Voislav Blagojevic

Identification of a Higher-Order Organozincate Intermediate Involved in Negishi Cross-Coupling Reactions by Mass Spectrometry and NMR Spectroscopy

Negishi cross-coupling reactions were analyzed in solution by mass spectrometry and NMR spectroscopy to identify both the effect of LiBr as an additive as well as the purpose of 3-dimethyl-2-imidazolidinone (DMI) as a co-solvent. The results suggest that the main role of DMI is to facilitate a higher order bromozincate formation during the addition of LiBr.

Catalytic oxidation of H2 by N2O in the gas phase: O-atom transport with atomic metal cations.

Twenty-five atomic cations, M (+), that lie within the thermodynamic window for O-atom transport catalysis of the oxidation of hydrogen by nitrous oxide, 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 4 of these 25 atomic cations were seen to be catalytic: Fe (+), Os (+), Ir (+), and Pt (+). Two of these, Ir (+) and Pt (+), are efficient catalysts, while Fe (+) and Os (+) are not. Eighteen atomic cations (Cr (+), Mn (+), Co (+), Ni (+), Cu (+), Ge (+), Se (+), Mo (+), Ru (+), Rh (+), Sn (+), Te (+), Re (+), Pb (+), Bi (+), Eu (+), Tm (+), and Yb (+)) react too slowly at room temperature either in their oxidation with N 2O to form MO (+) or in the reduction of MO (+) by H 2. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. Three alkaline-earth metal monoxide cations, CaO (+), SrO (+), and BaO (+), were observed to favor MOH (+) formation in their reactions with H 2. A potential-energy landscape is computed for the oxidation of H 2 with N 2O catalyzed by Fe (+)( (6)D) that vividly illustrates the operation of an ionic catalyst and qualitatively accounts for the relative inefficiency of this catalyst.

Nitrogen dioxide reactions with atomic lanthanide cations and their monoxides: gas-phase kinetics at room temperature

Results of experimental investigations are reported for the gas-phase kinetics of chemical reactions between nitrogen dioxide (NO(2)) and 14 different atomic cations of the lanthanide series, Ln(+) (Ln = La-Lu, excluding Pm), and their monoxides, LnO(+). Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer in helium buffer-gas at a pressure of 0.35 +/- 0.01 Torr and at 295 +/- 2 K. The atomic lanthanide cations were produced at ca. 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction with NO(2). The atomic ions were observed to react rapidly with NO(2) with large rate coefficients, k > 2 x 10(-10) cm(3) molecule(-1) s(-1), and almost exclusively by oxygen-atom abstraction to produce lanthanide-oxide LnO(+) cations. In contrast to results of previous studies with many other molecules, the reaction efficiency exhibits essentially no dependence upon the energy required to promote an electron to achieve a d(1)s(1) excited electronic configuration, in which two non-f electrons are available to Ln(+) for chemical bonding. Apparently the radical character of NO(2) (X (2)A(1)) leads to the efficient formation of LnO(+) by the end-on abstraction of an oxygen atom by Ln(+). In the reactions with La(+), Ce(+), Pr(+) and Gd(+) an additional minor channel (less than 2%) leads to the formation of NO(+). The LnO(+) product ions participate in various secondary and higher order reactions with NO(2) resulting in the formation of ions of the type LnO(x)(NO)(y)(NO(2))(z)(+) with x = 1-2, y = 0-2, and z = 0-2, as well as the ions NO(+) and NO(2)(+).

Structure of [Pb(Gly-H)]+ and the monosolvated water and methanol solvated species by infrared multiple-photon dissociation spectroscopy, energy-resolved collision-induced dissociation, and electronic structure calculations.

Infrared multiple-photon dissociation (IRMPD) spectroscopy, collision-induced dissociation mass spectrometry, and theoretical calculations are combined to provide new insights into the structure and dissociation of lead(II) complexed with the conjugate acid of the amino acid glycine ([Pb(Gly-H)](+)) in the presence and absence of solvent. Unexpectedly, these experiments show the main site of lead(II) coordination to be the deprotonated amino group of glycine, with additional coordination to the carbonyl group. In such a structure lead(II) can act as an effective conduit for proton/hydrogen shifts, making H(2)O loss competitive with that of CO in the [Pb(Gly-H)](+) complex and leading to solvent deprotonation and formation of [PbOR(Gly)](+) (R = H, CH(3)) ions when solvent is present in the complex. The structural assignments based on IRMPD spectroscopy are complemented with isotopic labeling experiments (H(2)(18)O) and experiments done on the ethyl ester of glycine.

Nitrogen dioxide reactions with 46 atomic main-group and transition metal cations in the gas phase: room temperature kinetics and periodicities in reactivity.

Experimental results are reported for the gas-phase room-temperature kinetics of chemical reactions between nitrogen dioxide (NO(2)) and 46 atomic main-group and transition metal cations (M(+)). Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer in helium buffer gas at a pressure of 0.35 ± 0.01 Torr and at 295 ± 2 K. The atomic cations were produced at ca. 5500 K in an ICP source and allowed to decay radiatively and to thermalize to room temperature by collisions with Ar and He atoms prior to reaction with NO(2). Measured apparent bimolecular rate coefficients and primary reaction product distributions are reported for all 46 atomic metal cations and these provide an overview of trends across and down the periodic table. Three main types of reactions were observed: O-atom transfer to form either MO(+) or NO(+), electron transfer to form NO(2)(+), and addition to form MNO(2)(+). Bimolecular O-atom transfer was observed to predominate. Correlations are presented between reaction efficiency and the O-atom affinity of the metal cation and between the prevalence of NO(+) product formation and the electron recombination energy of the product metal oxide cation. Some second-order reactions are evident with metal cations that react inefficiently. Most interesting of these is the formation of the MNO(+) cation with Rh(+) and Pd(+). The higher-order chemistry with NO(2) is very diverse and includes the formation of numerous NO(2) ion clusters and a number of tri- and tetraoxide metal cations. Group 2 metal dioxide cations (CaO(2)(+), SrO(2)(+), BaO(2)(+)) exhibit a unique reaction with NO(2) to form MO(NO)(+) ions perhaps by NO transfer from NO(2) concurrent with O(2) formation by recombination of a NO(2) and an oxide oxygen.

Role of (NO)2 dimer in reactions of Fe+ with NO and NO2 studied by ICP-SIFT mass spectrometry.

In a recent publication by J. J. Melko et al. (J. Phys. Chem. A2012, 116, 11500-11508) on the reactions of Fe(+) cations with NO and NO2, these authors made a number of assertions regarding the work previously published in our laboratory. Melko et al. assert that our previously reported data was erroneously analyzed, resulting in our misreporting of the Fe(+) + NO2 reaction branching ratio for NO(+). Also, they proposed that this alleged misreporting made it likely for the second-order chemistry observed in our Fe(+) + NO experiments to be a product of an impurity of NO2 in our NO reagent and, furthermore, that our reported rate coefficient for the effective second-order chemistry was unreasonably high on the basis of their model calculations. Despite extensive private communications in which we presented detailed data supporting our original data analysis to Melko et al., these authors proceeded to publish their critique without any reference to this data. Here, we present the data communicated by us to Melko et al. and show that our result reported earlier for the Fe(+) + NO2 reaction branching ratio to form NO(+) is accurate and, furthermore, that there is no evidence for a sufficient NO2 impurity in any of our NO experiments. We suggest that the discrepancy in the results observed by us and Melko et al. may be attributed to a reaction with the dimer (NO)2. This possibility was dismissed in our earlier work as the dimer concentration under the flow tube conditions was calculated to be below 10(-5)% of the monomer, but the new results of J. J. Melko et al. raise the dimer reaction as a real possibility. Finally, J. J. Melko et al. appear to have misunderstood the mechanism of the second-order NO chemistry that we had proposed.

Gas-phase kinetic measurements and quantum chemical calculations of the ligation of Ni+, Cu+, Ni+(pyrrole)1,2 and Cu+(pyrrole)1,2 with O2 and CO

The rate and equilibrium kinetics of the reactions of M + ,M + (pyrrole) and M + (pyrrole)2 (M = Ni, Cu) with the small diatomic ligands O2 and CO have been investigated in the gas phase at 295 ± 2 K in helium buffer gas at a pressure of 0.35 ± 0.01 Torr. The measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP-SIFT) tandem mass spectrometer. Only ligation was observed. While atomic Cu + was observed to bind up to two ligands of O2 and CO, atomic Ni + was observed to bind up to three. The presence of one molecule of pyrrole dramatically increases the gas-phase rate of metal-ion ligation except for the ligation of Cu + with O2 .N i + (pyrrole)2 and Cu + (pyrrole)2 were found to be unreactive with O2, k< 1.0 × 10 −13 cm 3 molecule −1 s −1 , but both ions were observed to ligate a single molecule of CO. While equilibrium was observed to be approached in several of the ligation reactions, an absolute value for the standard free energy of ligation could be obtained only for the ligation of Ni + (pyrrole)(CO) with CO. Quantum chemical calculations using density functional theory (DFT) with the B3LYP (Becke-3 Lee–Yang–Parr) hybrid functional have provided insight into the energetics and geometries of ligation. The bonding of pyrrole to either Ni + or Cu + is much stronger than bonding of either O2 or CO. This agrees with the failure to observe experimentally any ligand-switching reactions involving the pyrrole ligand. Also, the computations show that the ligation of pyrrole does not significantly change the ligation energy of O 2 and CO to the metal ions. The various isomers of CO-containing complexes were investigated and it was found that metal–C bonding was always thermodynamically favored over metal–O bonding. The computations also show that the addition of a ligand of O2 or CO can skew the symmetry inherent in M + –pyrrole complexes (but less so with O2) by shifting the position of the metal ion relative to the midline of the pyrrole molecule. The structures determined for the various metal ion–CO complexes were found to have a linear M–CO geometry, while structures of metal ion–O2 complexes were found to have a bent M–O2 geometry.

Locating Pb2+ and Zn2+ in Zinc Finger-Like Peptides Using Mass Spectrometry

AbstractThe binding preferences of Pb2+and Zn2+ in doubly charged complexes with zinc finger-like 12-residue peptides (Pep), [Mn(Pep-2(n-1)H)]2+ have been explored using tandem mass spectrometry. The peptides were synthesized strategically by blocking the N-terminus with an acetyl group and with four cysteine and/or histidine residues in positions 2, 5, 8, and 11, arranged in different motifs: CCHH, CHCH, and CCCC. The MS2 spectra of the Pb2+ and Zn2+ complexes show multiple losses of water and a single methane loss and these provide a sensitive method for locating the metal dication and so elucidating its coordination. The elimination of a methane molecule indicated the position of the metal at the Cys2 residue. Whereas lead was observed to preferentially bind to cysteine residues, zinc was found to primarily bind to histidine residues and secondarily to cysteine residues. Preferential binding of lead to cysteine is preserved in the complexes with more than one Pb2+. Key to the mechanism of the loss of water and methane is the metal dication withdrawing electrons from the proximal amidic nitrogen. This acidic nitrogen loses its hydrogen to an amidic oxygen situated four atoms away leading to formation of a five-member ring and the elimination of water.

Gas-Phase Kinetic Measurements of the Ligation of Ni+, Cu+, Ni(Pyrrole)1,2+ and Cu(Pyrrole)1,2+ with CO2, D2O, NH3 and NO

The rate and equilibrium kinetics of the reactions of the biologically important metal species M+, M+(pyrrole) and M+(pyrrole)2 (M = Ni, Cu) have been investigated with the biological gases CO2, D2O, NH3 and NO in the gas phase at 295 ± 2 K in helium buffer-gas at a pressure of 0.35 ± 0.01 Torr. The measurements were taken with an Inductively Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Only ligation was observed for the reactions of bare Ni+ and Cu+ with CO2, D2O and NH3 with rates consistent with the known strengths of the resulting ligand–metal bonds. Both metal cations appeared to be oxidized and produce N2O in interesting reactions that are second order in NO. One pyrrole ligand was observed to increase the rate of ligation by as much as a factor of 100 and to switch off the oxidation with NO. Equilibrium was achieved for the ligation of CO2, D2O and NO to both Ni+(pyrrole) and Cu+(pyrrole), and so it was possible to determine absolute values for the standard free energies of ligation. No ligand substitution was observed with M+(pyrrole). M+(pyrrole)2 was observed to be generally unreactive towards the small molecules investigated: a notable exception is ammonia. Very fast ligand substitution reactions were observed for reactions of M+(pyrrole)2 with NH3.

Atmospheric pressure chemical ionization mass spectrometry of pyridine and isoprene: potential breath exposure and disease biomarkers.

Volatile organic compounds (VOCs) in exhaled human breath can serve as potential disease-specific and exposure biomarkers and therefore can reveal information about a subjects health and environment. Pyridine, a VOC marker for exposure to tobacco smoke, and isoprene, a liver disease biomarker, were studied using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). While both molecules could be detected in low-ppb levels, interactions of the ionized analytes with their neutral forms and ambient air led to unusual ion/molecule chemistry. The result was a highly dynamic system and a nonlinear response to changes in analyte concentration. Increased presence of ambient water was found to greatly enhance the detection limit of pyridine and only slightly decrease that of isoprene. APCI-MS is shown to be a promising analytical tool in breath analysis with good detection limits, but its application requires a better understanding of the ion/molecule chemistry that may affect VOC quantification from a chemically complex system such as human breath.