Thermochemistry of C7H16 to C10H22 alkane isomers: primary, secondary, and tertiary C-H bond dissociation energies and effects of branching.

Abstract

Standard enthalpies of formation (ΔH°f298) of methyl, ethyl, primary and secondary propyl, and n-butyl radicals are evaluated and used in work reactions to determine internal consistency. They are then used to calculate the enthalpy of formation for the tert-butyl radical. Other thermochemical properties including standard entropies (S°(T)), heat capacities (Cp(T)), and carbon-hydrogen bond dissociation energies (C-H BDEs) are reported for n-pentane, n-heptane, 2-methylhexane, 2,3-dimethylpentane, and several branched higher carbon number alkanes and their radicals. ΔH°f298 and C-H BDEs are calculated using isodesmic work reactions at the B3LYP (6-31G(d,p) and 6-311G(2d,2p) basis sets), CBS-QB3, CBS-APNO, and G3MP2B3 levels of theory. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level for contributions to entropy and heat capacities. Enthalpy calculations for these hydrocarbon radical species are shown to have consistency with the CBS-QB3 and CBS-APNO methods using all work reactions. Our recommended ideal gas phase ΔH°f298 values are from the average of all CBS-QB3, CBS-APNO, and for G3MP2B3, only where the reference and target radical are identical types, and are compared with literature values. Calculated values show agreement between the composite calculation methods and the different work reactions. Secondary and tertiary C-H bonds in the more highly branched alkanes are shown to have bond energies that are several kcal mol(-1) lower than the BDEs in corresponding smaller molecules often used as reference species. Entropies and heat capacities are calculated and compared to literature values (when available) when all internal rotors are considered.