ReaxFF molecular dynamics simulations of oxidation of toluene at high temperatures.

Aromatic hydrocarbon fuels, such as toluene, are important components in real jet fuels. In this work, reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field have been performed to study the high-temperature oxidation mechanisms of toluene at different temperatures and densities with equivalence ratios ranging from 0.5 to 2.0. From the ReaxFF MD simulations, we have found that the initiation consumption of toluene is mainly through three ways, (1) the hydrogen abstraction reactions by oxygen molecules or other small radicals to form the benzyl radical, (2) the cleavage of the C-H bond to form benzyl and hydrogen radicals, and (3) the cleavage of the C-C bond to form phenyl and methyl radicals. These basic reaction mechanisms are in good agreement with available chemical kinetic models. The temperatures and densities have composite effects on toluene oxidation; concerning the effect of the equivalence ratio, the oxidation reaction rate is found to decrease with the increasing of equivalence ratio. The analysis of the initiation reaction of toluene shows that the hydrogen abstraction reaction dominates the initial reaction stage at low equivalence ratio (0.5-1.0), while the contribution from the pyrolysis reaction increases significantly as the equivalence ratio increases to 2.0. The apparent activation energies, E(a), for combustion of toluene extracted from ReaxFF MD simulations are consistent with experimental results.

Effects of fuel additives on the thermal cracking of n-decane from reactive molecular dynamics.

Thermal cracking of n-decane and n-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of n-decane are mainly through two pathways: (1) the cleavage of a C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH(3), and C(2)H(5). The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of n-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C-C or C-H bond-breaking in n-decane. The thermal cracking of n-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl tert-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure n-decane, in the following order: NP > TEMPO > DADP > DEE (∼MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level.

Multichannel photoinduced intramolecular electron-transfer excitations in a bis-naphthalimide spermine conjugate by time-dependent density functional theory.

Density functional theory was applied to the investigation of photoinduced electron transfer (ET) and the absorption spectrum for a bis-naphthalimide spermine conjugate. The multichannel feature of ET excitation in this system was focused on because four groups may act as electron donors and acceptors. The segment in this conjugate, N-(N-methylpropyl)-1,8-naphthalimide, which contains one donor and acceptor pair, was studied at first. Through theoretical calculation, the absorption band at 340 nm was assigned to the pi-->pi* transition. For the whole system involving four chromophores, this work suggested three types of ET. From the theoretical investigation, the naphthalimide radical anion turned out to be formed via intramolecular ET between the two terminal naphthalimide groups, rather than via the electron transfer between the dialkylamine moiety and the naphthalimide one. Furthermore, the electronic coupling matrix elements according to the generalized Mulliken-Hush theory were estimated and the detailed analyses showed that the strongest absorption was due to the local excitation of the naphthalimide chromophore.