Origin and use of quantum-rotor induced polarization in NMR
Theoretical Chemistry: Molecules, Materials, Surfaces
Final Report Abstract
Within this project, the concept of quantum-rotor induced polarization (QRIP) was put on significantly improved experimental and theoretical grounds, mainly in terms of the relationships between NMR polarization amplitude, tunnel frequency, and chemical environment of the involved methyl groups. Although several promising substances were identified by indicative theoretical findings and “chemical intuition”, the subsequent actual experimental tests on NMR signal enhancement did not yield the desired and anticipated effect. Ongoing measurements of the tunnel frequency by INS indeed suggest that these substances have a tunnel splitting that is too low, or equivalently, a rotational barrier that is too high. Therefore, our study confirms the dependence of the tunnel frequency on the “freedom” of the involved methyl group and show that an experimental determination of the tunnel splitting is required. Unfortunately, the machine time on neutron sources is rather limited, so that no further systematic screening of simple molecular compounds could be pursued. A promising theoretical finding was that explicit coupling of two methyl rotors can significantly increase the tunnel splittings. However, this idea could not be validated immediately via experiments, since such coupled methyl rotors are actually quite rare in nature. Considerable progress has been made in improving the experimental procedure: we have shown that dDNP setups are perfectly suitable for QRIP studies, and even a simple self-constructed setup improved the signal and the reliability significantly. Therefore, using such existing set ups, it might be possible to use g-picoline for selective signal enhancement and polarization transfer to near-by surfaces. Similar efforts, using the solid-state photo-CIDNP effect as polarization source, are presently going on in our laboratory.
Publications
- Methyl rotor quantum states and the effect of chemical environment in organic crystals: g-picoline and toluene. J. Chem. Phys. 145, 234506 (2016)
Khazaei, S. & Sebastiani, D.
(See online at https://doi.org/10.1063/1.4971380) - Tunneling of coupled methyl quantum rotors in 4-methylpyridine: Single rotor potential versus coupling interaction Tunneling of coupled methyl quantum rotors in 4- methylpyridine : Single rotor potential versus coupling interaction. J. Chem. Phys. 147, 194303 (2017)
Khazaei, S., Sebastiani, D., Khazaei, S. & Sebastiani, D.
(See online at https://doi.org/10.1063/1.5003081) - Simple device for dissolution and sample transfer for applications in spin-hyperpolarization. Mol. Phys. 8976, 1–5 (2018)
Dietrich, C., Wissel, J., Knoche, J., Lorenz, O. & Matysik, J.
(See online at https://doi.org/10.1080/00268976.2018.1550224)
