H2O@C60のオルト水とパラ水の電気化学的検出
水の２つの状態にあまり意識したことはなかった。オルト水素やパラ水素は有名であるが。
C60に水分子を包接させて、バルクの誘電率と言えるか？
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±0.05Debye 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.
Introduction
Water, like dihydrogen, has two different spin isomers, called ortho and para, which have different spin state symmetries.
In ortho-water, the spin state of the two proton nuclei is symmetric under particle exchange and the total nuclear spin has quantum number I = 1. In para-water, the spin state is antisymmetric and has nuclear spin quantum number I = 0.
Water spin isomerism is of relevance to a broad range of scientific fields from nuclear magnetic resonance (NMR) to astrophysics1,, and closely related to long-lived nuclear spin states, which also involve the slow interconversion of nuclear singlet and triplet states.
Physical properties of dihydrogen H2, such as heat capacity or thermal conductivity, depend on the concentration of ortho and para spin isomers.
Do the spin isomers of water also 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. This effect was predicted theoretically; and observed in beam experiments10, but no bulk properties have been reported.
Although it is feasible to separate ortho- and para-water molecules in rarified molecular beams, , it remains challenging to study the separated isomers in the condensed phase, since rapid proton exchange obscures the spin isomerism in bulk water, and strong intermolecular interactions usually quench the molecular rotation at low temperatures.
Spin isomer-enriched water may be captured in an inert gas matrix and studied using infrared spectroscopy, but this approach provides little control over the molecular environment.
In contrast, the supramolecular endofullerene H2O@C60, composed of C60 carbon cages that each encloses a single water molecule, forms a well-defined lattice.
The synthesis of this material provides macroscopic quantities of a stable substance that contains isolated and freely rotating water molecules. It has been studied under a very wide range of physical conditions using various spectroscopic techniques1. Dielectric measurements were made on a single crystal of H2O@C60, but without anticipating, or observing, a dependence on spin isomer composition.
Figure 1a,b shows the molecular structure of H2O@C60 and the four lowest rotational energy levels as determined by neutron scattering, neglecting the observed splitting of the ortho-water ground state1, .
The energy levels are similar to those of water in the gas phase indicating that the water rotation is unhindered even at cryogenic temperatures.
The thermal equilibrium fraction of ortho-water molecules as a function of temperature is shown in Fig. 1c, using the energy levels of Fig. 1b and taking into account the degeneracies of the rotational levels. The equilibrium fraction changes rapidly in the vicinity of 15 K. Ortho–para conversion may therefore be induced by (i) allowing the sample to reach complete equilibrium at a temperature >15 K, (ii) rapidly cooling to <15 K and (iii) studying the behaviour of the sample as a function of time at the constant low temperature.
Here we demonstrate that the bulk dielectric constant of H2O@C60 depends on the spin isomer composition of the encapsulated water molecules.
We find a time-dependent change in dielectric constant at 5 K that is due to different molecular polarizabilities of the ortho and para ground states.
The polarizabilities are extracted from the capacitance data and compared with a theoretical prediction that only requires knowledge of the dipole moment of H2O@C60 and the rotational constants of water.
The dipole moment is estimated from a high-temperature measurement of the molecular polarizability and found to be in very good agreement with recent predictions of 0.5±0.1 Debye.