NEARLY AB-INITIO THERMOCHEMISTRY - THE USE OF REACTION SCHEMES - APPLICATION TO IO AND HOI

Reaction energetics from ab initio calculations are almost always comb ined with experimental data to derive enthalpies of formation. Some po pular strategies are discussed, and a simple criterion is suggested fo r estimating the accuracy to be expected from using an arbitrary react ion, We recommend using more than one reaction scheme whenever possibl e and suggest how the results from different reactions may be combined in a weighted average. The uncertainties of the derived values can be estimated from the averaging procedure and by comparing the results o btained using different basis sets and treatments of electron correlat ion. As examples, we compute the gas-phase thermochemistry for IO-, IO , IO+, and HOI. As a check, analogous calculations are also done for t he corresponding bromine species. Finally, we recommend that appropria te experimental data be exploited whenever they are expected to be mor e accurate than their theoretical counterparts. While compiling the au xiliary data to support these calculations, we found that recent measu rements require that the experimental enthalpies of formation of many molecules be revised. We present values for HOCl, HOBr, Cl2O, and Br2O .