Combustion in oxygen is a chain reaction in which many distinct radical intermediates participate. The high energy required for initiation is explained by the unusual structure of the dioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a triplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, with spins aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum. Interaction between the two is quantum mechanically a "forbidden transition", i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or singlet oxygen. This intermediate is extremely reactive. The energy is supplied as heat, and the reaction then produces additional heat, which allows it to continue.
combustion in oxygen is a chain reaction in which many distinct radical intermediates participate the high energy required for initiation is explained by the unusual structure of the dioxygen molecule the lowest-energy configuration of the dioxygen molecule is a stable relatively unreactive diradical in a triplet spin state bonding can be described with three bonding electron pairs and two antibonding electrons with spins aligned such that the molecule has nonzero total angular momentum most fuels on the other hand are in a singlet state with paired spins and zero total angular momentum interaction between the two is quantum mechanically a "forbidden transition" ie possible with a very low probability to initiate combustion energy is required to force dioxygen into a spin-paired state or singlet oxygen this intermediate is extremely reactive the energy is supplied as heat and the reaction then produces additional heat which allows it to continue