ortho-Water and para-Water
Every water molecule in the universe is either a para-water
molecule or an ortho-water molecule
Each hydrogen atom in water has a magnetic moment, which is associated with the proton's spin of ½. As is found in molecular hydrogen (H2), the protons (within the hydrogen atoms) in water (H2O) may possess parallel
or antiparallel nuclear
spin (see right). When the spins are parallel, there is a paramagnetic state called ortho-H2O with a magnetic moment = 1. This is the high spin (triplet) state with three symmetric spin states +1 , 0 , -1 (↑↑, ↑↓+↑↓, ↓↓) where the three states have equal energy in a zero magnetic field. This spin state always possesses positive energy with a minimum energy level of 284.7 J mol-1 (23.794352 cm-1) H216O, [607c], 284.4 J mol-1 (23.773510 cm-1) H217O [607a] or 284.2 J mol-1 (23.754902 cm-1) H218O [607a]. When the spins are opposed there exists the nonmagnetic state called para-H2O with magnetic moment = 0 with just one antisymmetric spin state (↑↓-↑↓) and magnetic moment = 0. Some of the water molecules in this low spin (singlet) state will not be rotating even at room temperature.
Para-H2O does not interact with an external magnetic field, but ortho-H2O does. Conversion between these isomers is symmetry forbidden for isolated water molecules and they act as different molecular species. They can change spin state on interaction with another particle, including other water molecules. The equilibrium ratio of these nuclear spin states in H2O is all para at zero
Kelvin, where the molecules have no rotational spin in their ground state, shifting to the most stable ratio  of 3:1 ortho:para, in the relative amounts of the number of magnetic states, at less
cold temperatures (>50 K, see left ); the equilibrium taking months to establish itself in ice (or gas) and
nearly an hour in ambient water .
It is now thought that the ratio lies far from equilibrium and much closer to 1:1 in liquid water due to hydrogen bond formation . This means that liquid H2O effectively consists
of a mixture of non-identical molecules and the properties of pure liquid ortho-H2O or para-H2O are unknown. The differences in the properties of these two forms of water are expected to be greater in an electric field , which may be imposed externally, from surfaces or from water clustering itself. Many materials preferentially
adsorb para-H2O due to its non-rotation
ground state [410, 835].
The apparent difference in energy between the two states is
a significant 1-2 kJ mol-1, far greater than expected
from spin-spin interactions (< μJ
mol-1) . It has been suggested that structural rearrangements may be induced by ortho-H2O : para-H2O conversion , as it is possible that hydrogen bonds between para-H2O, possessing no ground state spin, are stronger and last longer than hydrogen bonds between ortho-H2O . It is thus possible that ortho-H2O and para-H2O form separate hydrogen bonded clusters  with para-H2O being preferred in the low density tetrahedrally coordinated clusters and ortho-H2O being preferred in the high density clusters , where their rotation is more easily accommodated. Picoliter samples of pure ortho-H2O and para-H2O may be separated in a strong dc electric field .
Ortho-H2O and para-H2O can be separately studied as isolated freely-rotating molecules down to 5 K, whilst contained inside fullerene molecules (see left). At this temperature it exists as 100% para but at 15 K it exists as a 50% mixture whilst reaching an equilibrium mixture of 75% ortho at 40 K .
Investigation of the ortho–para conversion of the single trapped water molecule has shown that metastable ortho-water molecules are present at low temperatures with breakage of its three-fold rotational degeneracy of its ground state .
Due to deuterium's
nuclear spin of 1 (compare 1/2 for H's spin; ortho D2O has two spin states 2 and 0; para D2O has one spin state 1), the lowest
energy form of D2O is ortho. D2O
converts to a 2:1 ortho:para ratio at higher temperatures.
The difference in energy between the lowest ortho state and the lowest para state is 12.1170191 cm-1 (D216O), 12.098600 cm-1 (D217O) and 12.082026 cm-1 (D218O) [607d].
HDO, having non-equivalent hydrogen atoms, does not possess
an ortho/para distinction. T2O
behaves similarly to H2O as tritium also possesses
a nuclear spin of 1/2.
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