Meier Benno, Mamone Salvatore, Concistrè Maria, Alonso-Valdesueiro Javier, Krachmalnicoff Andrea, Whitby Richard J, Levitt Malcolm H
School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK.
Nat Commun. 2015 Aug 24;6:8112. doi: 10.1038/ncomms9112.
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.
水以两种自旋异构体存在，即正水和仲水，它们具有不同的核自旋态。在大块水中，快速的质子交换和受阻的分子旋转使得两种自旋异构体的直接观测变得模糊。超分子内嵌富勒烯H₂O@C₆₀即使在低温下也能提供自由旋转的孤立水分子。在此我们表明，这种物质的体介电常数取决于正水/仲水比例，并且在温度突然跃升后，由于核自旋转换，其随时间缓慢变化。通过与核磁共振和量子理论的比较，验证了该效应归因于正 - 仲转换。介电常数的变化与被包裹水分子的电偶极矩为0.51±0.05德拜相一致，这表明被包裹的笼对水偶极有部分屏蔽作用。体介电常数对核自旋异构体组成的依赖性似乎是一种此前未被报道的物理现象。
