observed reactivity with infrared spectroscopy and DFT calculations suggesting little possibility for isomerization. To further explore the differences in reactivity, thiiranium ion 25 and sulfonium ion 27 were formed independently, with the latter ion reacting over 260 times faster toward cis-cyclooctene than the thiiranium ion rationalized by calculations suggesting a barrierless pathway for sulfonium ion 27 to react with the cycloalkene.

The gas-phase ion–molecule identity exchange reactions of phenyl chalcogen iranium ions with alkenes have been examined experimentally in a linear ion trap mass spectrometer by isotope labeling experiments. The nature of both the alkene and the chalcogen play crucial roles, with the bimolecular rates for π-ligand exchange following the order: [PhTe­(c-C6H10)]+ + c-C6D10 > [PhTe­(C2D4)]+ + C2H4 > [PhSe­(c-C6H10)]+ + c-C6D10, with no reaction being observed for [PhSe­(C2D4)]+ + C2H4, [PhS­(C2D4)]+ + C2H4, and [PhS­(c-C6H10)]+ + c-C6D10. The experimental results correlate with RRKM modeling and density functional theory (DFT) calculations, which also demonstrates that these reactions proceed via associative mechanisms. Natural bond orbital (NBO) analysis reveals a shift in the association complexes from a σ-hole interaction to ones mirroring the π–p+ and n−π* at the transition state in accordance with the rates of reaction.

Haliranium ion reactivity with cyclic alkenes in the gas phase was investigated by examining how the nature of the halogen (X = Br or I) and the effect of ring strain affected the partitioning between π-ligand exchange and addition.

Despite numerous computational and experimental studies on the π‐ligand exchange reactions of chalcogen iranium ions, a classification of the reaction class has yet to be made. The characteristics of the transition states presented thus far suggested a coarctate nature with two bonds breaking and forming simultaneously at the chalcogen centre. The change in barrier height, depending on the nature of the chalcogen and the alkene, was initially attributed as a shift in the degree of aromaticity moving from pseudocoarctate to coarctate reactions. However, this paper suggests that all 12 reactions under consideration are pseudocoarctate with a Hückel number of delocalised π electrons based on comprehensive studies of the orbital interactions and magnetic properties both at the transition state and along the reaction path. The change in barrier height was largely driven by the electrophilicity of the chalcogen and the strain present in the three‐membered ring rather than a shift in the degree of aromaticity.

Observation of the [M + H] + ion in electrospray ionisation mass spectrometry (ESI-MS) for either product identification or further unimolecular reactivity studies is not always guaranteed. When a suitable donor is β to a good leaving group such as protonated alcohols, neighbouring group participation (NGP) is one process that can lead to facile in-source decomposition leading to threemembered ring product ions. In this study, lone pair rich heteroatoms are explored computationally as donors for the intramolecular displacement of either water or ammonia in protonated 2-heteroethanols and ethanamines, respectively. Density functional theory (DFT) calculations showed that for the halogens (Cl, Br and I) both the loss of water and ammonia are overall endothermic processes with modest barriers for the 2-haloethanol fragmentation by NGP. The chalcogens (S, Se and Te) displayed a significant shift with the loss of water becoming exothermic and the enthalpic barrier approximately zero in most cases leading to little [M + H] + ion observation. The loss of ammonia remained endothermic for both the chalcogens (S, Se and Te) and pnictogens (P, As and Sb), but the 2-pnictoethanol fragmentation showed mixed results as two pathways were accessible due to the increased s character of the pnictogen lone pair. Examination of the donor-acceptor interactions by natural bond orbital (NBO) theory reflected an increase in the heteroatom lone pair participation at the transition state, but that stabilisation due to the polarisable C X bond dominated and should not be discounted in processes believed to proceed by nonvertical neighbouring group participation.