New Approach For Calculation Of Enthalpy Of Formation From Computational Thermochemistry

The Ochterski method (1) is often used for calculation of standard enthalpies of formation (D?? 0) from thermochemical data obtained by molecular modeling. The method considers that a molecule is formed by the junction of isolated atoms and that the molecular D?? 0 at 298.15 K is obtained by a thermochemical cycle involving the molecule and its free atoms at 298.15 K and 0 K. In this work, it was considered that the term describing the enthalpy of atoms at 0 K should be substituted to 298.15 K for the cycle be closed. Structures of chlorinated and brominated trihalomethanes (TCM, BDCM, DBCM and TBM) were optimized in vacuum using ab initio G4 theory (2) and the values of D?? 0 at 298.15 K were determined by both classical and modified Ochterski method. Results are presented in Table 1. One of the difficulties in calculating thermochemical parameters from molecular modeling is the quantum description of atoms with many electrons, as chlorine and bromine. In this work, the G4 method of optimization was used because it describes these atoms with accuracy. From Table 1, the values of DHf0 calculated by the Ochterski method were all endothermic and overestimated by about 100 times compared to the references. However, with the proposed correction it was obtained values of D?? 0 quite close to those of reference. For the TBM, containing three bromine atoms, the margin of error was only 11.3%, acceptable considering the complexity of the electronic structure.