Molecular Forms
Diradicals are molecular species with 2 electrons occupying 2 equal energy (degenerate) orbits. Diradicals vary by the spins of covalently bonded electrons. O2 and H2C (methylene) are exemplary diradicals.
Diradicals have 3 possible arrangements: singlet, doublet, and triplet.
The oxygen molecule’s ground state is triplet. Both electrons in the degenerate orbitals spin up. The parallel spins of the unpaired electrons make gaseous dioxygen paramagnetic (reactive to magnetic fields), which is quite unusual for a gas. Liquid oxygen is magnetic.
Singlet oxygen has 1 spin-up electron and 1 spin-down electron in 1 orbital, with an equal energy orbital empty. The singlet configuration has several species, all relatively high energy. Hence singlet O2 is much more reactive than triplet O2.
If the O2‘s ground state was singlet instead of triplet, life would be impossible, and any accumulation of organic matter unlikely. Singlet oxygen is used as an industrial-strength pesticide in buildings, exterminating even the most persistent critters.
The human immune system produces singlet O2 for weaponry: reactive oxygen species (ROS). Plants have an ROS response to attacking pathogens: strengthening the cell wall with superoxide or hydrogen peroxide to imprison the infection.
In a reaction powered by sunshine, singlet oxygen is formed from water during photosynthesis. Carotenoids in chloroplasts absorb energy from singlet oxygen using tetraterpenoids: a molecular skeletal structure comprising 40 carbon atoms. Carotenoids convert the highly reactive singlet to triplet ground state before it inflicts harm on tissues.
The ability to detoxify ROS evolved in the earliest life, prior to photosynthesis and aerobic respiration; otherwise, aerobic organisms would have poisoned themselves.
Photolysis of ozone by short-wavelength (high-energy) light in the troposphere produces singlet O2. The troposphere is the lowest layer of the Earth’s atmosphere, 17 kilometers thick in the middle latitudes, with about 75% of the atmosphere’s mass and 99% of its water vapor and aerosols.
Doublet O2, with one electron unpaired, is a simple free radical, and highly reactive.
O2‘s odd triplet ground state prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet state. But triplet oxygen will readily react with doublet molecules, such as radicals, to form a new radical. And the univalent pathway of adding electrons 1 at a time in series reactions is quite common.
