Mixed time-dependent density-functional theory/classical trajectory surface hopping study of oxirane photochemistry

Area of Science:

• Computational Chemistry
• Photochemistry
• Theoretical Chemistry

Background:

• The photochemical ring opening of oxirane is a key reaction in organic chemistry.
• Previous studies highlighted the importance of the Tamm-Dancoff approximation (TDA) in time-dependent density-functional theory (TDDFT) for describing oxirane's reaction pathways away from equilibrium.

Purpose of the Study:

• To investigate the photochemical ring opening mechanism of oxirane using a mixed time-dependent density-functional theory (TDDFT) and classical trajectory surface hopping (SH) approach.
• To provide state-specific insights into the reaction dynamics and product formation.
• To assess the performance of various functionals in describing the reaction pathway.

Main Methods:

• Mixed time-dependent density-functional theory (TDDFT) with the Tamm-Dancoff approximation (TDA) coupled with classical trajectory surface hopping (SH).
• Calculation of potential energy curves using TDDFT and diffusion Monte Carlo methods.
• Analysis of reaction dynamics, including non-minimum energy pathways and multiple surface hops.

Main Results:

• The TDDFT TDA/SH calculations successfully confirm the Gomer-Noyes mechanism for oxirane's photochemical ring opening.
• Excitation to one specific excited state leads to rapid ring opening, followed by hopping to the ground state.
• Hot dynamics on the ground state (4000 K) result in further reactions, producing acetaldehyde, methane, and carbon monoxide.
• The TDDFT TDA effectively approximates conical intersections in this system, even in the absence of true (S(0),S(1)) conical intersections in adiabatic TDDFT.

Conclusions:

• The study validates the Gomer-Noyes mechanism for oxirane photochemistry using advanced computational methods.
• It elucidates the crucial role of specific excited states and subsequent dynamics in product formation.
• The findings demonstrate the capability of TDDFT TDA/SH to accurately model photochemical reactions and approximate conical intersections.