Abstraction Kinetics of H-Atom by OH Radical from Pinonaldehyde (C₁₀H₁₆O₂): Ab Initio and Transition-State Theory Calculations

The kinetics and abstraction rate coefficients of hydroxyl radical (OH) reaction with pinonaldehyde were computed using G3­(MP2) theory and transition-state theory (TST) between 200 and 400 K. Structures of the reactants, reaction complexes (RCs), product complexes (PCs), transition states (TSs), and products were optimized at the MP2­(FULL)/6-31G* level of theory. Fifteen transition states were identified for the title reaction and confirmed by intrinsic reaction coordinate (IRC) calculations. The contributions of all the individual hydrogens in the substrate molecule to the total reaction are computed. The quantum mechanical tunneling effect was computed using Wigner’s and Eckart’s methods (both symmetrical and unsymmetrical methods). The reaction exhibits a negative temperature dependent rate coefficient, k(T) = (1.97 ± 0.34) × 10^–13 exp[(1587 ± 48)/T] cm³ molecule^–1 s^–1, k(T) = (3.02 ± 0.56) × 10^–13 exp­[(1534 ± 52/T] cm³ molecule^–1 s^–1, and k(T) = (4.71 ± 1.85) × 10^–14 exp[(2042 ± 110)/T] cm³ molecule^–1 s^–1 with Wigner’s, Eckart’s symmetrical, and Eckart’s unsymmetrical tunneling corrections, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the experimentally measured ones and other theoretical results. It is shown that hydrogen abstraction from −CHO position is the major channel, whereas H-abstraction from −COCH₃ is negligible. The atmospheric lifetime of pinonaldehyde is computed to be few hours and found to be in excellent agreement with the experimentally estimated ones.