Modeling the heat of formation of organic compounds using SPARC
Whiteside, Tad Sease
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Typically, the interaction of chemicals with the environment is governed through physic-ochemical properties. The Environmental Protection Agency has developed several models to predict the fate of chemicals in the environment. SPARC (SPARC Performs Automated Reasoning in Chemistry) has been developed as a method to predict the properties of environmentally sensitive compounds. SPARC uses computational algorithms based on chemical structure theory to calculate chemical properties, including the heat of formation. Molecular structures are broken into simple functional units (reactophores) with intrinsic properties. Each reactophore is analyzed and the effects of appended molecular structures are quantified through perturbation theory. Standard enthalpies of formation ( Hf ) were calculated with models developed using the computer program SPARC. The Hf models have been completely developed using all known data for saturated and unsaturated hydrocarbons and halogenated hydrocarbons. Basic models have also been developed for alcohols, aldehydes, and ketones. The structures of these compounds vary from chains and conjugated rings to poly-benzoic aromatic hydro-carbons. The 587 hydrocarbons have a SPARC calculated RMS of 4.50 kJ mol-1. Halogenated hydrocarbons have a calculated RMS deviation of 5.18 kJ mol-1 for 202 compounds. The effect of stereochemistry on the standard enthalpy of formation was also modeled. Chiral centers are found in a variety of molecules and help define the overall structure of a compound. The local atomic environment determines the strain energy in each chiral center. There are four local environments in which chiral centers are found: Linear, Single, Bridge, and SideShare. These are modeled independently and the total contribution of stere- ochemistry to the heat of formation is determined by summing the energy found in these environments. The 169 experimentally determined compounds with chiral centers were used to develop this model. To provide a benchmark of SPARC’s capabilities, the heat of formation of the compounds used to develop the models was also calculated using the semi-empirical PM3 method and the group additivity method developed by Benson, as implemented by NIST in their chemical webbook. SPARC outperforms both of these methods in terms of speed and accuracy.