Abstract
Chemiexcitation, the generation of electronic excited states by a thermal reaction initiated on the ground state, is an essential step in chemiluminescence, and it is mediated by the presence of a conical intersection that allows a nonadiabatic transition from ground state to excited state. Conical intersections classified as sloped favor chemiexcitation over ground state relaxation. The chemiexcitation yield of 1,2-dioxetanes is known to increase upon methylation. In this work we explore to which extent this trend can be attributed to changes in the conical intersection topography or accessibility. Since conical intersections are not isolated points, but continuous seams, we locate regions of the conical intersection seams that are close to the configuration space traversed by the molecules as they react on the ground state. We find that conical intersections are energetically and geometrically accessible from the reaction trajectory, and that topographies favorable to chemiexcitation are found in all three molecules studied. Nevertheless, the results suggest that dynamic effects are more important for explaining the different yields than the static features of the potential energy surfaces.
Highlights
Conical intersections (CIs) are known to play a key role in photochemistry.[1,2,3] Absorption of light can promote a molecule into an electronically excited state
Chemiexcitation, the generation of electronic excited states by a thermal reaction initiated on the ground state, is an essential step in chemiluminescence, and it is mediated by the presence of a conical intersection that allows a nonadiabatic transition from ground state to excited state
We find that conical intersections are energetically and geometrically accessible from the reaction trajectory, and that topographies favorable to chemiexcitation are found in all three molecules studied
Summary
Conical intersections (CIs) are known to play a key role in photochemistry.[1,2,3] Absorption of light can promote a molecule into an electronically excited state. Chemiluminescence can be seen as a reverse photochemical process: it is the emission of light as a result of a nonadiabatic chemical reaction.[4] More precisely, a thermally activated molecule reacts and, by doing so, undergoes a nonadiabatic transition from the reactant in the electronic ground state up to an electronic excited state of the product or an intermediate. The latter releases the excess energy in the form of a photon
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