Theoretical Calculations of Hydrogenation Enthalpies of Some Unsaturated Hydrocarbon Compounds

The standard enthalpies of hydrogenation of 29 unsaturated hydrocarbon compounds were calculated in the gaseous state by the CCSD(T) and M06-2X theories of levels with the complete basis 6-31g(d) and cc-pVXZ, where X =DZ, TZ, QZ, these enthalpies were calculated, as well as by complete basis set limit extrapolation using two different formulas. Geometries of compounds were optimized at the M06-2X/6-31g(d) level. This M06-2X geometries were used in the M06-2X, CCSD(T) and extrapolation calculations with cc-pVXZ basis sets. Comparison of calculation and experimental results shows that the median absolute deviations (MAD) between the calculated and experimental enthalpies of hydrogenation range from 25.1 to 5.1 kJ mol-1 at M06-2X calculations and from 8.8 to 4.1 kJ mol-1 at CCSD(T) calculations. for both calculations, where using complete basis set limit extrapolation E(X) = E¥ + A/X3, the MAD have decreased to 2.7 and 1.5 kJ mol-1 respectively. 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/TZ and CCSDT/TZ methods.