Chunxiang Yao

Hypervalent ammonium radicals. Competitive N–C and N–H bond dissociations in methyl ammonium and ethyl ammonium

The title hypervalent ammonium radicals were investigated by neutralization-reionization mass spectrometry and quantum chemical calculations. Methyl ammonium (1) forms a small fraction of metastable radicals from isotopomers CH3ND3 (la) and CD3NH3 (1b) when these are produced by femtosecond electron transfer to vibrationally excited precursor cations. The branching ratios for dissociations of the N-C and N-(H,D) bonds in 1 favor the latter, k(N-C)/k(N-H) = 0.39. The experimental results are in accord with ab initio/RRKM calculations that quantitatively reproduce the branching ratios for dissociations of 1. A small fraction of high-energy 1 dissociates to form ammonium methylide, -CH2NH3+. Ethyl ammonium (2) and its CH3CH2ND3 isotopomer (2a) dissociate completely on the microsecond time scale. The branching ratios for dissociations of the N-C and N-(H,D) bonds favor the former, k(N-C)/k(N-H) = 2.04. This result is incompatible with the calculated potential energy surface of the ground doublet electronic state in 2 and is attributed to the formation and dissociations of excited electronic states.

Histidine-containing radicals in the gas phase.

Radicals containing the histidine residue have been generated in the gas phase by femtosecond electron transfer to protonated histidine-N-methylamide (1H+), Nalpha-acetylhistidine-N-methylamide (2H+), Nalpha-glycylhistidine (3H+), and Nalpha-histidylglycine (4H+). Radicals generated by collisional electron transfer from dimethyldisulfide to ions 1H+ and 2H+ at 7 keV collision energies were found to dissociate completely on the microsecond time scale, as probed by reionization to cations. The main dissociations produced fragments from the imidazole side chain and the cleavage of the C(alpha)CO bond, whereas products of NCalpha bond cleavage were not observed. Electron transfer from gaseous potassium atoms to ions 3H+ and 4H+ at 2.97 keV collision energies not only caused backbone NCalpha bond dissociations but also furnished fractions of stable radicals that were detected after conversion to anions. Ion structures, ion-electron recombination energies, radical structures, electron affinities, and dissociation and transition-state energies were obtained by combined density functional theory and Møller-Plesset perturbational calculations (B3-PMP2) and basis sets ranging from 6-311+G(2d,p) to aug-cc-pVTZ. The Rice-Ramsperger-Kassel-Marcus theory was used to calculate rate constants on the B3-PMP2 potential energy surfaces to aid interpretation of the mass spectrometric data. The stability of Nalpha-histidylglycine-derived radicals is attributed to an exothermic isomerization in the imidazole ring, which is internally catalyzed by reversible proton transfer from the carboxyl group. The isomerization depends on the steric accessibility of the histidine side chain and the carboxyl group and involves a novel cation radical-COO salt-bridge intermediate.

Hydrogen Atom Addition to Cytosine, 1-Methylcytosine, and Cytosine−Water Complexes. A Computational Study of a Mechanistic Dichotomy

Combined ab initio and density functional theory calculations at the B3-MP2/6-311++G(3df,2p) level of theory are used to investigate the structures and energetics of radicals produced by hydrogen atom addition to cytosine tautomers, 1-methylcytosine, and cytosine−water complexes. H-atom adducts to the N-3 positions are the most stable radical isomers derived from cytosine tautomer (1), 1-methylcytosine, and cytosine−water complexes in the gas phase. Solvent effects on radical stabilities are addressed by calculations that use the polarizable continuum model. Solvation by bulk water favors C-5 and C-6 adducts which have free energies in water that are comparable to those of the N-3 adducts. H-atom additions to the C-5 positions have the lowest activation energies for all cytosine derivatives under study and are predicted to be kinetically predominant. H-atom additions to the N-3 and C-6 positions are solvent dependent. In the absence of solvation, N-3 is more reactive than C-6 in cytosine and 1-methylcytos...