A Quantum Chemical and TST Study of the OH Hydrogen-Abstraction Reaction from Substituted Aldehydes: FCHO and ClCHO Nelaine Mora-Diez, ²,‡ J. Rau ´ l Alvarez-Idaboy, ‡,§ and Russell J. Boyd* ,² Department of Chemistry, Dalhousie UniVersity, Halifax, NS, Canada B3H 4J3, Instituto Mexicano del Petro ´ leo, 07730 Me ´ xico, DF, Mexico, and Laboratorio de Quı ´mica Computacional y Teo ´ rica, Facultad de Quı ´mica, UniVersidad de La Habana, Habana 10400, Cuba ReceiVed: April 18, 2001; In Final Form: May 29, 2001 In the present study, ab initio methods have been used to study the OH hydrogen-abstraction reaction from two substituted aldehydes: FCHO and ClCHO. A complex mechanism in which the overall rate depends on the rates of two competitive reactions, a reversible step where a reactant (or prereactive) complex is formed, followed by the irreversible hydrogen abstraction to form the products, is corroborated. This mechanism was previously shown to describe accurately the kinetics of the OH hydrogen-abstraction reaction from formaldehyde and acetaldehyde. Classical transition state theory (TST) rate constants calculated with tunneling corrections, assuming an unsymmetrical Eckart barrier, agree very well with experimental upper bound values. Activation energy barriers and enthalpies of reaction have been estimated through CCSD(T) single point calculations using MP2 geometries and frequencies and the 6-311++G(d,p) basis set. Introduction The atmosphere is a very complex chemical system and of crucial importance to life on Earth. Aldehydes, known to play an important role in the chemistry of the polluted troposphere, 1 are emitted as primary pollutants from partial oxidation of hydrocarbon fuels and arise as secondary pollutants from the oxidation of volatile organic compounds. Once in the atmo- sphere, aldehydes may either photolyze or react further with OH radicals, the most important tropospheric daytime oxidant, or with NO 3 radicals during the nighttime. The chemistry of the atmosphere is quite complex. 2 The life cycles of the atmospheric species (including traces) are strongly coupled, and the results of this are often unexpected. Depending on their atmospheric lifetimes these species can exhibit an enormous range of spatial and temporal variability, but every substance emitted into the atmosphere is eventually removed so that a biogeochemical cycle is established. To estimate the lifetimes of pollutants in the atmosphere different removal options have to be considered, and for this, the development of a reliable database of atmospheric reactions is extremely important. However, such reactions are often difficult to study experimentally. Since a likely tropospheric removal route for aldehydes in the atmosphere is by the reaction with OH radicals, we focus on this reaction that occurs according to the following overall equation: Previous calculations on the reaction of formaldehyde with OH radicals 3 showed why the addition reaction of OH radicals to the carbonylic double bond does not occur. A discussion based on activation energy values and also on a comparison of structural parameters of the TS of this reaction with those of similar reactions clarified this topic. Earlier experimental and theoretical studies on reactions between OH radicals and aldehydes have been performed. 4,5 In a former study 3 the OH hydrogen-abstraction reaction from formaldehyde and acetaldehyde was examined by considering a complex mechanism in which the overall rate depends on the rates of two competitive reactions: a reversible step where a reactant (or prereactive) complex is formed, followed by the irreversible hydrogen abstraction to form the products. TST 6 was applied for the calculation of the rate constants with successful results. Tunneling corrections were incorporated assuming an unsymmetrical Eckart barrier. The consideration of the reactant complex formation has two important consequences in the kinetics calculations of these systems since it explains the negative activation barriers observed (especially for acetaldehyde) and also affects the rate constant calculations, as it determines the barrier height of the hydrogen-abstraction process and hence the value of the tunneling correction. In view of the previous successful results, we decided to extend these ideas to the OH hydrogen-abstraction reaction from FCHO and ClCHO, for which only experimental upper bound rate constants have been reported and activation energy values are unknown. 4a,7,8 Formyl fluoride (FCHO) is one of the halogenated molecules in the upper stratosphere and a major product of the degradation in the troposphere of CH 3 CFH 2 (HFC-134a). 9 It is also a product of the subsequent dissociation of fluorinated radicals that originate in the atmosphere. Formyl chloride (ClCHO) is a reactive molecule that forms as an atmospheric degradation intermediate of several chlorinated hydrocarbons such as CH 3 Cl, CH 2 Cl 2 , CHCl 3 , and hydrochlorofluorocarbons (HCFCs), 10 as well as from the tropospheric reaction of Cl atoms with volatile organic compounds such as isoprene. 11 The reaction of FCHO and ClCHO with OH radicals is supposed to be a tropospheric removal route for these compounds. * To whom correspondence should be addressed. E-mail: boyd@is.dal.ca. ² Dalhousie University. ‡ Universidad de La Habana. § Instituto Mexicano del Petro ´leo. 9034 J. Phys. Chem. A 2001, 105, 9034-9039 10.1021/jp011472i CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001