Effects of molecular structure on oxidation reactivity of cyclic hydrocarbons: Experimental observations and conformational analysis

This work concerns the pre-ignition reactivity of cyclic hydrocarbons and its dependence on cyclic structures. In the first part, global reactivity of five cyclic hydrocarbons, methylcyclopentane (MCP), cyclohexane (CH), methylcyclohexane (MCH), decahydronaphthalene (decalin), and 1,2,3,4-tetrahydronaphthalene (tetralin), whose detailed product analyses were recently reported [Y. Yang, A.L. Boehman, Proc. Combust. Inst. 32(1) (2009) 419–426; Y. Yang, A.L. Boehman, Combust. Flame 157(3) (2010) 495–505], were compared over a range of compression ratio and intake temperature in a motored engine. Molecular structure exerts a profound effect on low temperature oxidation reactivity. Decalin is the most reactive compound whose extent of oxidation increases monotonically with increasing temperature and pressure. MCH shows higher low temperature reactivity than CH, and both show distinct negative temperature coefficient behavior. MCP and tetralin exhibit little low temperature reaction before critical conditions are reached for autoignition. In the second part, conformational analysis is conducted to understand how molecular structures affect the low temperature oxidative reactivity, in particular the (1,5) H-shift of fuel peroxy radicals (ROO→(1,5)QOOH), the key step in low temperature chain branching. In comparison with open-chain structures, cyclic structures significantly reduce the total number of hydrogens that can be abstracted in the (1,5) H-shift. This is because the (1,5) H-shift of a cyclohexylperoxy radical requires both the peroxy group and the to-be-abstracted hydrogen locate at an axial position on the cyclohexane ring. The total number of available hydrogens in decalin, MCH, CH, and tetralin is 14, 11, 6, and 4, respectively. Also important is the number of hydrogens available for the (1,5) isomerization of a given peroxy group, i.e., the degeneracy of the (1,5) H-shift. Degeneracy in (1,5) H-shift for decalin, MCH, CH, and tetralin is ∼3, 2–3, 2, and 1, respectively. These numbers are in accord with the relative reactivity observed for these compounds. The higher reactivity of MCH relative to CH also results from the equatorial preference of the methyl group which forces the peroxy group to stay at an axial position and facilitates (1,5) H-shift. The last argument is confirmed by quantum mechanical calculations.