The Enthalpies of Formation for Polychlorinated Dibenzofurans with Use of G3XMP2 Model Chemistry and Density Functional Theory

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.

Bond dissociation enthalpies in chlorinated benzenes and phenols and enthalpies of formation of their free radicals: A Gaussian-4 prediction

“…For chlorobenzenes, inspection of BDEs listed in Table II shows that the C H bonds are strengthened upon Cl substitution, whereas the C Cl BDEs vary within a narrow range of 402-408 kJ/mol. Linear regression on G4 BDEs shows that the substitution effect is additive: The expression for BDE (C Cl) is consistent with our previous conclusion that B3LYP overestimates the ortho-ClCl repulsions in polychlorinated aromatics [26,30,31]. [32], which, however, has no intramolecular hydrogen bond as 2-C 6 H 4 ClOH does.…”

Predicted specific heat behaviour of the polychlorinated dibenzofuran family from DFT analysis

“…The number of isomers of PCDF is even greater than those of PCDD, and it was shown that, for a limited set of these [6] it is possible to reproduce the heats of formation predicted by DFT with good accuracy using a simple algorithm. Wang and He [7] have recently applied a number of model chemistries and basis sets to the calculation of heats of formation of the PCDF family and the results from the present study compare closely with these.…”

“…The method used in estimating the standard heats of formation of the PCDF molecules has been previously described [6,7] and is based on the use of a calculated heat of reaction involving the target molecule combined with the known heats of formation of the other reaction species. The heats of reaction are calculated from the vacuum energies of the individual molecules involved in each reaction.…”

The enthalpies of formation of brominated benzenes and phenols: A theoretical prediction

“…On the other hand, there have been two theoretical predictions on the thermodynamic properties of PBDD/Fs [22,23] and PBDEs [24] using density functional theory (DFT) at B3LYP/6-31G(d) level. Our previous theoretical studies on PCDD/DF/Ns showed that DFT methods greatly over-estimated the Cl-Cl repulsions when comparing to Gaussian-3X (G3X) method, and the discrepancy of DFT-B3LYP to G3X was as large as 30 kJ/mol for C 6 Cl 6 even after homodesmic reaction correction [25][26][27]. Therefore, the DFT results for PBDD/ DF/DEs are suspected being too high.…”