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Michael T Bowers - One of the best experts on this subject based on the ideXlab platform.

  • Sigma Bond activation by transition metal ions the co ch4 n systems revisited
    International Journal of Mass Spectrometry, 2001
    Co-Authors: Qiang Zhang, Paul R Kemper, Seung Koo Shin, Michael T Bowers
    Abstract:

    Abstract Measurements are reported for sequential clustering of CH 4 to Co + ions under equilibrium conditions. The CH 4 cluster Bond strengths show a pairwise behavior: −Δ H 0 0 = 23.1 and 25.3 kcal/mol for n = 1 and 2; 7.3 and 5.2 kcal/mol for n = 3 and 4; and ∼2 kcal/mol for both n = 5 and 6. This pairwise behavior is well reproduced by large basis set density functional theory calculations. These calculations indicate n = 1 and n = 2 add on opposite sides of the Co + ion in η 2 configuration and induce significant s/d hybridization on Co + . This hybridization both reduces Pauli repulsion and fosters Sigma donation into the 4 s orbital on Co + . Clusters n = 3 and n = 4 add at 90° to the n = 1 and 2 line of centers forming a planar system. The s/d hybridization is unfavorable for these clusters resulting in longer Co + –C Bond lengths and substantially reduced binding energies. To n = 5 and 6 ligands probably complete a pseudo octahedral complex and are very weakly bound, perhaps defining a second solvation shell. An impurity contributed substantially to the experimental peak at m/z = 123 corresponding to Co + (CH 4 ) 4 . The impurity was tentatively identified as O 2 Co + (CH 4 ) 2 and experimental protocals were developed to eliminate its impact on the data reported here. It is suggested this impurity could be responsible for published guided ion beam results that found a substantially larger binding energy for n = 4 than for n = 3 in contrast to what is reported here.

  • electronic state effects in Sigma Bond activation by first row transition metal ions the ion chromatography technique
    1996
    Co-Authors: Petra A M Van Koppen, Paul R Kemper, Michael T Bowers
    Abstract:

    The ability of transition metal ions to activate C—H and C—C Bonds of small saturated hydrocarbons has been attributed to the high density of lowlying excited states available to the transition metal center. The details of the interactions of these metal ion states with the reactant molecule, however, are not well understood and the desire to understand these details has stimulated significant experimental and theoretical interest. On the experimental side, gas-phase transition metal ion chemistry has been explored extensively [1–3] and recently state specific studies have been carried out [4–17]. Theoretically, ab initio electronic structure calculations provided insight regarding the nature of the Bonding and trends in Bond energies [18–25]. In addition, ab initio potential energy surfaces allow chemists to explore the interaction of the low-lying excited states of metal ions with reactant molecules [26]. Both theory and experiment indicate the reactivity of a given state of the metal ion will depend on its interaction with other nearby electronic states. Surface crossings are commonplace and “spin-forbidden” reactions are often observed for transition metal ions (spin is generally conserved for bimolecular reactions involving light elements, but only the total angular momentum must be rigorously conserved for heavier elements) [27]. Because reactions involving atomic transition metal ions are inherently complex, quantitative state specific experimental studies along with theoretical calculations are necessary to understand their reaction energetics and mechanisms.

  • cluster ions carbon met cars and Sigma Bond activation
    Accounts of Chemical Research, 1994
    Co-Authors: Michael T Bowers
    Abstract:

    Cluster ion chemistry is reviewed here on three fronts. These are: the formation of fullerenes, the formation of met-cars, and the {Sigma} Bond activation by transition metal clusters.

Gregory I Gellene - One of the best experts on this subject based on the ideXlab platform.

  • Sigma Bond activation by cooperative interaction b ch4 nh2
    Journal of Physical Chemistry A, 2003
    Co-Authors: Gregory I Gellene
    Abstract:

    The reactions of B+ + CH4 + nH2 (n = 1, 2) to produce B+ Sigma-Bonded insertion products have been studied by high-level ab initio computational techniques. The results demonstrate that the mechani...

  • Sigma Bond activation by cooperative interaction with s2 atoms b nch4 n 1 2
    Faraday Discussions, 2001
    Co-Authors: Janeanne C Bell, Gregory I Gellene
    Abstract:

    Ab initio investigations at the MP2 and CCSD(T) level with augmented double and triple zeta basis sets have identified various stationary points on the B+/nCH4, n = 1, 2 hypersurfaces. The electrostatic complexes show a strong variation in the sequential binding energy with De for the loss of one CH4 molecule calculated to be 16.5 and 6.8 kcal mol-1 for the n = 1 and n = 2 complexes, respectively. The covalent molecular ion, CH3BH+, is found to have the expected C3 nu geometry and to be strongly bound by 84.0 kcal mol-1 with respect to B+ + CH4. The interaction of CH4 with CH3BH+ is qualitatively very similar to the interaction of CH4 with HBH+, however, the binding is only about 50% as strong due to the electron donating characteristic of the methyl group. Of particular interest are the insertion transition states which adopt geometries allowing the B+ ion to interact with multiple Sigma Bonds. In the n = 1 case, the interaction with two CH Bonds lowers the insertion activation energy by about 25 kcal mol-1 from that expected for a mechanism involving only one Sigma Bond. For n = 2, B+ interacts with two CH Sigma Bonds from one CH4 and one CH Sigma Bond from the other CH4 leading to an additional activation energy decrease of about 15.7 kcal mol-1 relative to B+ + nCH4.

  • Sigma Bond activation by cooperative interaction with ns2 atoms b nh2
    Journal of Physical Chemistry A, 1998
    Co-Authors: Stephanie B Sharp, Blake Lemoine, Gregory I Gellene
    Abstract:

    The reactions of B+ + nH2 to produce BH2+(H2)n-1 have been studied by high-level ab initio techniques. The reaction mechanism and associated activation energy is found to depend dramatically on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise:  first breaking the H2 Bond and forming one BH Bond followed by forming the second BH Bond. This process has an activation energy of about 57 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism though a planar cyclic transition state where two H2 Bonds are broken while simultaneously two BH Bonds and one new H2 Bond are formed. The activation energy for this process decreases dramatically from the n =1 value to only about 11 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism; however, the actual insertion occurs late in the reaction after over 75% of the exothermicity has been realized. The addition of the third H2 molecule decreases the activation energy to only about 3.4 kcal/mol. For n = 4,...

Christopher J Douglas - One of the best experts on this subject based on the ideXlab platform.

Ashley M Dreis - One of the best experts on this subject based on the ideXlab platform.

Hee Gweon Woo - One of the best experts on this subject based on the ideXlab platform.

  • Sigma Bond metathesis mechanism for dehydropolymerization of silanes to polysilanes by d0 metal catalysts
    Journal of the American Chemical Society, 1992
    Co-Authors: Hee Gweon Woo, John F Walzer, Don T Tilley
    Abstract:

    A mechanism for the dehydropolymerization of hydrosilanes to polysilanes, as catalyzed by early-transition-metal metallocene derivatives, is proposed. This mechanism is based on two a-Bond metathesis reactions that pass through four-center transition states: (1) the dehydrometalation of silane, H(SiHR) n H, with a metal hydride to give hydrogen and a silyl derivative, M(SiHR) n H, and (2) coupling of the metal silyl derivative with more hydrosilane, H(SiHR) m H, to produce H(SiHR) n (SiHR) m H and regenerate the active metal hydride catalyst

  • Sigma Bond metathesis reactions for d0 metal silicon Bonds that produce zirconocene and hafnocene hydrosilyl complexes
    Journal of the American Chemical Society, 1992
    Co-Authors: Hee Gweon Woo, Richard H Heyn, T D Tilley
    Abstract:

    Reactions of zirconocene and hafnocene silyl derivatives CpCp'M(SiR 3 )Cl (Cp=η 5 -C 5 Me 5 ; M=Zr or Hf; R=Me or SiMe 3 ) with hydrosilanes have been investigated. The observed products depend on the nature of the starting materials, since in some cases the initial σ-Bond metathesis products react further via dehydrocoupling processes. For example, Cp Z r(SiMe 3 )Cl reacts with PhSiH 3 to gtive the direct products Me 3 SiH 2 Ph)Cl, which then combines rapidly with PhSiH 3 to produce [Cp 2 ZrHCl] n ,PhH 2 Si-SiH 2 Ph, and PhH 2 Si-SiH 2 Ph