Andreas J. Illies

Theoretical study of the proton affinity of N2O

The proton affinity of N2O has been calculated with the combination of theoretical methods called Gaussian-1 theory (567·7 kJ mol-1) and Gaussian-2 theory (570·4 kJ mol-1). The site of protonation at the highest levels of theory (G1 and G2 theory) is predicted to be oxygen; however, this prediction is very sensitive to the level of electron correlation. At MP3, MP4SDQ, and QCISD(T) levels of electron correlation, O protonation is preferred, whereas at MP2 and MP4SDTQ levels, N protonation is preferred. At the MP4 level, the effect of triple excitations on relative energies is overestimated and strongly favors the HNNO+ isomer over the HONN+ isomer. At the QCISD level, the effect of triple excitations also favors the HNNO+ isomer, but it is not sufficiently large to cause the N protonated isomer to become more stable. Evidence is presented indicating that biradical character in HNNO+ leads to a nonlinear NNO bond angle.

Experimental and theoretical investigation into the structural and thermodynamic properties of the mixed methyl halide association radical cations

An experimental and theoretical study of the ion/molecule association reactions of methyl halide radical cations with mixed methyl halides (X, Y = I, Br, Cl, F) has been carried out. MS/MS unimolecular and collision-induced dissociation experiments were performed on the association products and provide strong evidence for formation of a two-center three-electron (2c-3e) bonded structure, [CH3X∴YCH3]·+, for all association products excluding [C2H6FCl]·+. The [C2H6FCl]·+ results suggest the following atomic connectivity for this association product: [CH2Cl–H–FCH3]·+. Two metastable fragmentation pathways were observed for [CH3I∴BrCH3]·+ and [CH3Br∴ClCH3]·+: direct cleavage of the 2c-3e bond and elimination of a methyl group. Only direct cleavage was observed in [CH3I∴ClCH3]·+, [CH3I∴FCH3]·+, and [CH3Br∴FCH3]·+. Kinetic energy release distributions were measured and unimolecular kinetic modeling studies were performed on the metastable reaction pathways. Unimolecular kinetic modeling was carried out using phase space calculations and molecular parameters from density functional theory (DFT) calculations. DFT was also used to investigate the potential energy surfaces for each system.

On the structures and fragmentation of [Me2S∵SMe2]+ and [Et2S∵SEt2]+: Tandem mass spectrometry metastable and collision-induced dissociation results

An experimental study of the metastable and collision-induced dissociation (CID) spectra of [Me2S∵SMe2]+ and [Et2S∵SEt2]+ has been carried out. These ions are examples of species that contain two-center-three-electron (2c–3e) sulfur-sulfur bonds. The metastable and CID spectra provide experimental evidence of the atomic connectivity and of the 2c–3e bonds. The metastable cleavage of the S∵S 2c–3e bonds appears to occur with no reverse activation barriers and to result in small average kinetic energy releases. Fragmentation of the same bonds by CID results in the most intense product for both ions. Comparisons with the metastable and CID spectra of [MeSSMe]+, an ion with a two-center-two-electron (2c–2e) sulfur-sulfur bond, are made and strongly support the difference in the sulfur-sulfur bonding.