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.