Gas phase isomerization enthalpies of organic compounds: A semiempirical, density functional theory, and ab initio post-Hartree–Fock theoretical study

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A database of gas phase standard state isomerization enthalpies was constructed for 562 pure and nitrogen-, oxygen-, sulfur-, and halogen-containing hydrocarbon reactions. The PM6 and PDDG semiempirical methods, B3LYP and M062X hybrid density functionals with the 6-311++G(d,p) and 6-311+G(3df,3p) basis sets, and the CBS-Q//B3 and G4MP2 ab initio post-Hartree–Fock composite methods were examined for prediction accuracy within each class of isomerization reactions. At much lower computational cost, the PM6 and PDDG semiempirical methods offer modest isomerization enthalpy prediction performance approximately comparable to the B3LYP density functional. The M062X density functional provides nearly equivalent accuracy to the higher level CBS-Q//B3 and G4MP2 methods across all hydrocarbon classes. Increasing basis set size from 6-311++G(d,p) to 6-311+G(3df,3p) with the B3LYP and M062X density functionals does not influence their respective isomerization enthalpy prediction accuracies. Using the 6-311+G(3df,3p) basis set, the M062X functional also achieves near CBS-Q//B3 quality accuracy for enthalpies of formation using the atomization approach.