Detailed Kinetic Study of the Ring Opening of Cycloalkanes by CBS-QB3 Calculations

Abstract

This work reports a theoretical study of the gas phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data (∆H°f, S°, C°p) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the 1 energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed to validate the theoretical approach. An important result is that the rotational barriers between the conformers, which are usually neglected, are of importance in decomposition rate of the largest biradicals. Ring strain energies (RSE) in transition states for ring opening have been estimated and show that the main part of RSE contained in the cyclic reactants is removed upon the activation process.

An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

“…Many experiments have been carried out to investigate pyrolysis [3][4][5][6], oxidation [7][8][9][10][11][12][13][14][15][16][17] and combustion [18][19][20][21][22][23][24][25] of cyclohexane. In addition to experimental investigations, some theoretical studies have been performed by Handford-Styring and Walker [26], Sirjean et al [27] and Cavallotti et al [28].…”

Formation of H‐atoms in the pyrolysis of cyclohexane and 1‐hexene: A shock tube and modeling study

“…To take a closer look at the kinetics of this retro-ene process, King [9] has performed another VLPP study on the thermal decomposition of 1-hexene with temperatures ranging from 915 to 1153 K. Rice-RamspergerKassel-Marcus (RRKM) calculations were done, and King's results confirmed that at temperatures above 1000 K 1-hexene preferably decomposes via reaction (R2). Sirjean et al [10] presented a theoretical study dealing with the quantum chemical exploration of ring-opening reactions of common cyclic alkanes like cyclohexane. For the ring-opening step yielding the biradical 1,6-hexadiyl (…”

“…According to Sirjean et al [10], the subsequent formation of 1-hexene involves a six-membered ring as a slightly constrained transition state for H-abstraction. For this process, an activation energy of 1.8 kcal·mol .…”

Chemical activation reactions of cyclic alkanes and ethers and tricyclodecane ring‐opened diradicals with O2: Thermochemistry, reaction paths, kinetics, and modeling

“…We estimate approximately five collisions (A = 9 × 10 13 cm 3 (mol s) −1 ) and a small barrier (E a = 1.5), for the electronic state crossing, collision of diradical with bath gas, or chemical activation of diradical in association reaction with O 2 , as there is ∼35 kcal mol The intramolecular hydrogen transfer reactions were estimated from literature calculations on fourto six-membered ring-opened systems [1] at the CBS-QB3 level, and a computational study on the thermal decomposition of tricyclodecane [14]. The rate constants for the unimolecular β-scission reactions have been estimated from a literature evaluation of the experimental data on β-scission reactions of hydrocarbons [15].…”

“…Sirjean et al have studied thermochemistry and kinetics of ring-opening reactions of cyclic alkanes using high-level computational chemistry on both singlet and triplet [1,2]. To our knowledge, there are no studies on the reactions of the cyclic alkyl diradicals or on the ring-opening products from cyclic ethers that are formed at early time by reactions of the radical pool.…”