The heat of formation of 2-H heptafluoropropane by ab initio calculations E.P.F. Lee a,b,c, * , J.M. Dyke c , W.-K. Chow a, * , F.-T. Chau b , D.K.W. Mok b a Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, SAR, Hong Kong b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, SAR, Hong Kong c School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK Received 18 November 2004; in final form 1 December 2004 Available online 18 December 2004 Abstract The heat of formation of the fire suppressant, 2-H Heptafluoropropane, which is a replacement for ozone depleting halons, DH f, 298 K (CF 3 CHFCF 3 ), has been calculated at the B3LYP, MP2 and CCSD(T) levels with basis sets of up to cc-pVQZ quality, and with the model chemistry methods: G2(MP2), G2, G3 and CBS-Q, employing three reaction schemes. The CBS limit value of 370.6 ± 2.3 kcal/mol obtained is believed to be the most reliable value currently available. This is recommended to be used in thermochemical/kinetic modelling of the inhibition chemistry of CF 3 CHFCF 3 in flames. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Since hydrofluorocarbons and perfluorocarbons were proposed as replacements for ozone depleting halons as fire-extinguishing agents a decade ago, 2-H heptafluoro- propane, CF 3 CHFCF 3 (HFC-227ea), has received much attention as an environmentally friendly substitute for halons and is now widely used. However, although the inhibition chemistry of CF 3 CHFCF 3 in flames has been studied experimentally and theoretically on numerous occasions (see for example, [1–7] and references therein), its heat of formation, DH f, 298 K , appears not to be well established. Hynes et al. [1] used a value of DH f, 300 K (CF 3 CHFCF 3 )= 365.1 kcal/mol in kinetic modelling of the inhibition chemistry of CF 3 CHFCF 3 in premixed hydrogen-air flames. Liu et al. [4] quoted an experimental value of DH f, 298 K (CF 3 CHFCF 3 )= 365.55 kcal/mol in their density functional and reac- tion rate study of the reaction, CF 3 CHFCF 3 + O ! CF 3 CFCF 3 + OH. In fact, these two values of the heat of formation of CF 3 CHFCF 3 used in [1,4] have come from the same and only available source, which is the thermochemical database for combustion com- piled by Burcat and McBride [8]. However, the most re- cent compilation of Burcat gives a value of 374.0 kcal/ mol (1564.8 kJ/mol) for DH f, 298 K (CF 3 CHFCF 3 ) [9] (no experimental error quoted). This value is more neg- ative than the earlier value by nearly 10 kcal/mol. This difference between the two available values of DH f, 298 K (CF 3 CHFCF 3 ) is significant and hence, a reli- able derivation of DH f, 298 K (CF 3 CHFCF 3 ) from an independent source seems appropriate. In the present study, we propose to calculate DH f, 298 K (CF 3 CHFCF 3 ) by high-level ab initio calculations. 2. Theoretical considerations and computational details 2.1. Reaction schemes and available heats of formation The basic aim was to use some suitable theoretical methods to calculate the enthalpies of some selected reactions, which involve CF 3 CHFCF 3 . Then using the 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.004 * Corresponding authors. E-mail address: e.p.lee@soton.ac.uk (E.P.F. Lee). www.elsevier.com/locate/cplett Chemical Physics Letters 402 (2005) 32–36