Calculations of hydrogenation enthalpies of hydrocarbons by M06-2X/CBS extrapolated level in the gas phase

Abstract

The standard enthalpies of hydrogenation of 29 unsaturated hydrocarbon compounds were calculated in the gas phase by M06-2X theory with the 6-31g(d) and cc-pVXZ, where X = DZ, TZ, QZ, as well as by complete basis set extrapolated level. Geometries of compounds were optimized at the M06-2X/6-31g(d) level. These M06-2X geometries were used in the M06-2X, and extrapolation calculations with cc-pVXZ basis sets. Comparison of calculation and experimental results shows that the mean absolute deviations between the calculated and experimental enthalpies of hydrogenation range from 25.1 to 5.1 kJ mol−1 at M06-2X calculations, and when using cc-pV(DT)Z extrapolated level, the mean absolute deviations have decreased to 2.7. The results of some calculations showed that the deviations from experimental values are located inside the “chemical accuracy” (±1 kcal mol−1≈±4.2 kJ mol−1). Very good linear correlations between experimental and calculated enthalpies of hydrogenation have been obtained at M06-2X/cc-pVTZ and cc-pV(DT)Z extrapolated levels (standard deviation = 3.2 and 3.4 kJ mol−1, respectively).