This work presents a thoroughgoing theoretical study on the OH-initiated combustion chemical kinetics and atmospheric degradation of C4F9N by employing high-level quantum chemical methods and RRKM/master-equation theory. All the stationary points on potential energy surface were cautiously investigated at B3LYP/6-311++G(d,p) level for geometry optimizations, and thereby their single-point energies were refined by applying CCSD(T)/6-311++G(d,p) method. Based on quantum calculations, kinetics and branching ratios for the major channels were predicted within 300-3000 K and 0.01-100 atm by solving the RRKM/master-equations. The addition of OH to C4F9N generating M1 dominates the overall kinetics at low temperatures. Subsequently, its two β-scission channels of C-C bonds forming CF3CF2N=CF(OH)+CF3 (P8) and CF2=NCF(OH)CF3+CF3 (P9) become competitive and play a lead role in whole C4F9N+OH system at the corresponding high temperatures and elevated pressures. The formation of CF3 radical prompts two routes to potentially have the significant contribution to flame inhibition in actual applications. Additionally, the complex degradation pathways of C4F9N were also looked into by successively reacting with various oxides, including OH, O2, NO, HO2, to finally generate the removal products CF3CF2N(OOH)CF(OH)CF3 (Pd2), CF3CFO (Pd3-2), and CF3CF2NO (Pd4). The atmospheric lifetime of C4F9N is evaluated as 72 years regarding to one step addition between C4F9N and OH radical.
Cyclohexyl radicals are crucial primary intermediates in combustion of fossil and alternative fuels. They would present the inherent conformation feature, i.e. diverse conformers retained in inversion-topomerization pathways, jointly controlled by the varying radical site and specific spatial positions of alkyl side chains on “easy-distortion” cyclic ring. These conformers for one radical have different energies and thermodynamics, and are highly expected to influence their subsequent decomposition reactions in terms of energetics and kinetics. To reveal such impact, all conformational structures and their interconversion mechanisms for trans-1,2-dimethylcyclohexyl isomers were explored by employing quantum chemical calculations coupled with transition state theory. Originated from distinct conformers, all accessible transition states were explicitly identified in different reaction paths for each type of intramolecular H-transfer or β-scission, and then were carefully used in computing rate coefficients. The kinetic predictions demonstrate that the fairly speedy equilibrium among conformers would be established for one isomer via conformation before they proceed the initial decomposition over 300-2500 K. This allows thoroughly evaluating the contribution of various conformers to the kinetics for multiple paths in one reaction regarding to their thermodynamic properties. Moreover, conformational analysis elucidates that H-transfers exhibit strong structure dependence. Note that the most favorable 1,5 H-transfer is only feasible for one twist-boat with radical site in axial side chain accompanied by one isoclinal methyl group. The results for β-scissions are affected by steric energies and substituent effects remained in conformational structures. These findings facilitate to finally suggest the proper kinetic parameters for each decomposition reaction with the aim of their potential implication in kinetic modelling.
This work concerns the typical conformational behaviors for di-substituted cyclohexanes that inherently depend on spatial orientations of side chains in flexible cyclic ring. The 1,3-dimethylcyclohexane and 1,4-dimethylcyclohexane in both cis- and trans-configurations were focused here to unravel their conformational inversion-topomerization mechanisms. Full geometry optimizations were performed at B3LYP/6-311++G(d,p) level of theory to explicitly identify all distinguishable molecular structures, and thus explore potential energy surfaces (PES) of the complete interconversion routes for two stereoisomers of 1,3-dimethylcyclohexane and 1,4-dimethylcyclohexane. Additional quantum calculations were carried out by separately applying MP2/6-311++G(d,p), G4, and CCSD(T)/6-311++G(d,p) methods to further refine all PES’ stationary points. With respect to quantum results, the conformational analysis was conducted to gain insight into the determination, thermodynamic stabilities, and relative energies of distinct molecular geometric structures. On base of highly biased conformational equilibria, the temperature-dependent populations of stable local minima for four studied dimethylcyclohexanes were obtained by utilizing Boltzmann distribution within 300-2500 K. Moreover, two unique interconversion processes for them, including inversion and topomerization, were fully investigated, and their potential energy surfaces were illustrated with the rigorous descriptions in two or three-dimensional schemes for clarify.