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| Titel: |
Spin-lattice relaxation in 13CH3 compounds: application to 13C enriched aspirin |
| Auteur: |
KANKAANPAA, M. PUNKKINEN, M. YLINEN, E.E. |
| Verschenen in: |
Molecular physics |
| Paginering: | Jaargang 100 (2002) nr. 17 pagina's 2877-2893 |
| Jaar: |
2002-09-10 |
| Inhoud: |
Spin-lattice relaxation processes in 13CH3 groups in methyl compounds are studied both theoretically and experimentally. The four spin-½ 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 13C magnetization MC, proton magnetization MH, tunnelling energy TE, rotational polarization RP and dipolar energy DE. Their spin-lattice relaxation via the transitions induced by the 13C-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 13C spin-lattice relaxation at 75.4 MHz in 99% enriched (only methyl carbons enriched) single crystal of aspirin. Above 40 K, the MC 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 ωt, + ωC = ωH, where the angular frequencies represent rotational tunnelling and carbon and proton resonances, respectively. The resonant transitions produce couplings between MC, MH and TE that are described quite accurately by the present model. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
Details van artikel 47 van 64 gevonden artikelen
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