First‐principles based group additivity values for thermochemical properties of substituted aromatic compounds

Abstract

A set of 7 Benson group additive values (GAV) together with 15 correction terms for non-nearest neighbor interactions (NNI) is developed to calculate the gas phase standard enthalpies of formation, entropies and heat capacities of monocyclic aromatic compounds containing methyl, ethyl, vinyl, formyl, hydroxyl, and methoxy substituents. These GAVs are obtained through least squares regression of a database of thermodynamic properties of 143 molecules, calculated at the post-Hartree-Fock G4 composite method. Out of the 15 NNIs, which account for several well-known substituent effects in aromatic molecules, 13 have been determined for the first time. All but two group additively calculated standard enthalpies of formation agree within 4 kJ mol 21. The entropies and the heat capacities generally deviate less than 4 J mol 21 K 21 from the ab initio results. Natural bond orbital analysis is utilized to identify the underlying causes of the observed NNIs.

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

“… in which i and j run over all atom types and is the number of bonds between atoms of type i and j . The required BACs for the G4 level of theory are taken from an earlier study …”

“…This demonstrates that the change in BDE upon the introduction of a substituent group is only a function of the difference between substituent effects in the radical and the corresponding parent molecule. Equation lays the foundation for identification in this study as the ΔBDEs are calculated with the G4/BAC methodology and the terms are already known from the study of Ince et al…”

“…One obstacle in developing such models for biomass is that reliable thermochemical data for a large number of intermediates and products including those related to lignin are unknown . Therefore, a recent study employed electronic structure methods to generate a large set of thermochemical data for single, double, and various triple substituted MAHs . A similar comprehensive thermochemical data set for MARs is still unavailable.…”

“…This implies that (1) GAVs are defined and parameters are assigned in a systematic way resolving all linear dependencies, (2) the set of NNIs are comprehensive and well‐defined, and (3) all parameters are determined from reference data of equal quality. Marin and coworkers have begun to create such an internally consistent GAV/NNI database based on ab initio results at either the CBS‐QB3 or the G4 level of theory. Currently this database contains GAV and NNI values that allow calculation of thermodynamic properties for aliphatic hydrocarbons and their corresponding radicals, oxygenated hydrocarbons including radicals, organic sulfur compounds, and MAHs .…”

“…Marin and coworkers have begun to create such an internally consistent GAV/NNI database based on ab initio results at either the CBS‐QB3 or the G4 level of theory. Currently this database contains GAV and NNI values that allow calculation of thermodynamic properties for aliphatic hydrocarbons and their corresponding radicals, oxygenated hydrocarbons including radicals, organic sulfur compounds, and MAHs . The purpose of this study is to extend the GAV and NNI database to those substituted MARs, which are expected to play a significant role in lignin pyrolysis.…”