Performance of the major semiempirical, ab initio, and DFT methods for isomerization enthalpies of linear to branched heptanes

ABSTRACT

The gas phase standard state (298.15 K, 1 atm) isomerization enthalpy (Δ_isomH°_(g)) prediction performance of the major semiempirical, ab initio, and density functional levels of theory for environmentally relevant transformations was investigated using the linear to branched heptanes as a representative case study. The M062X density functional, MP2 (and higher) levels of Moller-Plesset perturbation theory, and the CBS and Gaussian-n composite methods are well suited for investigating the thermodynamic properties of environmentally interesting isomerizations, although the M062X functional may not be appropriate for all systems. Where large molecular systems prohibit the use of higher levels of theory, the PM6 and PDDG semiempirical methods may offer an appropriate computational cost-accuracy compromise.

KEYWORDS:

• Isomerization enthalpies
• heptanes
• theoretical methods
• environmental fate

Acknowledgments

This work was made possible by the facilities of the Western Canada Research Grid (WestGrid:www.westgrid.ca; project 100185), the Shared Hierarchical Academic Research Computing Network (SHARCNET:www.sharcnet.ca; project aqn-965), and Compute/Calcul Canada.