Chemiluminescent decomposition of 1,2-dioxetanes: an MC-SCF/MP2 study with VB analysis

MC-SCF (at the 4-31G level) geometry optimizations and analytical hessian (frequencies) for ground-state (S0) and excited state (T1 and S1) surfaces that are necessary in the description of the mechanism of the chemiluminescent decomposition of 1,2-dioxetane are reported. The energetics have been confirmed by multi-reference MP2 computations at selected critical points. The origin of the S0/T1 surface intersection and the different barrier heights in S0/T1 fragmentation are rationalized using a rigorous VB model. The computed results suggest a novel mechanism where the rate-determining step occuss on the excited T, surface. The computed activation energy is 25.3 kcal mol-1 at the MC-SCF/MP2/6-31G* level (21.3 kcal mol-1 with zero-point correction from 4-31G level MC-SCF) which is in acceptable agreement with the experimental activation energy of 22.1 ± 0.3 kcal mol-1. Our computed value of ΔS‡ is 5 cal mol−1 K−1 in agreement with the experimental result which is small or negative depending upon solvent. Our results suggest that the thermal S0 ring opening of dioxetane to produce a biradical can occur almost without activation energy. The S0–T1 avoided crossing is shown to occur along an O–O bond rupture coordinate in the region just before the biradical minimum and is controlled by strong spin-orbit coupling. After passage (via C–C stretching) through a second real T1–S0 crossing immediately after the biradical minimum the rate-determining step involves a transition state for C–C fragmentation on the T1 surface to produce triplet and ground-state formaldehyde.