Abstract
Water exists in two spin isomers, ortho and para, that have different nuclear spin states. In bulk water, rapid proton exchange and hindered molecular rotation obscure the direct observation of two spin isomers. The supramolecular endofullerene H2O@C60 provides freely rotating, isolated water molecules even at cryogenic temperatures. Here we show that the bulk dielectric constant of this substance depends on the ortho/para ratio, and changes slowly in time after a sudden temperature jump, due to nuclear spin conversion. The attribution of the effect to ortho–para conversion is validated by comparison with nuclear magnetic resonance and quantum theory. The change in dielectric constant is consistent with an electric dipole moment of 0.51±0.05 Debye for an encapsulated water molecule, indicating the partial shielding of the water dipole by the encapsulating cage. The dependence of bulk dielectric constant on nuclear spin isomer composition appears to be a previously unreported physical phenomenon.
Highlights
Water exists in two spin isomers, ortho and para, that have different nuclear spin states
Water spin isomerism is of relevance to a broad range of scientific fields from nuclear magnetic resonance (NMR) to astrophysics[1,2,3,4,5], and closely related to long-lived nuclear spin states, which involve the slow interconversion of nuclear singlet and triplet states[6,7]
Do the spin isomers of water have different bulk properties? Since water, unlike dihydrogen, possesses an electric dipole moment, the spin isomers of ortho and para water are expected to display a distinct response to electric fields
Summary
Water exists in two spin isomers, ortho and para, that have different nuclear spin states. The thermal equilibrium fraction of ortho-water molecules as a function of temperature is shown, using the energy levels of Fig. 1b and taking into account the degeneracies of the rotational levels[2,16]. We demonstrate that the bulk dielectric constant of H2O@C60 depends on the spin isomer composition of the encapsulated water molecules.
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