Extensive Theoretical Study of the Thermochemical Properties of Unsaturated Hydrocarbons and Allylic and Super-Allylic Radicals: The Development and Optimization of Group Additivity Values

In this study, the thermochemistry of C₂–C₇ unsaturated hydrocarbons (22 alkene and 6 diene molecules) and 16 allylic and 5 super-allylic radicals is determined using high-accuracy quantum chemistry calculations. In addition, the group additivity values (GAVs) of a total of 19 relevant groups are systematically optimized on the basis of the calculated thermochemistry of species clusters. The M06-2X method using the 6-311++G­(d,p) basis set is used for the geometry optimizations, vibrational frequency calculations, and internal rotation scans for lower-frequency modes. The composite compound methods, CBS-APNO, G3, and G4, are utilized to derive the average atomization formation enthalpies. The entropy and temperature-dependent heat capacity values of all species are calculated using statistical thermodynamics in MultiWell. These results are in good agreement with literature data. A GAVs optimization is performed on the basis of a statistical analysis: a Bland–Altman plot, which is employed to visualize the agreement between the results from the quantum chemical calculations and the GA method. It is found that the 298 K entropies of the CD/C2, C/CD2/H2, C/C/CD2/H, and C/CD3/H groups disagree by more than 5 cal K^–1 mol^–1 compared to existing values, while the values for the ALLYLS and ALLYLT radical groups also differ by ∼2.4 and 4.1 cal K^–1 mol^–1, respectively. The 298 K formation enthalpies of the C/CD2/H2, C/C/CD2/H, C/CD3/H, and ALLYLT groups are modified by more than 1 kcal mol^–1, compared to existing values. The updated GAVs can be used with increased confidence to estimate the thermochemical properties of combustion-relevant unsaturated hydrocarbon molecules and their radicals which are critical for the development of accurate chemical kinetic models describing the pyrolysis and oxidation of hydrocarbon and oxygenated hydrocarbon fuels