text: Deuterium Diatomic deuterium () has nuclear spin isomers like diatomic hydrogen, but with different populations of the two forms because the deuterium nucleus (deuteron) is a boson with nuclear spin equal to one. There are six possible nuclear spin wave functions which are ortho or symmetric to exchange of the two nuclei, and three which are para or antisymmetric. Ortho states correspond to even rotational levels with symmetric rotational functions so that the total wavefunction is symmetric as required for the exchange of two bosons, and para states correspond to odd rotational levels. The ground state (J = 0) populated at low temperature is ortho, and at standard temperature the ortho:para ratio is 2:1.
  Other substances with spin isomers
  Other molecules and functional groups containing two hydrogen atoms, such as water and methylene (CH2), also have ortho- and para- forms (e.g. orthowater and parawater), but this is of little significance for their thermal properties. Their ortho:para ratios differ from that of dihydrogen. The ortho and para forms of water have recently been isolated. Para water was found to be 25% more reactive for a proton-transfer reaction.
  Molecular oxygen () also exists in three lower-energy triplet states and one singlet state, as ground-state paramagnetic triplet oxygen and energized highly reactive diamagnetic singlet oxygen. These states arise from the spins of their unpaired electrons, not their protons or nuclei.