Ring opening pathways in the reaction of morpholinyl radicals with O2

   In this research, we have explored the ring opening pathways in the reaction of morpholinyl radicals with O2 using high level ab initio methods. Here morpholine (a 6-membered cyclic hydrocarbon with nitrogen and oxygen heteroatoms) is used to mimic fuels that may be derived from non-traditional sources such as biomass, sand and shale oils. These fuels often contain N and O atoms, and have higher concentration of cyclic compounds than current fuels. The calculations show that morpholinyl radicals have a strong reactivity with O2, as the addition reaction of O2 to the radicals has no apparent activation barrier, and is considerably exothermic. The resulting adduct has two main paths to decomposition. A 1,4-H shift isomerization reaction is likely to produce a stable cyclic epoxy intermediate, whereas a 1,5-H shifting isomerization leads to ring-opened products.

Relative energy diagram at the CCSD(T)/aug-cc-pVTZ level for the reaction of morpholinyl radical (2●) with O2 through a low barrier pathway for the ring opening process.

 These two isomerization reactions will compete owing to their comparable barrier heights along the reaction pathway. In particular, as shown in the figure, the 1,5-H shifting reaction pathway involves an energy barrier only 4.85 kcal/mol higher than the reactant dissociation limit, and the overall ring opening reaction can produce a significant amount of heat. These novel findings provide valuable insights for the development of mechanisms for low-temperature combustion of morpholine and related cyclic O- and N-containing fuels. 

 P.P. Dholabhai and H.-G. Yu, J. Phys. Chem. A, 116, 7123 (2012).

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Last Modified: September 6, 2013
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