Thermochemistry of the higher chlorine oxides ClO x (x ¼ 3, 4) and Cl 2 O x (x ¼ 3–7) † J.E. Sicre, C.J. Cobos * Instituto de Investigaciones Fisicoquı ´micas Teo ´ricas y Aplicadas (INIFTA), Departamento de Quı ´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CONICET, CICPBA, Casilla de Correo 16, Sucursal 4, 1900 La Plata, Argentina Received 10 June 2002; accepted 14 August 2002 Abstract Heats of formation for ClO 3 , ClO 4 , Cl 2 O 3 , Cl 2 O 4 , Cl 2 O 5 , Cl 2 O 6 and Cl 2 O 7 molecules are determined at the B3LYP, B3PW91, mPW1PW91 and B1LYP levels of the density functional theory employing a series of extended basis sets, and using Gaussian-3 model chemistries. Modified Gaussian-3 calculations, which employ accurate B3LYP/6-311 þ G(3d2f) molecular geometries and vibrational frequencies, were also performed. Heats of formation were calculated from both total atomization energies and isodesmic reaction schemes. The latter method in conjunction with Gaussian-3 models leads to the most reliable results. The best values at 298 K for ClO 3 , ClO 4 , Cl 2 O 3 and Cl 2 O 4 as derived from an average of G3//B3LYP and G3//B3LYP/6-311 þ G(3d2f) calculations are 43.1, 54.8, 31.7 and 37.4 kcal mol 21 . From calculations carried out at the G3(MP2)//B3LYP and G3(MP2)// B3LYP/6-311 þ G(3d2f) levels, heats of formation for Cl 2 O 5 , Cl 2 O 6 and Cl 2 O 7 are predicted to be 53.2, 52.2 and 61.5 kcal mol 21 . All best values are reproduced within 1 kcal mol 21 by using mPW1PW91/6-311 þ G(3d2f) isodesmic energies. Enthalpy changes for relevant Cl – O bond fission reactions are reported. Comparisons with previous thermodynamics data are made. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Chlorine oxides; Heats of formation; Ab initio; Density functional theory 1. Introduction It is at present largely accepted that simple chlorine oxides are key participants in the destruction of strato- spheric ozone. Due to this, the photochemistry, chemical kinetics and thermochemistry of these species [1,2] have occupied center stage in the discussion of ozone loss since the discovery of the Antarctic ‘ozone hole’ [3]. However, the detection in polar stratosphere of elevated concentrations of ClO and OClO has increased the interest for the possible formation of higher oxides such as Cl 2 O 3 (ClOClO 2 ) Cl 2 O 4 (ClOClO 3 ), Cl 2 O 5 (ClOOClO 3 ), Cl 2 O 6 (O 2- ClOClO 3 ) and Cl 2 O 7 (O 3 ClOClO 3 ), acting as tempor- ary reservoirs of these and other species. In particular, the potential ozone-depleting catalytic cycle Cl 2- O 4 þ 2O 3 ! Cl 2 O 6 þ 2O 2 has been recently suggested [4]. In addition, transient species as ClO 3 and ClO 4 radicals could also play a role in stratospheric mechanisms. Laboratory kinetic studies show that Cl 2 O 6 and Cl 2 O 7 and small amounts of OClO, Cl 2 and O 2 are formed in the reaction between Cl atoms and Cl 2 O 4 [5], while the photolysis of OClO produces 0166-1280/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0166-1280(02)00602-4 Journal of Molecular Structure (Theochem) 620 (2003) 215–226 www.elsevier.com/locate/theochem † Dedicated to the memory of Dr Marı ´a Ine ´s Lo ´pez. * Corresponding author. Tel.: þ 54-221-4257430; fax: þ 54-221- 4254642. E-mail address: cobos@inifta.unlp.edu.ar (C.J. Cobos).