Reaction enthalpies core correlation

Since the core correlation is largely unaffected by chemical reactions, it is worthwhile to investigate whether its contribution to the reaction enthalpies can be obtmned at less demanding levels of... 

Let us investigate whether the core correction can be reliably estimated in a basis smaller than that needed for the dominant valence-correlation contribution to the reaction enthalpy. From Section 15.7.4, we recall that the core correction to the atomization energy may be estimated... 

Fig. 15.18. The errors relative to experiment in CCSD(T) reaction enthalpies (kJ/mol) calculated with core correlation included (full line) and without core correlation included (dotted line).

Table 1535 Errors in the core-correlation contributions to the reaction enthalpies relative to the CCSD(T)/cc-pCV5Z level (kJ/mol)...

Intramolecular nucleophilic substitution to form thiiranes was studied by means of ab initio MO computations based on the 6-31G basis set <1997JCC1773>. Systems studied included the anions SCH2CH2F and CH2C(=S)CH2F which would afford thiirane and 2-methylenethiirane, respectively (Equations Z and 3). It was important to include electron correlation which was done with the frozen-core approximation at the second-order Moller-Plesset perturbation level. Optimized structures were confirmed by means of vibrational frequency calculations. The main conclusions were that electron correlation is important in lowering AG and AG°, that the displacements are enthalpy controlled, and that reaction energies are strongly dependent on reactant stabilities. 

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