Abstract
The water-endofullerene H2O@C60 provides a unique chemical system in which freely rotating water molecules are confined inside homogeneous and symmetrical carbon cages. The spin conversion between the ortho and para species of the endohedral H2O was studied in the solid phase by low-temperature nuclear magnetic resonance. The experimental data are consistent with a second-order kinetics, indicating a bimolecular spin conversion process. Numerical simulations suggest the simultaneous presence of a spin diffusion process allowing neighbouring ortho and para molecules to exchange their angular momenta. Cross-polarization experiments found no evidence that the spin conversion of the endohedral H2O molecules is catalysed by (13)C nuclei present in the cages.
Highlights
Small molecules with sufficient symmetry display the phenomenon of spin isomerism, in which the nuclear spin states and the spatial quantum states are entangled as a consequence of the Pauli principle
0.0 t /103s thermal equilibrium is reached at the relatively high sample temperature of 32.5 K. These results demonstrate that H2O nuclear spin conversion in H2O@C60 may be observed with excellent signal-tonoise ratio and time resolution by monitoring the 1H nuclear magnetic resonance (NMR)
The second-order kinetic rate law suggests a bimolecular process in which neighbouring ortho-H2O pairs interact at close range
Summary
Small molecules with sufficient symmetry display the phenomenon of spin isomerism, in which the nuclear spin states and the spatial quantum states are entangled as a consequence of the Pauli principle. This paper concerns nuclear magnetic resonance (NMR) observations of spin isomer conversion in H2O@C60. A disadvantage of NMR with respect to infrared spectroscopy and neutron scattering is that the para spin isomer has nuclear spin I = 0 and cannot be observed. The kinetic evidence from NMR indicates that the ortho to para conversion of H2O@C60 is a bimolecular process, involving the interaction between pairs of neighbouring ortho spin isomers. NMR is able to probe the environment of the water molecules through double-resonance experiments This allows evaluation of the role of nearby magnetic nuclei in spin-isomer conversion, as has been postulated in related systems.[43,44]. Cross-polarization experiments indicate that the interactions between the H2O protons and the 13C nuclei in the enclosing fullerene cage play a negligible role in the ortho-para conversion of water in H2O@C60. Some observations on the possible spin-isomer conversion mechanism in H2O@C60 are given at the end of this paper
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
