Theoretical Study of Thermal
Rearrangements of 1-Hexen-5-yne,
1,2,5-Hexatriene, and 2-Methylenebicyclo[2.1.0]pentane
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Abstract
In this research, a comprehensive theoretical investigation
of
the thermal rearrangements of 1-hexen-5-yne, 1,2,5-hexatriene, and
2-methylenebicyclo[2.1.0]pentane is carried out employing density
functional theory (DFT) and high level <i>ab initio</i> methods,
such as the complete active space self-consistent field (CASSCF),
multireference second-order Møller–Plesset perturbation
theory (MRMP2), and coupled-cluster singles and doubles with perturbative
triples [CCSD(T)]. The potential energy surface (PES) for the relevant
system is explored to provide a theoretical account of pyrolysis experiments
by Huntsman, Baldwin, and Roth on the target system. The rate constants
and product distributions are calculated using theoretical kinetic
modelings. The rate constant for each isomerization reaction is computed
using the transition state theory (TST). The simultaneous first-order
ordinary-differential equations are solved numerically for the relevant
system to obtain time-dependent concentrations, hence the product
distributions at a given temperature. Our computed energy values (reaction
energies and activation parameters) are in agreement with Roth’s
experiments and the product distributions of Huntsman’s experiments
at 340 and 385 °C with various reaction times, while simulated product fractions are in qualitative accordance with Baldwin’s experiment