Vinylidene: Potential energy surface and unimolecular reaction dynamics

Abstract

New quantum chemistry calculations (with a triple zeta plus polarization basis set, and a single and double configuration interaction) have been carried out to determine the equilibrium points and the transition state for the vinylidene (H2C=C:)→acetylene (HC≡CH) isomerization. A classical barrier height (i.e., with no zero point energy effects) of 6.3 kcal/mol is obtained, and application of the Davidson correction for unlinked clusters reduces this to 5.4 kcal/mol. Our best estimate is that the true classical barrier lies in the range 2–4 kcal/mol. The dynamics of the vinylidene/acetylene isomerization is described with the framework of the reaction path Hamiltonian. The lifetime of vinylidene (in its ground vibrational state) with respect to this process is calculated to be 0.24 to 4.6 ps for a classical barrier of 2 to 4 kcal/mol. This lifetime decreases by a factor of ∼2 if one quantum of the CH2 scissors mode of vinylidene is excited, but is predicted to increase somewhat if a quantum of the C–C stretch is excited. These results are all consistent with the recent experimental observation of vinylidene via photodetachment of C2H−2.

Application of semiclassical methods to reaction rate theory

“…}.i, which adiabatically connect to the states of the activated complex where the square of the probability amplitude from a state of the activated complex to the continuum, Nf'}:, is identified by the semiclassical transmission probability in the deep tunneling regime, e-26 • At higher energies, Carrington et al 156 extended this result to approximately include the coupling between the reaction coordinate and the perpendicular modes in the reaction path Hamiltonian. 132 Within the Feshbach formalism described in Sec.…”

Accurate Quantum Dynamics Study on the Resonance Decay of Vinylidene

“…3) The semiclassical result (0.24-4.6 ps) of Carrington et al [7] using reaction path Hamiltonian may have evidently underestimated the RD lifetime for the ground state. 4) The result from recent 5D calculations [10] (293 ps) is consistent with ours, though it should be noted that, the neglect of the out-of-plane mode in the 5D calculations overestimated the VA barrier, while the PES calculations underestimated the classical barrier, of which errors canceled each other out.…”

“…The isomerization between vinylidene and acetylene is an important prototype of intramolecular H migration, and has attracted much attention; [3][4][5][6][7][8][9][10][11][12][13][14][15] however, the key issue related to the lifetime of vinylidene has not been resolved so far. For example, the lifetime of vinylidene was estimated to be 0.04-0.2 ps based on a simulation of line shapes by the Lineberger group using photoelectron spectroscopy of vinylidene anion;[4] while a very long lifetime of at least 3.5 ms was claimed by Vager et al in their coulomb explosion imaging (CEI) experiments.[5] Approximate QD calculations [7,8,10,15] also yielded lifetimes with large discrepancies. The present QD calculations also make it possible to clarify the inconsistency among previous studies on the lifetime of vinylidene.…”

Thermal and state‐selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs

“…An alternative method [37,38], which is also useful in many cases but is not applied here, is to directly interpolate parameters in the reaction-path Hamiltonian rather than to use a globally or semiglobally defined potential.…”