2 research outputs found
High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane
Unimolecular dissociation of 1,3,5-trioxane
was investigated experimentally
and theoretically over a wide range of conditions. Experiments were
performed behind reflected shock waves over the temperature range
of 775–1082 K and pressures near 900 Torr using a high-repetition
rate time of flight mass spectrometer (TOF-MS) coupled to a shock
tube (ST). Reaction products were identified directly, and it was
found that formaldehyde is the sole product of 1,3,5-trioxane dissociation.
Reaction rate coefficients were extracted by the best fit to the experimentally
measured concentration–time histories. Additionally, high-level
quantum chemical and RRKM calculations were employed to study the
falloff behavior of 1,3,5-trioxane dissociation. Molecular geometries
and frequencies of all species were obtained at the B3LYP/cc-pVTZ,
MP2/cc-pVTZ, and MP2/aug-cc-pVDZ levels of theory, whereas the single-point
energies of the stationary points were calculated using coupled cluster
with single and double excitations including the perturbative treatment
of triple excitation (CCSDÂ(T)) level of theory. It was found that
the dissociation occurs via a concerted mechanism requiring an energy
barrier of 48.3 kcal/mol to be overcome. The new experimental data
and theoretical calculations serve as a validation and extension of
kinetic data published earlier by other groups. Calculated values
for the pressure limiting rate coefficient can be expressed as log<sub>10</sub> <i>k</i><sub>∞</sub> (s<sup>–1</sup>) = [15.84 – (49.54 (kcal/mol)/2.3<i>RT</i>)] (500–1400
K)
Theoretical Study of the Reaction Kinetics of Atomic Bromine with Tetrahydropyran
A detailed theoretical analysis of
the reaction of atomic bromine
with tetrahydropyran (THP, C<sub>5</sub>H<sub>10</sub>O) was performed
using several ab initio methods and statistical rate theory calculations.
Initial geometries of all species involved in the potential energy
surface of the title reaction were obtained at the B3LYP/cc-pVTZ level
of theory. These molecular geometries were reoptimized using three
different meta-generalized gradient approximation (meta-GGA) functionals.
Single-point energies of the stationary points were obtained by employing
the coupled-cluster with single and double excitations (CCSD) and
fourth-order Møller–Plesset (MP4 SDQ) levels of theory.
The computed CCSD and MP4Â(SDQ) energies for optimized structures at
various DFT functionals were found to be consistent within 2 kJ mol<sup>–1</sup>. For a more accurate energetic description, single-point
calculations at the CCSDÂ(T)/CBS level of theory were performed for
the minimum structures and transition states optimized at the B3LYP/cc-pVTZ
level of theory. Similar to other ether + Br reactions, it was found
that the tetrahydropyran + Br reaction proceeds in an overall endothermic
addition–elimination mechanism via a number of intermediates.
However, the reactivity of various ethers with atomic bromine was
found to vary substantially. In contrast with the 1,4-dioxane + Br
reaction, the chair form of the addition complex (<i>c</i>-C<sub>5</sub>H<sub>10</sub>O–Br) for THP + Br does not need
to undergo ring inversion to form a boat conformer (<i>b</i>-C<sub>4</sub>H<sub>8</sub>O<sub>2</sub>–Br) before the intramolecular
H-shift can occur to eventually release HBr. Instead, a direct, yet
more favorable route was mapped out on the potential energy surface
of the THP + Br reaction. The rate coefficients for all relevant steps
involved in the reaction mechanism were computed using the energetics
of coupled cluster calculations. On the basis of the results of the
CCSDÂ(T)/CBS//B3LYP/cc-pVTZ level of theory, the calculated overall
rate coefficients can be expressed as <i>k</i><sub>ov.,calc.</sub>(<i>T</i>) = 4.60 × 10<sup>–10</sup> expÂ[−20.4
kJ mol<sup>–1</sup>/(<i>RT</i>)] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> for the temperature range of
273–393 K. The calculated values are found to be in excellent
agreement with the experimental data published previously