Heats of formation of the Criegee formaldehyde oxide and dioxirane Minh Tho Nguyen a, * , Thanh Lam Nguyen a , Vu Thi Ngan a,b , Hue Minh Thi Nguyen b a Department of Chemistry and Mathematical Modeling and Computational Science Centre, University of Leuven, B-3001 Leuven, Belgium b Faculty of Chemistry, Hanoi National University of Education, Hanoi, Vietnam Received 19 August 2007; in final form 5 October 2007 Available online 12 October 2007 Abstract CCSD(T)/CBS calculations plus corrections predict the heats of formation of formaldehyde O-oxide 1 and dioxirane 2: DH 0 f ðCH 2 OO; 1Þ¼ 28:1 and 26.4 kcal/mol, and DH 0 f ðCH 2 O 2 ; 2Þ¼ 3:0 and 1.2 kcal/mol at 0 K and 298 K, respectively. The adiabatic ionization energies are IE a (1) = 9.98 eV and IE a (2) = 10.82 eV. Protonation of carbonyl oxide takes place at terminal oxygen with a pro- ton affinity of PA(1) = 203.5 ± 1.0 kcal/mol. The vertical triplet state 3 A 0 of 1 is located at 1.82 eV above the ground state (with errors of ±1.0 kcal/mol or ±0.05 eV). The parent Criegee intermediate is unstable with respect to O–O bond cleavage but becomes more stable upon ionization. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Nearly 60 years ago, Criegee [1,2] postulated a mecha- nism, summarized in Scheme 1, to rationalize the isolation of 1,2,4-trioxolanes from the reaction of ozone with olefins and the incorporation of external aldehydes into these five- membered products. This mechanism has rapidly been endorsed by chemists, and the molecules COO formed by cyclo-reversions of the primary ozonides were named as ‘Criegee carbonyl oxides’ [3]. While formation of Criegee intermediates (CI’s) in solution has been demonstrated by trapping agents [2], the cyclic isomer dioxiranes have often been identified in the gas phase. The first evidence for the existence of the CI’s in the gas phase was obtained in 1983 from time-resolved laser spectroscopic studies [4], and the primary ozonides were only more recently trapped by using strained allylic alcohols [5]. It now appears that both the CI’s and dioxiranes, are involved as short-lived intermediates in a number of photo-oxidation reactions and enzymatic processes [6–9]. When formed from fast reaction of a carbene with molecular oxygen without the presence of another substrate, a CI could undergo a multi-step unimolecular rearrangement forming first a cyclic dioxirane, and then a carboxylic acid and finally CO 2 or CO [10]. In fact, the ozonolysis of olefins remains the main source of carboxylic acids in the atmosphere [8]. In the presence of hydrogen compounds, carbonyl oxides contribute to production of the OH and HO 2 radicals, which undergo important chemical processes in the tropo- sphere [11]. Due to the fact that the CI’s were not directly detected and characterized during the ozonolyses, most information on their molecular structure, basic spectro- scopic and thermochemical properties have been derived from quantum chemical calculations. The simplest parent CI, formaldehyde carbonyl O-oxide CH 2 O 2 , has been the subject of several theoretical studies [12–19], but these rather disagree with each other about its heat of formation. In fact, the reported theoretical val- ues for DH f (CH 2 O 2 ) at 298 K range from 27 to 48 kcal/ mol [12–19]. As far as we are aware, the experimental heat of formation of dioxirane is not available, and the theoret- ical values vary from 0.3 to 6.0 kcal/mol [15]. Although relative energies between both isomers with respect to for- mic acid have repeatedly been calculated using different lev- els of theory, their heats of formation have been determined with high accuracy. Note that formic acid is 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.10.033 * Corresponding author. Fax: +32 16 32 79 92. E-mail address: minh.nguyen@chem.kuleuven.be (M.T. Nguyen). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 448 (2007) 183–188