(758c) Accurate Estimation of Thermochemical Parameters for Branched and Strained Organics: The Enthalpy of Formation of 2,2,4,4,6,8,8-Heptamethylnonane

Accurate thermochemistry is critical for many applications, particularly predictive kinetic modeling. The heat of formation of highly-branched hydrocarbons, such as the diesel primary reference fuel 2,2,4,4,6,8,8-heptamethylnonane (HMN, or “isocetane”), is significantly affected by steric crowding. Multiple approaches have been devised to account for these effects with group-additive methods. Several such approaches were investigated, with focus on two of the most widely-used approaches: Benson’s approach (using group parameters from 1992) and the “methyl repulsion” method of Domalski and Hearing (1988). Our study of several highly branched alkanes suggests that Benson’s approach to steric corrections is at least comparable, and possibly superior, in accuracy to the “methyl repulsion” approach.

The investigation, which also includes supporting evidence from MM4 and CBS-QB3 calculations, suggests that gas-phase ΔH_f(298 K) values for HMN used in the literature and reported by some group-additivity approaches are as much as 3-8 kcal/mol too low. The use of accurate thermodynamic parameters for HMN will be important in ongoing efforts to construct detailed chemical kinetic models for its combustion.

This work is part of a larger effort to improve parameter estimation during automated construction of detailed chemical kinetic models, particularly for combustion and pyrolysis. Although Benson’s method is excellent for branched aliphatics, Benson’s method for accounting for ring strain, particularly for fused ring systems, is less satisfactory. Our methods for estimating the thermochemistry of strained ring species using explicit three-dimensional molecular structures in the context of automated reaction mechanism generation will also be summarized.