Enhanced H-atom abstraction from pinonaldehyde, pinonic acid, pinic acid, and related compounds: theoretical study of C–H bond strengths

Abstract

The experimentally observed rate of the pinonaldehyde + OH reaction is substantially higher than expected from established structure activity relationships (SARs). In this study the C–H bond strengths in pinonaldehyde and in related, structurally similar compounds are investigated theoretically using B3LYP-DFT/6-31G(d,p). We find bond strengths in agreement with the literature values for smaller reference compounds, except for the abstraction of hydrogens from the exocyclic CH₂ group, which has a significantly lower bond strength due to strain-enhanced hyperconjugation. The rate constant for H-abstraction from this group in pinonaldehyde is estimated at 8 × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The C–H bond strengths of the two tertiary hydrogens on the carbons in the four-membered ring are found to have normal values, in contrast to the high values for secondary hydrogens in (substituted) cyclobutane(s); the summed rate coefficient for abstracting these two tertiary hydrogens in pinonaldehyde is estimated at 5 × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The resulting total rate constant estimated for H-abstraction from pinonaldehyde by OH is 3.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, with a ca. 59∶23∶14 ratio of aldehyde-H, exocyclic-CH₂-hydrogen, and tertiary-H abstraction. This predicted branching has important implications for the ultimate product yields in the OH-initiated atmospheric oxidation of pinonaldehyde. Similar effects are found for pinonic acid, pinic acid and related compounds, and are predicted to entail rate constants of the gas-phase reactions of these compounds with OH radicals of 1 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, i.e. 3 times higher than expected from the current standard SAR.