The enthalpies of formation for polychlorinated dibenzofurans with use of G3XMP2 model chemistry and density functional theory

Liming Wang; Yi-Liang He

doi:10.1021/jp802091z

The enthalpies of formation for polychlorinated dibenzofurans with use of G3XMP2 model chemistry and density functional theory

J Phys Chem A. 2009 Jan 8;113(1):238-45. doi: 10.1021/jp802091z.

Authors

Liming Wang¹, Yi-Liang He

Affiliation

¹ College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China. wanglm@scut.edu.cn

PMID: 19072627
DOI: 10.1021/jp802091z

Abstract

The standard gas-phase enthalpies of formation of polychlorinated dibenzofurans (PCDFs) have been predicted by using G3XMP2 model chemistry, density functional theory (DFT), and second-order Muller-Plesset (MP2) theory, coupled with isodesmic reactions. The results show a large difference between G3XMP2 and DFT methods with 6-31G(2df,p) and 6-311++G(3df,3pd) basis sets, while MP2/G3MP2Large calculations agree closely with G3XMP2. Two isodesmic reaction schemes are used for better prediction of formation enthalpies. The first (IR1) employs monochlorobenzene as a reference species and the second (IR2) employs polychlorinated benzenes as reference species. The relative stability of PCDFs is rationalized by positional interactions. While the Cl-substitution at position 1/9 leads to the most stable isomers, the simultaneous substitutions at positions 1 and 9 result in a strong repulsion between Cl atoms. Failure of DFT-B3LYP is due to the overestimation of o-ClCl repulsion. For 1,9-PCDFs, the torsion motions of the benzene rings have extremely low harmonic vibrational frequencies. Their contributions to entropy, heat capacity, and thermal corrections have been calculated by using the numerically evaluated energy levels. The PCDF isomer patterns are also discussed based on the calculated thermodynamic parameters.