Département de chimie moléculaire

The authors present a mixed time-dependent d.-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochem. ring opening in oxirane. Previous preparatory work limited to the sym. CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approxn. (TDA) is important for improving the performance of TDDFT away from the equil. geometry. This observation is supported by the present TDDFT TDA/SH calcns. which successfully confirm the main exptl. derived Gomer-Noyes mechanism for the photochem. CO ring opening of oxirane and provide important state-specific information not easily accessible from expts. In particular, while one of the lowest two excited states is photochem. relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C2H4O → .bul.CH2CH2O.bul.. This is followed by hopping to the electronic ground state where hot (4000 K) dynamics leads to further reactions, namely, .bul.CH2CH2O.bul. → CH3CHO → .bul.CH3+.bul.CHO and CH4+CO. In the dynamics, the authors are not limited to following min. energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S0,S1) conical intersections are expected to be absent in adiabatic TDDFT, the TDDFT TDA is able to approx. a conical intersection in this system. (c) 2008 American Institute of Physics. [on SciFinder(R)]