Experimental and theoretical studies of OH-initiated reactions of isoprene

Abstract

Photochemical oxidation of isoprene plays a significant role in tropospheric ozone production. The OH-initiated reactions of isoprene are the dominant daytime removal pathway for isoprene. The OH-isoprene reactions proceed through multiple steps and pathways. In each step of the chain reaction, organic radical intermediates are produced and they further propagate or terminate the oxidation process. Currently, there is very little understanding of the chemistry of these organic radicals. This study investigates the OH-initiated reactions of isoprene using combined experimental and theoretical approaches, focusing on the chemistry of the organic intermediate radicals. Kinetic studies of the OH-isoprene-O₂-NO reaction system were performed using a fast-flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. Intermediate species such as OH-isoprene adduct and hydroxy-isoprene peroxy radical were detected directly with the CIMS method. Bimolecular reaction rate constants were determined by monitoring the intermediate species. The measured rate constants are: (1.0 ± 0.1) x 10⁻¹⁰ for the addition of OH to isoprene, (7 ± 3) x 10⁻¹³ for the reaction of O₂ with the OH-isoprene adduct, and (9 ± 3) x 10⁻¹² cm³ molecule⁻¹ s⁻¹ for the reaction of the hydroxy-isoprene peroxy radical with NO. In addition, quantum-chemical studies using density functional theory (DFT) and ab initio methods were conducted to investigate the hydroxy-isoprene peroxynitrite intermediate. Structures and energetics were obtained for the six peroxynitrite isomers. Geometry optimizations of the peroxynitrite isomers were performed at B3LYP/6-31G(d,p) level of theory. Single-point energies were calculated using MP2 and CCSD(T) methods with various basis sets. This work provides further understanding on the tropospheric photochemical oxidation mechanism of isoprene and their impacts in regional and global air quality.