Mechanistic Molecular Motion of Transition-Metal Mediated Beta-Hydride Transfer: Quasiclassical Trajectories Reveal Dynamically Ballistic, Dynamically Unrelaxed, Two Step, and Concerted Mechanisms

<div>The transfer of a -hydrogen from a metal-alkyl group to ethylene is a fundamental</div><div>organometallic transformation. Previously proposed mechanisms for this transformation involve either a</div><div>two-step -hydrogen elimination and migratory insertion sequence with a metal hydride intermediate</div><div>or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct</div><div>dynamics trajectories that reveal new dynamical mechanisms for the -hydrogen transfer of</div><div>[Cp*RhIII(Et)(ethylene)]</div><div>Despite the DFT energy landscape showing a two-step mechanism with a Rh-H</div><div>intermediate, quasiclassical trajectories commencing from the -hydrogen elimination transition state</div><div>revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast</div><div>(typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a</div><div>dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all</div><div>trajectories initiated at the Rh-H intermediate show continuation along the reaction coordinate. All of</div><div>these trajectory outcomes are consistent with the Rh-H intermediate <1 kcal/mol stabilized relative to</div><div>the -hydrogen elimination and migratory insertion transition states. For Co, which on the energy</div><div>landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the</div><div>transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh</div><div>ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed</div><div>mechanisms, trajectories also stopped at the Ir-H intermediate. This is consistent with an Ir-H</div><div>intermediate that is stabilized by ~3 kcal/mol relative to the -hydrogen elimination and migratory</div><div>insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the</div><div>relationship between the energy surface shape and resulting dynamical mechanisms of an</div><div>organometallic transformation.</div>

Mechanistic Molecular Motion of Transition-Metal Mediated Beta-Hydride Transfer: Quasiclassical Trajectories Reveal Dynamically Ballistic, Dynamically Unrelaxed, Two Step, and Concerted Mechanisms

<div>The transfer of a -hydrogen from a metal-alkyl group to ethylene is a fundamental</div><div>organometallic transformation. Previously proposed mechanisms for this transformation involve either a</div><div>two-step -hydrogen elimination and migratory insertion sequence with a metal hydride intermediate</div><div>or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct</div><div>dynamics trajectories that reveal new dynamical mechanisms for the -hydrogen transfer of</div><div>[Cp*RhIII(Et)(ethylene)]</div><div>Despite the DFT energy landscape showing a two-step mechanism with a Rh-H</div><div>intermediate, quasiclassical trajectories commencing from the -hydrogen elimination transition state</div><div>revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast</div><div>(typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a</div><div>dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all</div><div>trajectories initiated at the Rh-H intermediate show continuation along the reaction coordinate. All of</div><div>these trajectory outcomes are consistent with the Rh-H intermediate <1 kcal/mol stabilized relative to</div><div>the -hydrogen elimination and migratory insertion transition states. For Co, which on the energy</div><div>landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the</div><div>transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh</div><div>ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed</div><div>mechanisms, trajectories also stopped at the Ir-H intermediate. This is consistent with an Ir-H</div><div>intermediate that is stabilized by ~3 kcal/mol relative to the -hydrogen elimination and migratory</div><div>insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the</div><div>relationship between the energy surface shape and resulting dynamical mechanisms of an</div><div>organometallic transformation.</div>

Mechanistic molecular motion of transition-metal mediated β-hydrogen transfer: quasiclassical trajectories reveal dynamically ballistic, dynamically unrelaxed, two step, and concerted mechanisms

Quasiclassical direct dynamics reveal new dynamical mechanisms for metal-alkyl to ethylene β-hydrogen transfer.

Extended Hamiltonian molecular dynamics: semiclassical trajectories with improved maintenance of zero point energy

We present a new semiclassical molecular dynamics method designed to improve the treatment of the zero-point energy in quasiclassical trajectories.

Reactive collisions for NO(2Π) + N(4S) at temperatures relevant to the hypersonic flight regime

Rate coefficients for the NO(2Π) + N(4S) reaction at high temperatures from quasiclassical trajectories using MRCI+Q PESs of the lowest triplet states.

Implications of the O + OH reaction in hydroxyl nightglow modeling

The hydroxyl nightglow has been examined anew using calculated rate constants for the key reactive and inelastic O + OH(v') quenching processes. These constants have been obtained from quasiclassical trajectories run on the adiabatic ab initio-based double many-body expansion-IV potential energy surface for the ground state of the hydroperoxil radical. Significant differences in the vertical profiles of vibrationally excited hydroxyl radicals are obtained relative to the ones predicted by Adler-Golden (1997) when employing an O + OH(v') effective rate constant chosen to be twice the experimental value for quenching of OH(v' = 1). At an altitude of 90 km, such deviations range from ~ 80% for v' = 1 to only a few percent for v' = 9. Other mechanisms reported in the literature have also been utilized, in particular those that loosely yield lower and upper limits in the results, namely sudden-death and collisional cascade. Finally, the validity of the steady-state hypothesis is analysed through comparison with results obtained via numerical integration of the master equations.