Room-temperature reactions of 46 different atomic cations with N 2 O have been surveyed systematically using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations are produced at about 5500 K in the ICP source and are allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic cations from K + to Se + , of second-row atomic cations from Rb + to Te + (excluding Tc + ), and of third-row atomic cations from Cs + to Bi + . Primary reaction channels were observed corresponding to O-atom transfer, N-atom transfer, and N 2 O addition. Periodicities were observed in overall reaction efficiency, and these are scrutinized in terms of overall exothermicity, the presence of an activation barrier in the reaction coordinate, and the overall conservation of spin. N-atom transfer was observed to compete with O-atom transfer in the reactions of N 2 O with La + (4%), Ti + (22%), Zr + (45%), Nb + (35%), Ta + (40%), and Os + (60%). Up to three N 2 O molecules were observed to add sequentially to selected atomic cations as well as several monoxide and dioxide cations. A second O-atom transfer was observed with the group 4, 5, and 6 transition-metal ions (except Mo + ) as well as the third-row cations Re + , Os + , Ir + , and Pt + . The atomic ions W + , Os + , and Ir + formed trioxides in sequential O-atom transfer reactions, and Os + even formed the tetroxide OsO 4 + . Multicollision-induced dissociation studies with Ar buffer gas indicated thermodynamically controlled dissociation of TiO 2 + , ZrO 2 + , HfO 2 + , VO 2 + , NbO 2 + , TaO 2 + , and WO 2 + by the consecutive detachment of O-atoms while CrO 2 + , ReO 2 + , and PtO 2 + decomposed primarily by loss of O 2 molecules.

Molecular radical cations, M •+ , of amino acids and oligopeptides are produced by collision-induced dissociation of mixed complex ions, [Cu II (dien)M] •2+ , that contain Cu II , an amine, typically diethylenetriamine (dien), and the oligopeptide, M. With dien as the amine ligand, abundant M •+ formation is observed only for the amino acids tryptophan and tyrosine, and oligopeptides that contain either the tryptophanyl or tyrosyl residue. Dissociation of the M •+ ion is rich and differs considerably from that of protonated amino acids and peptides. Facile fragmentation occurs around the R-carbon of the tryptophanyl residue. Cleavage of the N-C R bond and proton transfer from the exocyclic methylene group in the side chain to the N-terminal residue results in formation of the [z n -H] •+ ion and elimination of the N-terminal fragment as ammonia or an amide, depending on the position of the tryptophanyl residue. Cleavage of the C R -C bond of an oligopeptide containing a C-terminal tryptophan residue and proton transfer from the carboxylic group to the N-terminal fragment (a carbonyl oxygen atom) results in formation of the [a n + H] •+ ion and elimination of carbon dioxide. Both types of fragmentation have no analogous reactions in protonated peptides. For the M •+ of tryptophanylglycylglycine, WGG, elimination of the tryptophanyl side chain results in GGG •+ . This radical cation fragments by eliminating its C-terminal glycine to give the [b 2 -H] •+ ion, which is an oxazolone and shares much of the structure and reactivity of the b 2 + ion from protonated triglycine. Density functional theory shows the mechanism of forming the [b 2 -H] •+ ion is similar to that of the b 2 + ion, although the free-energy barrier at 29.4 kcal/mol is lower. The [b 2 -H] •+ ion eliminates CO readily to give the [a 2 -H] •+ ion, which is an iminium radical ion.

Atomic cations (26), M+, have been shown to lie within a thermodynamic window for O-atom transport catalysis of the reduction of N2O by CO and have been checked for catalytic activity at room temperature with kinetic measurements using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Only 10 of these 26 atomic cations were seen to be catalytic: Ca+, Fe+, Ge+, Sr+, Ba+, Os+, Ir+, Pt+, Eu+, and Yb+. The remaining 16 cations that lie in the thermodynamic window (Cr+, Mn+, Co+, Ni+, Cu+, Se+, Mo+, Ru+, Rh+, Sn+, Te+, Re+, Pb+, Bi+, Tm+, and Lu+) react too slowly at room temperature either in the formation of MO+ or in its reduction by CO. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. A new measure of efficiency is introduced for catalytic cycles that allows the discrimination between catalytic cations on the basis of the efficiencies of the two legs of the catalytic cycle. Also, a potential-energy landscape is computed for the reduction of N2O by CO catalyzed by Fe+(6D) that vividly illustrates the operation of an ionic catalyst.

Reaction mechanisms for the formation of the keto-form of oxyluciferin (OxyLH(2)) from the luciferin of fireflies via a dioxetanone intermediate are predicted using the B3LYP/6-31G theoretical method. The ring opening of a model dioxetanone and the decarboxylation proceed in one step via a singlet diradical transition structure with an activation barrier of 18.1 and an exothermicity of 90.8 kcal/mol. The S(0) --> S(1) vertical excitation energies predicted with time dependent density functional theory, TDDFT B3LYP/6-31+G, for the anionic and neutral forms of OxyLH(2) are in the range of 60 to 80 kcal/mol. These energetic results support the generally accepted theory of chemically initiated electron exchange luminescence (CIEEL). The chemical origin of the multicolor bioluminescence from OxyLH(2) is examined theoretically using the TDDFT B3LYP/6-31+G, ZINDO//B3LYP/6-31+G, and CIS/6-31G methods. A change in color of the light emission upon rotation of the two rings in the S(1) excited state of OxyLH(2) is unlikely because both possible emitters, the planar keto- and enol-forms, are minima on the S(1) potential energy surface. The participation of the enol-forms of OxyLH(2) in bioluminescence is plausible but not required to explain the multicolor emission. According to predictions at the TDDFT B3LYP level, the color of the bioluminescence depends on the polarization of the OxyLH(2) in the microenvironment of the enzyme-OxyLH(2) complex.

Gas-phase dissociation reactions of protonated amino acidsphenylalanine, tyrosine, tryptophan, and histidineare rich and diverse. Considerable similarities exist among the four amino acids, but there are also significant differences. Facile reactions include the elimination of NH3, common to all aromatic amino acids except histidine, and the concomitant elimination of H2O and CO. Labeling experiments with deuteriums show considerable H/D scrambling prior to dissociation involving N−H, O−H, and C−H (both aliphatic and aromatic hydrogens). Mechanisms of this scrambling are proposed. At higher collision energies, eliminations of H2O, CO, CO2, and CH2CO occur after that of NH3. Similarly, eliminations of HCN, HCNH2, and NH3 occur after that of H2O and CO. The elimination of CH2CO is preceded by migration of the hydroxyl ion from the carboxylic group to the exocyclic carbon on the side chain. Aromatic amino acids, with the exception of tyrosine, were observed to yield cationic radical fragments by eliminating small radicals, including H•, CH3 •, and NHCH•.