Understanding the thermal dehydrochlorination reaction of 1-chlorohexane. Revealing the driving bonding pattern at the planar catalytic reaction center

Abstract

The nature of bonding along the gas-phase thermal decomposition of 1-chlorohexane to produce 1-hexene and hydrogen chloride has been examined at the DFT M05-2X/6-311+G(d,p) level of theory. Based on results both from energetical and topological analysis of the electron localization function (ELF), we propose to rationalize the experimental available results not in terms of a decomposition via a four-membered cyclic transition structure (TS), but properly as a two stage one step reaction mechanism featuring a slightly asynchronous process associated to the catalytic planar reaction center at the TS. In this context, the first electronic stage corresponds to the ‫ܥ‬ ఋା ••• ‫݈ܥ‬ ఋି bond cleavage, which take place on the activation path earlier the transition structure be reached. There is no evidence of a Cl-C bond at the TS configuration. The second stage, associated to the top of the energy barrier, includes the TS and extends beyond on the deactivation pathway towards the products. The existence of both bonding and nonbonding non covalent interactions (NCI) are also revealed for the first time for the TS configuration. 17 47 to point -9 along the IRC reaction pathway. Arbitrarily the second stage concerning main electronic changes will be associated to points -9 to +9. Both stages refer to regions where key electronic changes driving the transformation take place. Note that from point -34 to point -5, the population associated to the disynaptic basin V(C α ,C β ) increases from 1.99e to 2.20e, whereas the population integrated in the disynaptic protonated basin V(H β ,C β ) decreases from 1.98e to 1.72e. The attractor corresponding to the forming double bond localizes out of the line connecting the carbon center cores. This topology for the C  -C  region remains unchanged until point 12 in region g. At the top of the energy barrier, within the entire interval IRC=(-0.97962,+0.97971), i.e., point -9 to point 9, we observe an isolated electron localization region integrating to 7.64e which is associated to the migrating chlorine center. This picture, on the top of the barrier, corresponds to a lonely chlorine atom bearing an electronic charge of -0.64 while evolving to the encounter of the H β center.Thereafter, within the tiny region d four valence monosynaptic basins V(Cl) associated to the negatively charged chlorine center adopt a tetrahedral conformation, with an increasing in the population from 7.64e to 7.70e. In this region the valence basins populations for V(C α ,C β ) and V(H β ,C β ) varies from 2.41e to 2.34e, and from 1.81e to 1.51e, respectively. The migrating chlorine center develops thus a maximum charge of -0.71 at the transition structure (point 0). ELF picture of bonding reveals that activation events are mainly associated to conformational changes in preparation of the reaction center (region a), followed by the breaking of the Cl-C α bond (regions b and c), and the start of the migration of H  atom with the associated bonding changes at C α -C β -H β moiety (regions c and d). Regarding t...

Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

“…Similar conclusions are derived when the preexponential factors are examined. In fact, our estimations agree reasonably well with the value of 1.23 × 10 14 s −1 obtained by Chuchani et al and are significantly higher than the value of 6.92 × 10 9 s −1 from Nisan et al Also our results are in reasonable agreement with the values of E a ∞ = 55.5 kcal mol −1 and A ∞ = 1.2 × 10 14 s −1 derived by Lopez et al with the M05–2X/6–311+G(d,p) method, and with the typical Arrhenius parameters derived for the 1‐chloroalkanes from chloroethane to 1‐chloropentane of E a = 54–58 kcal mol −1 and A = (0.2–4) × 10 14 s −1 . It is important to note that the estimations considering tunneling effect are in better agreement with the experimental results of Chuchani et al than the results without this correction.…”

“…B. The structure here calculated for the transition state is similar to that estimated by López et al with the M05–2X/6–311+G(d,p) level of theory.…”

“…Although both investigations concluded that the unimolecular reaction is homogeneous, independent of the initial pressure, and follows a first‐order rate law, discrepancies of about 14 kcal mol −1 in the activation energy and of five orders of magnitude in the preexponential factor were found. On the other hand, during the development of the present work, Lopez et al published a theoretical investigation on the mechanism of reaction (1) . Based on an energetical and topological analysis of the electron localization function, they pointed to characterize the electronic patterns governing cyclic transition states.…”

“…Chlorinated compound have received special attention because they could produce Cl atoms or reactive chlorinated species which could be involved in atmospheric reactions. There are numerous related investigations of the thermal decomposition of chloroethane , 1‐chloropropane , 1‐chlorobutane , and 1‐chloropentane , but for 1‐chlorohexane, which has extended use in industry as a solvent; there exists limited kinetic information . Studies and reviews of the thermal decomposition of 1‐chloroalkanes, from chloroethane to 1‐chloropentane, show that the corresponding activation energies are about 54–58 kcal mol −1 , and the preexponential factors are (0.2–4) × 10 14 s −1 .…”

Theoretical Study on the Mechanism of the Thermal Retro-Cycloaddition of Isoxazolinofullerenes

“…The blue area presented in the RDG figures between O–C C60 and C2–C C60 represents strong attractive bonding noncovalent interactions , associated with the breaking bond region, and the intensity of the blue color is related to the distance between the 1,3-dipole and the C60 species. This fact is clearly shown by the less-intense blue color presented among O–C C60 in TS1_d (light blue color) and in TS1_e.…”