Entropy data are reported using different calculation methods for internal rotors on n-heptane, 2-methylhexane, and 2,3-dimethylpentane and on the different radical sites of each species corresponding to the loss of a hydrogen atom for temperatures between 298 and 1500？K. Structures, moments of inertia, vibration frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level of theory. Comparisons with experimental literature data suggest limitations inuse of the rigid-rotor harmonic-oscillator (HO) approximation and advantages to the use of internal rotation contributions for entropy relative to torsion frequencies. The comparisons suggest the need to include contributions from all internal rotors where the barriers are at or below those of the above molecules. Calculation of entropy from the use of internal rotor contributions provides acceptable approximations to available literature values. Entropy values for radicals corresponding to carbon sites on these hydrocarbons are presented. 1. Introduction Over the past decades advancements made in quantum chemistry, including the evolution of density functional theory (DFT) and compound methods, coupled with increasing computer processing power, have allowed for substantial advancements in estimating thermochemical and other chemical properties and systems [1]. Thermochemical properties including enthalpies, entropies, and heat capacities for stable molecules and for radicals corresponding to the loss of H atom through abstraction reactions are important and necessary for building and analyzing detailed chemical kinetic models. Several research studies have focused on linear and branched alkanes, including n-heptane, [2–6] because it has been considered as an important surrogate for major components in diesel and spark ignition transportation fuels. The parent hydrocarbons might be stable alkanes that have been studied experimentally, but oxidation under both atmospheric and combustion conditions creates hundreds of intermediate species that are fragmented, partially oxygenated, and/or unsaturated. The stability, thermochemical properties, and chemical kinetics of these intermediates is needed for the model used for optimization. Enthalpy, entropy, and heat capacity are all important because of the influence they have on reactions’ paths. The TΔS entropy term in the Gibbs Energy influences the equilibrium constant and calculation of reverse reaction rate constants with a large lever because of the temperature multiplier. Entropy also has a large contribution to preexponential
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