A1 Refereed original research article in a scientific journal
Spin-lattice relaxation in (CH3)-C-13 compounds: application to C-13 enriched aspirin
Authors: Kankaanpaa M, Punkkinen M, Ylinen EE
Publisher: TAYLOR & FRANCIS LTD
Publication year: 2002
Journal:: Molecular Physics
Journal name in source: MOLECULAR PHYSICS
Journal acronym: MOL PHYS
Volume: 100
Issue: 17
First page : 2877
Last page: 2893
Number of pages: 17
ISSN: 0026-8976
DOI: https://doi.org/10.1080/00268970210123955
Abstract
Spin-lattice relaxation processes in (CH3)-C-13 groups in methyl compounds are studied both theoretically and experimentally. The four spin-1/2 nuclei in such methyl groups give rise to 16 spin-rotational states, which are split by rotational tunnelling. From the corresponding populations (15 independent) five long lived combinations are formed: the C-13 magnetization M-C, proton magnetization M-H, tunnelling energy T E, rotational polarization RP and dipolar energy DE. Their spin-lattice relaxation via the transitions induced by the C-13-proton dipolar interaction is studied in detail. Direct relaxation rates and coupling terms between these combinations are derived. Predictions are compared with experimental data for C-13 spin-lattice relaxation at 75.4 MHz in 99% enriched (only methyl carbons enriched) single crystal of aspirin. Above 40 K, the M-C recovery is exponential and describable in terms of the direct relaxation transitions without couplings. The same is true for the initial relaxation in the region of non-exponential relaxation between 30 K and 40 K. The orientation dependence of the initial relaxation rate agrees with the theoretical calculations. The non-exponentiality is related to resonant level-crossing transitions with omega(t) + omega(C) = omega(H), where the angular frequencies represent rotational tunnelling and carbon and proton resonances, respectively. The resonant transitions produce couplings between M-C, M-H and T E that are described quite accurately by the present model.
Spin-lattice relaxation processes in (CH3)-C-13 groups in methyl compounds are studied both theoretically and experimentally. The four spin-1/2 nuclei in such methyl groups give rise to 16 spin-rotational states, which are split by rotational tunnelling. From the corresponding populations (15 independent) five long lived combinations are formed: the C-13 magnetization M-C, proton magnetization M-H, tunnelling energy T E, rotational polarization RP and dipolar energy DE. Their spin-lattice relaxation via the transitions induced by the C-13-proton dipolar interaction is studied in detail. Direct relaxation rates and coupling terms between these combinations are derived. Predictions are compared with experimental data for C-13 spin-lattice relaxation at 75.4 MHz in 99% enriched (only methyl carbons enriched) single crystal of aspirin. Above 40 K, the M-C recovery is exponential and describable in terms of the direct relaxation transitions without couplings. The same is true for the initial relaxation in the region of non-exponential relaxation between 30 K and 40 K. The orientation dependence of the initial relaxation rate agrees with the theoretical calculations. The non-exponentiality is related to resonant level-crossing transitions with omega(t) + omega(C) = omega(H), where the angular frequencies represent rotational tunnelling and carbon and proton resonances, respectively. The resonant transitions produce couplings between M-C, M-H and T E that are described quite accurately by the present model.
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