Benchmark Thermochemistry of the CnH2n+2 Alkane Isomers (n = 2−8) and Performance of DFT and Composite Ab Initio Methods for Dispersion-Driven Isomeric Equilibria

The thermochemistry of linear and branched alkanes with up to eight carbons has been reexamined by means of W4, W3.2lite and W1h theories. "Quasi-W4" atomization energies have been obtained via isodesmic and hypohomodesmotic reactions. Our best atomization energies at 0 K (in kcal/mol) are 1220.04 for n-butane, 1497.01 for n-pentane, 1774.15 for n-hexane, 2051.17 for n-heptane, 2328.30 for n-octane, 1221.73 for isobutane, 1498.27 for isopentane, 1501.01 for neopentane, 1775.22 for isohexane, 1774.61 for 3-methylpentane, 1775.67 for diisopropyl, 1777.27 for neohexane, 2052.43 for isoheptane, 2054.41 for neoheptane, 2330.67 for isooctane, and 2330.81 for hexamethylethane. Our best estimates for DeltaH(f,298K)(o) are -30.00 for n-butane, -34.84 for n-pentane, -39.84 for n-hexane, -44.74 for n-heptane, -49.71 for n-octane, -32.01 for isobutane, -36.49 for isopentane, -39.69 for neopentane, -41.42 for isohexane, -40.72 for 3-methylpentane, -42.08 for diisopropyl, -43.77 for neohexane, -46.43 for isoheptane, -48.84 for neoheptane, -53.29 for isooctane, and -53.68 for hexamethylethane. These are in excellent agreement (typically better than 1 kJ/mol) with the experimental heats of formation at 298 K obtained from the CCCBDB and/or NIST Chemistry WebBook databases. However, at 0 K, a large discrepancy between theory and experiment (1.1 kcal/mol) is observed for only neopentane. This deviation is mainly due to the erroneous heat content function for neopentane used in calculating the 0 K CCCBDB value. The thermochemistry of these systems, especially that of the larger alkanes, is an extremely difficult test for density functional methods. A posteriori corrections for dispersion are essential. Particularly for the atomization energies, the B2GP-PLYP and B2K-PLYP double hybrids and the PW6B95 hybrid meta-GGA clearly outperform other DFT functionals.