Adiabatic and nonadiabatic dissociation of ethyl radical

Abstract

Direct ab initio molecular dynamics using the trajectory surface hopping method with Tully's fewest switches simulates the photodissociation dynamics of ethyl radical, C(2)H(5), following electronic excitation to the A-state. Nonadiabatic dissociation dominates and produces ground state ethylene and fast hydrogen atoms with an anisotropic angular distribution. Surface hopping also generates hot ground state ethyl radicals followed ultimately by unimolecular dissociation to C(2)H(4)+H. The calculated excited state lifetime and the product recoil energy distribution obtained from an ensemble of trajectories are consistent with previous experiments and suggest that a strictly nonadiabatic mechanism can account for nonradiative decay. This process is in competition with adiabatic dissociation producing electronically excited state ethylene and H, a dissociation channel that has not yet been experimentally observed. The branching ratio between adiabatic and nonadiabatic dissociation pathways depends sensitively on the quality of the potential energy surfaces. At the multireference configuration interaction with singles and doubles level of theory, 15% of all trajectories dissociate adiabatically.

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

“…From studies of partially deuterated ethyl radicals, it has been deduced that it is the methyl hydrogen that dissociates in all cases [146,147]. Because of its fundamental importance in combusion, several theoretical articles discussed the electronic structure and dissociation mechanism of the radical [152][153][154][155][156][157].…”

“…In a very recent investigation, Hostettler et al [157] exploited direct ab initio dynamics calculations on MR-CISD surfaces using the trajectory surface-hopping method with Tully's [158] fewest switches to simulate the photodissociation dynamics. They used an International Reviews in Physical Chemistry 299 ensemble of about 5000 trajectories and deduced that the lifetime of the excited state was $20 fs.…”

Trajectory integration with potential energy discontinuities

“…In SA-MCSCF calculations, often used in ab initio multistate molecular dynamics, a further source of discontinuities in the solution is the occurrence of root-switching, i.e. a change in the composition of the subspace of electronic states that are optimized [4]. Notice that root-switching does not affect the FO-SCF-CI results.…”

“…When a direct strategy is used, the potential energy surface (PES) and its gradient are computed at every time step of the trajectory, i.e. for a given molecular geometry by means of quantum chemistry methods [1][2][3][4]. The alternative is to use analytic potential energy functions, either of standard molecular mechanics (MM) type or devised ad hoc for specific systems and processes.…”

“…The SCF, DFT and MCSCF methods provide good examples of such behaviour [6][7][8][9][10][11][12]4]. In CASSCF, a variant of the general MCSCF approach, the definition of the active space (number of active electrons and orbitals) suffices to determine the global minimum.…”