Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.

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.

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a-c and 11a-c) provides strong evidence for C-Se hyperconjugation (σ(C-Se)-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (n(Se)-π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (n(Se)-σ*(CC)) involving the selenium p-type lone pair electrons and the polarized CH(2)-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C-Se hyperconjugation (σ(C-Se)-π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (n(Se)-π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (n(Se)-σ*(CC)) is relatively weak in all ions.

2,4-Difluoro-, 2,4,6-trifluoro-, and 2,3,4,6,tetrafluoronitrobenzenes undergo nucleophilic aromatic substitution, once, twice, and three times with a variety of amine substituents with a high degree of regiochemical control to provide a range of electron-rich nitrobenzene derivatives. In these structures the nitro group proves a useful structural probe to reveal the varying extents of electron donation from the varying number of amino substituents onto the nitro group as revealed by accurate low temperature X-ray crystal structure analysis, thus increasing electron donation manifests in a decrease in the Ar–NO2 distance consistent with increased double bond character, while the N–O bond distance increases as the oxygens accept the electron density. The effect of delocalization of the aniline nitrogen lone pair onto the nitro group impacts on the geometry and hybridization of the nitrogen substituent and also impacts on the ability of the nitrogen lone pair electrons to participate in other competing electronic interactions, such as the nN–σ*C–S anomeric effect as demonstrated by the thiazolidine substituted derivatives 3c, 4c, and 6c.