Multiplexed photoionization mass spectrometry (MPIMS) is used to determine branching fractions of conjugate alkenes from R + O2 oxidation reactions for cyclohexane (c-C6H12) and tetrahydropyran (c-C5H10O), a lignocellulosic-derived oxygenated biofuel. Because conjugate alkene formation is coincident with the formation of HO2, the results reveal the temperature- and pressure-dependent influence of the oxygen heteroatom on chain-termination propensity. The experiments were conducted using Cl-initiated oxidation at 10 and 1520 Torr and from 500 to 700 K, and ab initio calculations of bond energies, saddle point energies, and rate coefficients of unimolecular decomposition of α-tetrahydropyranyl were conducted to complement the experiments.
Relative to the initial radical concentration [R]0 the trend in conjugate alkene branching fraction exhibits monotonic positive temperature dependence in both cyclohexane and tetrahydropyran, except for the latter species at 10 Torr where increasing the temperature to 700 K caused an appreciable decrease. The decrease in the branching fraction with increasing temperature in tetrahydropyran oxidation occurs because ring-opening rates of α-tetrahydropyranyl radicals, enabled by weak C–O bond dissociation energies, exceed rates of O2-addition. Ring-opening rate coefficients for α-tetrahydropyranyl, computed from stationary points at the CCSD(T)-F12a/cc-PVDZ//M06-2X/6-311++G** level of theory, confirm that ring-opening at 700 K is favorable and higher oxygen concentrations are required for O2-addition rates to be significant. With increased temperature and lower [O2], α-tetrahydropyranyl radicals preferentially undergo unimolecular decomposition into the linear radical . Conjugate alkene branching fractions measured at 1520 Torr for both cyclohexane and tetrahydropyran followed monotonic positive temperature dependence. The change in the tetrahydropyran trend at 1520 Torr relative to the 10 Torr measurements is ascribed to the increase in oxygen concentration mitigating ring-opening reactions of the initial R radicals.
In contrast to the results at higher temperature, where ring-opening of tetrahydropyranyl radicals interrupts R + O2 chemistry and reduces the formation of conjugate alkenes, branching fractions measured below 700 K were higher in tetrahydropyran compared to cyclohexane at 10 Torr. The difference suggests that the ether group renders chain-termination pathways more-facile. Saddle point energy calculations on the surfaces of equatorial ROO conformers reveal that the barrier height to direct HO2 formation from α-tetrahydropyranylperoxy is lower by ca. 5 kcal/mol compared to cyclohexylperoxy at the CBS-QB3 level of theory, which facilitates low-temperature chain-termination.