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Theoretical thermochemistry. The enthalpies of formation of some XHn and XYHn compounds

1991; Elsevier BV; Volume: 226; Issue: 3-4 Linguagem: Inglês

10.1016/0166-1280(91)85010-5

1872-7999

Michel Sana, Georges Leroy,

Nuclear Materials and Properties

Ab initio molecular orbital theory (Moller-Plesset to fourth order with 6–31 +G(2df,p) and 6–311 + + G(3df,2p) basis sets) was used to compute the total energies of a few XHn and XYHn compounds. In each case, the fully optimized structures at the full second order of the Møller-Plesset perturbation theory were employed and the thermal corrections were included using analytical harmonic frequencies at the Hartree-Fock level. We discuss the choice of the reaction to evaluate the enthalpies of formation as accurately as possible. For XHn compounds, the isogyric reaction of Pople leads to valuable results (± 1 kcal mol−1). For XYHn systems, we propose the use of bond hydrogenation reactions. In any case, the isodesmic reaction remains a fairly good choice when feasible (for instance with radicals). It appears that some corrections proposed in the recent G1 procedure of Pople are not crucial for saturated molecules as long as very extended basis sets are used (without combination of separately treated corrections) or with hydrogenation reactions. Nevertheless, Pople's ΔE (QCI) term becomes important for unsaturated radicals. This term permits the incorporation of correlation effects beyond the MP4 level.