Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene ${\mathrm{H}}_{2}\mathrm{O}@{\mathrm{C}}_{60}$

Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene $H_{2} O @ C_{60}$

Benno Meier, Karel Kouřil, Christian Bengs, Hana Kouřilová, Timothy C. Barker, Stuart J. Elliott, Shamim Alom, Richard J. Whitby, and Malcolm H. Levitt

Phys. Rev. Lett. 120, 266001 – Published 29 June 2018

Abstract

Water exists in two forms, para and ortho, that have nuclear spin states with different symmetries. Here we report the conversion of fullerene-encapsulated para water to ortho water. The enrichment of para water at low temperatures is monitored via changes in the electrical polarizability of the material. Upon rapid dissolution of the material in toluene the excess para water converts to ortho water. In $H_{2} {}^{16}O @ C_{60}$ the conversion leads to a slow increase in the NMR signal. In $H_{2} {}^{17}O @ C_{60}$ the conversion gives rise to weak signal enhancements attributed to quantum-rotor-induced nuclear spin polarization. The time constants for the para-to-ortho conversion of fullerene-encapsulated water in ambient temperature solution are estimated as $30 \pm 4 s$ for the ${}^{16}O$ isotopolog of water, and $16 \pm 3 s$ for the ${}^{17}O$ isotopolog.

Received 2 February 2018
Revised 24 April 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.266001

Physics Subject Headings (PhySH)

Dielectric properties Spin relaxation

Nuclear magnetic resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Benno Meier^*, Karel Kouřil, Christian Bengs, Hana Kouřilová, Timothy C. Barker, Stuart J. Elliott, Shamim Alom, Richard J. Whitby, and Malcolm H. Levitt^†

School of Chemistry, University of Southampton, Southampton, SO17 1BJ, United Kingdom

^*b.meier@soton.ac.uk
^†mhl@soton.ac.uk

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Vol. 120, Iss. 26 — 29 June 2018

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Figure 1
(a) Molecular structure of $H_{2} O @ C_{60}$ . (b) ${}^{1}H$ NMR spectrum of $H_{2} O @ C_{60}$ , with a ${}^{17}O$ labeling level of 85%, dissolved in deuterated ortho dichlorobenzene. The proton resonance is split into a sextet due to the $J$ coupling to the spin- $5 / 2$ oxygen. The central line is due to the 15% of water bearing the ${}^{16}O$ isotope. (c) Equilibrium ortho fraction as a function of temperature. At high temperatures, 75% of the molecules are in the ortho state. At 5 K and in thermal equilibrium, the ortho state is almost completely depleted.
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Figure 2
Apparatus for rapid dissolution. Approximately $50 μ L$ of sample (red) are pipetted into a Teflon bullet which is then immersed in liquid nitrogen. For loading, the tube at the top of the union (2) is disconnected, the ball valve (4) is opened, and the bullet is pushed to the bottom of the steel tube. A small flow of helium gas, applied via the needle valve (1), prevents contamination of the system with air. The capsule is ejected by opening valves (3) and (4) and is propelled into the NMR magnet (right), where a 3D printed receiver (blue) retains the capsule and vents the helium gas. The sample travels further into the 5 mm NMR tube where it dissolves in warm solvent.
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Figure 3
Intensity of the ${}^{1}H$ NMR signal of $H_{2} {}^{16}O @ C_{60}$ in solution as a function of time (blue curve). Acquisition starts prior to arrival of the sample. The time origin is chosen to coincide with the first NMR signal after dissolution of the sample. The initial sharp peak is due to initial ortho-water magnetization (the cryostat is located in a 6.7 Tesla magnet), and decays rapidly with the proton $T_{1}$ . Following the rapid decay the signal intensity increases with time as para water converts to ortho water. The black curve shows the result of a spin dynamical simulation with a time constant $T_{S} = 30 s$ for the conversion of para water to ortho water (see text). The inset shows a spectrum (lower, blue curve) obtained from averaging 20 transients recorded 8 min after dissolution of the sample, and a Lorentzian mask (upper, orange curve) that was applied prior to integration.
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Figure 4
QRIP Experiment on $H_{2} {}^{17}O @ C_{60}$ . (a) Thermal equilibrium ${}^{1}H$ spectrum, recorded after equilibration of the sample (1024 transients averaged). (b) Averages of 20 transients during consecutive intervals after dissolution of the sample. A clear antiphase signal is obtained immediately after the dissolution (0–5 s). The antiphase signal decays, and after approximately one minute the thermal signal is restored. (c) The individual spectra are multiplied with Lorentzian masks that correspond to each of the six peaks, and the product is integrated. (d) Results of the integration after applying a tricube bandwidth reduction of width 2 s. The integrals are normalized by the intensity of the respective thermal ${}^{17}O$ peaks. (e)–(j) The same data as in (d), with spin dynamical simulations (black lines).
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Physical Review Letters

Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene H2O@C60

Benno Meier, Karel Kouřil, Christian Bengs, Hana Kouřilová, Timothy C. Barker, Stuart J. Elliott, Shamim Alom, Richard J. Whitby, and Malcolm H. Levitt

Phys. Rev. Lett. 120, 266001 – Published 29 June 2018

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Spin-Isomer Conversion of Water at Room Temperature and Quantum-Rotor-Induced Nuclear Polarization in the Water-Endofullerene $H_{2} O @ C_{60}$