Thermochemistry of the hypobromous and hypochlorous acids, HOBr and HOCl.
The enthalpies of formation of HOBr and HOCl have been estimated by employing coupled cluster theory in conjunction with the correlation consistent basis sets and corrections for core-valence, relativistic, and anharmonic effects. We have employed three different reactions to estimate the DeltaH(o)(f,298)(HOBr), namely, the atomization reaction and two homodesmic reactions. Our best estimation is DeltaH(o)(f,298) (HOBr) = -15.3 +/- 0.6 kcal/mol and is very likely to lie toward the more negative values. The present value is 1.4 kcal/mol lower than the widely used experimental determination of Ruscic and Berkowitz (J. Chem. Phys. 1994, 101, 7795), DeltaH(o)(f,298)(HOBr) > -13.93 +/- 0.42 kcal/mol. However, it is closer to the more recent measurement of Lock et al. (J. Phys. Chem. 1996, 100, 7972), DeltaH(o)(f,298)(HOBr) = -14.8 +/- 1 kcal/mol. In the case of HOCl we have determined DeltaH(o)(f,298)(HOCl) = -18.1 +/- 0.3 kcal/mol, just in the middle of the two experimental values proposed, -17.8 +/- 0.5 kcal/mol (JANAF), obtained from equilibrium constant measurements, and -18.36 +/- 0.03 kcal/mol (Joens, J. A. J. Phys. Chem. A 2001, 105, 11041), determined from the measurements of the Cl-OH bond energy. If our conclusions are correct, several enthalpies of formation that have been determined by experimental chemists, Orlando and Burholder (J. Phys. Chem. 1995, 99, 1143), and theoretical chemists, Lee (J. Phys. Chem. 1995, 99, 15074), need to be revised, since a larger value was used for DeltaH(o)(f,298)(HOBr). Employing the results obtained by Orlando and Burkholder for Br(2)O we propose DeltaH(o)(f,298)(Br(2)O) = 24.9 +/- 0.6 kcal/mol, and employing Lee's enthalpies of reaction we propose the following DeltaH(o)(f,298): for BrBrO, HBrO, ClOBr, ClBrO, BrClO, BrCN, BrNC, BrNO, BrON, FOBr, and FBrO, 39.5 +/- 1, 41.0 +/- 1, 22.7 +/- 1.5, 34.2 +/- 1.5, 40.9 +/- 1.5, 43.7 +/- 1.5, 80.1 +/- 1.5, 22.3 +/- 1, 46.2 +/- 1, 17.3 +/- 1.5, and 6.3 +/- 1.5 kcal/mol, respectively. We expect that this work will stimulate new experimental measurements of the thermodynamic properties of HOBr and HOCl.
Denis
Pablo A
PA
DEQUIFIM, Facultad de Química, UDELAR, CC 1157, Montevideo, Uruguay. pablod@fq.edu.uy