H2 Elimination Products Research Articles

Low‐Energy Pathways Found for the NH3 Activation and H2 Elimination by the Werner‐Type Complexes M(NH3)4+ (M=Fe, Ru and Os).

AbstractThe interactions of NH3 with the transition metal complexes M(NH3)n+ (M=Fe, Ru, Os; n=0−4) were investigated theoretically with the aim of analyzing the factors that determine their reactivity patterns and product distributions. The reactions of ammonia with the low‐coordination complexes of the three metallic ions (n=1−3) lead only to the addition product M(NH3)n+1+. However, once the highest‐coordinated complex M(NH3)4+ is reached, each of the reactions evolves to metal‐inserted species through low energy channels. For osmium, the hydrogen abstraction from an additional NH3 molecule by the inserted intermediate H−Os(NH3)4‐NH2+ leads to H2 elimination products along a favorable pathway. The facile breaking of the strong N−H bond of ammonia by these Werner‐type complexes is by far the most striking finding emerging from this study. Hopefully, this could inspire future contributions in the areas of chemical and bioinorganic catalysis.

Density functional investigation of reaction of borohydride cation BH2+ with propylene.

The reactions of BH2+ with propylene (CH2=CHCH3) to form both the adducts BC3H8+ and the H2-elimination products BC3H6+ + H2 have been investigated at the density functional B3LYP/6-311G(d,p) level of theory. It is shown that the electrophilic attacks of BH2+ towards two olefinic carbons of H2C=CHCH3 and two subsequent 1,3-H-shifts may form four low-lying BC3H8+ isomers (with the relative energies in parentheses in kcal/mol): 1 BH2+.CH2CHCH3 (0.0), 1' BH2+.CH3CHCH2 (6.3), 3 BHCH2CH2CH3+ (4.3), and 4 BHCH(CH3)2+ (5.0), respectively. On the other hand, further H2-eliminations may also occur easily between B-C bonds of isomers 1 and 1' and between C-C bonds of isomers 3 and 4 to form two dissociation products (P1) HBCHCHCH3+ + H2 and (P2) HBC(CH3)CH2+ + H2, with H2-elimination from isomer 1 to be energetically most favorable. According to our calculated mechanism, the collisional stabilization processes of low-lying isomers 1, 1', 3, and 4 may compete extensively with their H2-eliminations processes for the title reaction, leading mainly to some linear carborane cations. This study may be helpful for understanding the stereochemical aspects of borohydride cations towards alkylenes.

Gas Phase Reactions Between SiH4 and B2H6: A Theoretical Study.

Gas-phase reactions of silane (SiH4) and diborane (B2H6) are investigated using ab initio calculations at the MP2/6-311++g** level. Initially SiH4 and B2H6 are only weakly attracted to each other. Under thermal activation, the two molecules can overcome an energy barrier of 33.87 kcal/mol to associate into a complex SiH4−BH3−BH3 with a hydrogen-bridged Si−H−B bond. Upon bonding, one of the two hydrogen-bridged bonds B−H−B in B2H6 is broken and the other becomes polarized. Started from the SiH4−BH3−BH3 complex, three comparable fragmentation pathways involving BH3 and H2 elimination produce several silaboranes with various silicon−boron−hydrogen ratios. A much higher barrier exists between the initial loosely bonded SiH4−B2H6 system and a direct H2 elimination product SiH3−B2H5 with Cs symmetry. The bonding nature in the species are further elucidated through topological analysis of electron density using the AIM theory. These intermediate silaboranes are possible precursors for chemical vapor deposition i...

Gas Phase Reactions between SiH4and B2H6: A Theoretical Study

Gas-phase reactions of silane (SiH4) and diborane (B2H6) are investigated using ab initio calculations at the MP2/6-311++g** level. Initially SiH4 and B2H6 are only weakly attracted to each other. Under thermal activation, the two molecules can overcome an energy barrier of 33.87 kcal/mol to associate into a complex SiH4−BH3−BH3 with a hydrogen-bridged Si−H−B bond. Upon bonding, one of the two hydrogen-bridged bonds B−H−B in B2H6 is broken and the other becomes polarized. Started from the SiH4−BH3−BH3 complex, three comparable fragmentation pathways involving BH3 and H2 elimination produce several silaboranes with various silicon−boron−hydrogen ratios. A much higher barrier exists between the initial loosely bonded SiH4−B2H6 system and a direct H2 elimination product SiH3−B2H5 with Cs symmetry. The bonding nature in the species are further elucidated through topological analysis of electron density using the AIM theory. These intermediate silaboranes are possible precursors for chemical vapor deposition i...

Velocity map imaging of ion-molecule reaction products: Co+(3F4)+isobutane

The velocity map imaging technique is applied to mass-selected CoC3H6++CH4 and CoC4H8++H2 elimination products from the Co+(3F4)+isobutane reaction studied under crossed-beam conditions at 0.21 eV collision energy. For both reactions we obtain the joint scattering probability distribution P(E,Θ), where E and Θ are the product translational energy and scattering angle. The fraction of available energy deposited into product translation is 0.4 for H2, compared with 0.1 for CH4. For the CH4 product, the angular distribution is forward-backwards symmetric and sharply peaked at Θ=0 and 180°. P(E,Θ) is not separable into the product of an energy and an angular function; rather, the angular distribution peaks more sharply at higher translational energy. Evidently, incipient CoC3H6++CH4 products equilibrate in the Co+(C3H6)(CH4) exit-channel well, from which they decay statistically. The product translational energy distribution P(E) is consistent with orbiting-transition state phase-space theory with no exit-channel barrier. In addition, the energy-integrated angular distribution T(Θ) is consistent with the predictions of the early statistical complex decay model of Miller and Herschbach for fragmentation from a transition state that is a prolate top. In sharp contrast, P(E) for the CoC4H8++H2 products exhibits a substantial hot, nonstatistical tail towards high energy. Perhaps the H2 channel has a late potential energy barrier some 0.5 eV above products, but we view this explanation as highly unlikely. Instead, we suggest that the potential energy from an earlier multicenter transition state is funneled efficiently, and highly nonstatistically, into product translation. This surprising conclusion may apply to H2 products for the entire family of reactions of the late-3D series transition metal cations Fe+, Co+, and Ni+ with alkanes.

Nonstatistical Translational Energy Distribution of H2 Elimination Products from Co+(3F4) + Propane

The velocity map imaging technique is applied to mass-selected CoC2H4+ + CH4 and CoC3H6+ + H2 elimination products from the Co+(3F4) + propane reaction studied under crossed-beam conditions at 0.19 eV collision energy. For both products, we obtain the joint scattering probability distribution P(E,ϑ), where E and ϑ are the product translational energy and scattering angle. Both angular distributions are forward-backward symmetric and only modestly anisotropic. Both product translational energy distributions P(E) are in fairly good agreement with earlier results of Bowers and co-workers, indicating that the earlier distributions were dominated by reaction of ground-state Co+. The fraction of the total available energy deposited into product translation is 0.13 for CH4 and 0.44 for H2. For CH4 products, P(E) is cold and would be readily fit by orbiting-transition-state phase-space theory (OTS−PST) without an exit-channel barrier. Evidently, incipient CoC2H4+ + CH4 products equilibrate in the Co+(C2H4)(CH4) e...

Crossed-beam study of Co+(3F4)+propane: experiment and density functional theory

A pulsed beam of Co+(3F4) crosses a pulsed beam of C3H8 or C3D8 gas under single collision conditions at collision energies of 0.01 eV and 0.21 eV. After a variable time delay t(ext) = 1-8 micros a fast high voltage pulse extracts product ions into a field-free flight tube for mass analysis. Consistent with earlier work, we observe prompt CoC3H6+ +H2 elimination products in 3:1 excess over CoC2H4+ +CH4 products at 0.21 eV on a 2-10 micros time scale. Long-lived CoC3H8+ complexes fragment predominantly back to Co+ +C3H8 reactants and to H2 elimination products on a 6-24 micros time scale. Density functional theory (B3LYP) calculations provide energies, geometries, and harmonic vibrational frequencies at key stationary points for use in a statistical rate model of the reaction. By adjusting two key multicenter transition state (MCTS) energies downward by 4-7 kcal mol(-1), we obtain good agreement with our decay time results and with the cross section versus collision energy of Armentrout and co-workers from 0.1-1.0 eV. B3LYP theory succeeds in finding relative energies of the MCTSs leading to CH4 and H2 in the proper order to explain the different product branching ratio for Co+ (which favors H2 over CH4) compared with its nearest neighbors Fe+ and Ni+ (which favor CH4 over H2).

Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H2 and NbC2+H2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%–50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5±2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y–H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D0(Y–CCH)=110.2±2.0 kcal/mol.

Bimolecular Ni+(2D5/2) + C3H8 Reaction Dynamics in Real Time

A beam of Ni+(2D5/2) is formed at a sharp zero of time by resonant two-photon ionization with a nanosecond dye laser pulse and crossed with a beam of propane gas under single-collision conditions at collision energies of 0.01 and 0.21 eV. The ion−molecule reaction occurs in field-free space in the extraction region of a time-of-flight mass spectrometer. After a variable time delay text = 1−8 μs, a fast high-voltage pulse extracts product ions and residual reactant ions into a field-free flight tube for mass analysis. In contrast with many earlier studies of this reaction under more energetic conditions, at 0.01 eV collision energy we find that Ni+(2D5/2) reacts with C3H8 to form long-lived NiC3H8+ complexes almost exclusively (≥96%) on the time scale 0−25 μs after initiation of the collision. Retarding field analysis of the decay of the long-lived NiC3H8+ complexes reveals that on a 6−24 μs time scale 28% revert to Ni+ + C3H8 and 6% form NiC2H4+ + CH4 elimination products; the remaining complexes have not yet decayed at t = 25 μs. At 0.21 eV collision energy, both CH4 and H2 elimination products are formed promptly (in less than 1 μs) and also over the entire range of time scales studied, 0.5−25 μs. Even at this higher collision energy, about 25% of the long-lived complexes survive beyond t = 25 μs. The apparent energetic threshold observed here for the first time provides new evidence of a potential energy barrier to elimination products comparable to the energy of ground-state reactants. In addition, direct measurement of the time scale of the reaction under carefully controlled conditions provides new dynamical information that serves as a benchmark for the theoretical treatment presented in the accompanying paper.

Statistical Modeling of Gas-Phase Organometallic Reactions Based on Density Functional Theory: Ni+ + C3H8

Density functional theory in its B3LYP variant has been used to explore quantitative details of the adiabatic potential energy surface leading from Ni+ + C3H8 reactants through a deep Ni+(C3H8) well to NiC2H4+ + CH4 and NiC3H6+ + H2 elimination products. The lowest energy path to CH4 elimination involves facile CC bond insertion followed by a high multicenter transition state (MCTS) leading directly to the exit-channel complex Ni+(C2H4)(CH4). The lowest energy path to H2 elimination involves comparably facile secondary CH insertion followed by a comparably high MCTS leading directly to the Ni+(C3H6)(H2) complex. Primary CH insertion leads to significantly higher barriers to both CH4 and H2 elimination; in particular, β-methyl migration is energetically very costly. These results support a mechanism significantly different from the stepwise mechanisms invoked earlier but the same as that found in recent calculations on the Fe+ + C3H8 reaction by Holthausen and Koch. The geometries suggest that agostic inte...

Total reaction cross sections of electronic state-specified transition metal cations: V++C2H6, C3H8, and C2H4 at 0.2 eV

We describe a crossed beam experiment which measures total cross sections for reaction of electronic state-specified V+ with small hydrocarbons at well-defined collision energy E=0.2 eV. The V+ state distribution created at each ionizing wavelength is directly measured by angle-integrated photoelectron spectroscopy (preceding paper). Reactant and product ions are collected and analyzed by pulsed time-of-flight mass spectrometry following a reaction time of 6 μs. Tests of the performance of the apparatus are described in detail. Our experiment defines the reactant V+ electronic state distribution and the collision energy much more precisely than previous work. For all three hydrocarbons C2H6, C3H8, and C2H4, H2 elimination products dominate at 0.2 eV. We observe a dramatic dependence of cross section on the V+ electronic term. The second excited term 3d34s(3F) is more reactive than either lower energy quintet term 3d4(5D) or 3d34s(5F) by a factor of ≥270, 80, and ≥6 for the C2H6, C3H8, and C2H4 reactions, respectively. The 3d34s(3F) reaction cross sections at 0.2 eV are 20±11 Å2, 37±19 Å2, and 2.7±1.6 Å2, respectively, compared with Langevin cross sections of ∼80 Å2. For the C2H6 and C3H8 reactions, cross sections are independent of initial spin–orbit level J within the 3F term to the limits of our accuracy. Comparison with earlier work by Armentrout and co-workers shows that electronic excitation to d3s(3F) is far more effective at promoting H2 elimination than addition of the same total kinetic energy to reactants. Electron spin is clearly a key determinant of V+ reactivity with small hydrocarbons. We suggest that triplet V+ reacts much more efficiently than quintet V+ because of its ability to conserve total electron spin along paths to insertion in a C–H bond of the hydrocarbon.