Criegee Intermediate Reaction with Alcohol Is Enhanced by a Single Water Molecule Yen-Hsiu Lin, †,‡ Cangtao Yin, † Wei-Hong Lin, † Yu-Lin Li, †,‡ Kaito Takahashi,* ,† and Jim Jr-Min Lin* ,†,‡ † Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ‡ Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan * S Supporting Information ABSTRACT: The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn- CH 3 CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0-80%) under near-ambient conditions (298 K, 250-755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH 3 CHOO + CH 3 OH + H 2 O → products, has been determined to be (1.95 ± 0.11) × 10 -32 cm 6 s -1 at 298 K, with no observable pressure dependence for 250-755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H 2 O is reformed and acts as a catalyst. B eing both a hydrogen-bond donor and acceptor, water may form complexes with molecules and affect their reactivity. 1-4 However, most of the literature discussions on water-mediated reactions in the gas phase are mainly based on theoretical results; experimental evidence is relatively rare. As discussed in the literature, 1-4 a number of theoretical works suggest that the introduction of water could reduce the reaction barriers, but because the water complex formation is entropically unfavorable under typical atmospheric or labo- ratory conditions, it is not common to observe water-enhanced reactions. In fact, only a limited number of reactions have been reported to exhibit significant water enhancement, 1 with the observed enhancement factors around 2 (e.g., ≤2.44 for HO 2 + HO 2 , 1.96 for CH 3 CHO + OH at 60 K, ≤1.67 for HO 2 + NO 2 ). These results are adapted from table 2 of a comprehensive review article by Buszek et al., who have reviewed the effect of water on a number of atmospheric reactions, including 5 radical-radical reactions, 10 radical- molecule reactions, 3 molecule-molecule reactions, and 9 unimolecular reactions. 1 Water may act as both a reactant and a catalyst in a single reaction. For example, Kolb et al. reported that the hydration of SO 3 to form H 2 SO 4 exhibits second-order kinetics on the concentration of water vapor ([H 2 O]), indicating the participation of two water molecules, 5 consistent with theoretical works. 6 Vö hringer-Martinez et al. reported water enhancement (about a factor of 2) in the reaction of OH radical with CH 3 CHO at 60 K, but this enhancement becomes negligible at room temperature. 7 The decrease in water enhancement at higher temperatures may indicate that the entropy cost (TΔS) of the water-assisted reaction prevails over its energy gain (-ΔH) at the reaction barrier. More recently, Jara-Toro et al. reported water catalysis of the reaction of CH 3 OH with OH at near room temperature (294 K). 8 However, because the rate coefficients of that work were determined by a relative measurement (CH 3 OH+OH versus C 5 H 12 +OH) in a simulation chamber (80 L collapsible Teflon bag), in which the reaction time (>10 2 s) was much longer than the lifetime of the involved radical species (<10 -3 s), a complex mechanism (e.g., reaction with byproducts) may introduce an additional complication in determining the rate coefficients. In this regard, direct kinetic measurements would be highly desired. Criegee intermediates are very reactive carbonyl oxides, which are formed in the ozonolysis of alkenes and may play important roles in atmospheric chemistry, for example, OH formation and oxidation of atmospheric gases (SO 2 , NO 2 , organic and inorganic acids, water, alcohols, etc.). 9-17 Criegee intermediates have strong UV absorption, which permits their direct kinetic monitoring in real time. 13-16 Data obtained in direct kinetic experiments are more reliable and have revised a number of rate coefficients obtained by relative rate methods. 9-14 Our group and others have observed that the rate of the simplest Criegee intermediate (CH 2 OO) reaction with water vapor exhibits second-order dependence on [H 2 O]; the kinetic Received: November 3, 2018 Accepted: December 4, 2018 Published: December 4, 2018 Letter pubs.acs.org/JPCL Cite This: J. Phys. Chem. Lett. 2018, 9, 7040-7044 © XXXX American Chemical Society 7040 DOI: 10.1021/acs.jpclett.8b03349 J. Phys. Chem. Lett. 2018, 9, 7040-7044 Downloaded via COLUMBIA UNIV on December 7, 2018 at 10:55:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.