Theoritical Kinetic Study of the Ring Opening of Cyclic Alkanes

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 CBSQB3 level of theory. Thermochemical data (H°f, S°, C°p) for all the involved species have been obtained by means of isodesmic reactions. Rate constants have been derived for all elementary reactions using transition state theory at 1 atm and temperatures ranging from 600 to 2000 K. These values have been compared with the few data available in the literature and showed a rather good agreement. * Corresponding author : [email protected] Associated Web site: http://www.ensic.inpl-nancy.fr/DCPR/ DCPR-ENSIC, BP 20451 1, rue Grandville 54001 NANCY Cedex Introduction Among fuels components, cyclic hydrocarbons, particularly cycloalkanes, represent an important class of compounds. These molecules are produced during the gas-phase processes though they can also be present in the reactants in large amounts; for example, a commercial jet fuel contains about 26% of naphtenes and condensed naphtenes, while in a commercial diesel fuel, this percentage reaches 40 % [1]. During combustion or pyrolysis processes, cycloalkanes can lead to the formation of toxics or soot precursors. Several experimental and modeling studies have been carried out on the oxidation of cyloalkanes in gas phase [2-8]. However, the modeling of their combustion remains difficult due to the low number of kinetic and thermodynamic data for the relevant species. Thermal decomposition of cyclobutane has been experimentally studied and rate constants for the ring opening yielding two ethylene molecules have been measured [9-12]. Theoretical studies on cyclobutane have mainly focused on the reverse reaction of cycloaddition of two ethylene molecules [13-16]. Moreover, Pedersen et al. [17] have showed the validity of the biradical hypothesis by direct femtosecond studies of the transition-state structures. Theses studies have highlighted the fact that cycloaddition of two ethylene molecules can proceed through a tetramethylene biradical intermediate or a concerted reaction that directly leads to cyclobutane formation. However, the latter reaction has been shown to have a high activation energy due to steric effects and to be much less favorable than the biradical process. The study of ring opening of cyclobutane is interesting from a theoretically point of view, but larger cycloalkanes such as cyclopentane or cyclohexane are mainly involved in fuels. In spite of that, the unimolecular initiation of these molecules has been little investigated. Ring opening of cyclopentane and cylohexane has been experimentally studied by Tsang [18,19] who reported the main routes of decomposition of these molecules. For cyclohexane, a pathway leading to the formation of 1-hexene has been considered only whereas for cyclopentane, the processes leading to either 1-pentene or to cyclopropane + ethylene have both been investigated, in accordance with the products experimentally detected. In addition, Tsang [18] has shown that the experimental global rate parameters are consistent with a biradical mechanism for ring opening: The present work analyzes the ring opening of cyclobutane, cyclopentane, and cyclohexane by means of high level quantum chemical calculations in order to obtain accurate rate constants for elementary reactions. We explore several plausible pathways that could be involved in the decomposition of the biradicals. Computational method Quantum chemical computations were performed on an IBM SP4 computer with the Gaussian03 software package [20]. The high-level composite method CBSQB3 has been used. Analysis of vibrational frequencies confirmed that all transition structures (TS) have one imaginary frequency. Intrinsic Reaction Coordinate (IRC) calculations have been systematically performed at the B3LYP/6-31G(d) level to ensure that the TSs connect the desired reactants and products. Only singlet states have been considered for the biradicals and their study at the composite CBS-QB3 level requires two specific comments. First, geometry optimization is performed at the density functional theory (DFT) level using an unrestricted onedeterminantal wavefunction (UB3LYP/CBSB7 method). Though such an approach might be questionable for describing open-shell singlet biradicals, previous studies have shown that the obtained ha l-0 03 77 65 4, v er si on 1 27 A pr 2 00 9 Author manuscript, published in "European Combustion Meeting, Chania : Greece (2007)"