Reaction Mechanism and Kinetics of Criegee Intermediate Formation in the Ozonolysis of a Marine Chlorinated Monoterpene: A Three-Channel Master-Equation Analysis

Abstract

Atmospheric ozonolysis of halogenated monoterpenes from marine red macroalgae proceeds through pre-reactive complexes, primary ozonides, Criegee intermediates, and carbonyl products. Structure–activity relationship (SAR) schemes, such as that of Khan et al. (2022) for nineteen Plocamium-derived monoterpenes, return only a single rate coefficient per channel, with no information on intermediates, energy partitioning or pressure dependence. Here all three ozone-attack channels of Compound 1, 5,6-dichloro-2-chloromethylene-6-methyl-octa-3,7-dienal (C₁₀H₁₁Cl₃O) — the terminal Δ⁷ vinyl, the internal Δ³ double bond, and the chloromethylene Δ¹ group — are characterized with a quantum-chemical/master-equation workflow (ωB97X-D3BJ/def2-TZVP; DLPNO-CCSD(T)/def2-TZVPP; ORCA 6.1; MESMER 7.1) on a single workstation. All three channels proceed over tight, first-order cycloaddition transition states to deep primary ozonides and on to chlorinated carbonyl + Criegee pairs. The internal Δ³ cycloaddition, initially suspected to be barrierless along the single-bond approach coordinate, is shown by a synchronous two-bond scan, transition-state optimization and intrinsic-reaction-coordinate analysis to possess a genuine concerted ring-closure saddle (one imaginary mode, −310 cm⁻¹) at +9.57 kcal mol⁻¹ — the highest of the three entrance barriers, above the terminal Δ⁷ (+7.02) and chloromethylene Δ¹ (+6.67 kcal mol⁻¹) channels. The resulting 298 K, 760 Torr rate coefficients are kΔ⁷ = 2.50 × 10⁻²¹, kΔ¹ = 1.45 × 10⁻²² and kΔ³ = 1.53 × 10⁻²³ cm³ molecule⁻¹ s⁻¹, giving a channel ordering Δ⁷ > Δ¹ > Δ³ (≈94 : 5 : 1). This confirms the SAR/MCM picture of Khan et al. on the dominant site — the terminal Δ⁷ vinyl — but reassigns the minor chemistry: the internal Δ³ double bond, treated by SAR/MCM as the secondary site (~7%), is found to be the slowest channel (~1%), while the chloromethylene Δ¹ group not considered by SAR emerges as the true secondary channel (~5%). Notably, the ordering is not set by barrier height alone. Although Δ¹ has the lowest barrier, its tighter transition state (a smaller rovibrational partition function) makes it slower than Δ⁷, so the channel selectivity reflects a balance of entropic and enthalpic factors rather than barrier height in isolation. Pressure-dependent collisional stabilization of the primary ozonides — inaccessible to SAR — is significant at atmospheric pressure, with prompt Criegee yields of ~28% (Δ⁷). Benchmarking the identical workflow against ethene ozonolysis reproduces the experimental barrier to within 0.4 kcal mol⁻¹ (5.3 versus ~4.95 kcal mol⁻¹), indicating that the total rate, which lies about three orders of magnitude below the group-additivity SAR estimate, reflects overestimation by the SAR scheme for this chlorinated, conjugated substrate rather than an offset in the computed barriers.