Spin-lattice relaxation of (CH3)-C-13 groups in C-13-enriched aspirin after proton saturation



: Kankaanpaa M, Ylinen EE, Punkkinen M

Publisher: ACADEMIC PRESS INC ELSEVIER SCIENCE

: 2003

Solid State Nuclear Magnetic Resonance

SOLID STATE NUCLEAR MAGNETIC RESONANCE

: SOLID STATE NUCL MAG

: 23

: 4

: 224

: 242

: 19

: 0926-2040

DOI: https://doi.org/10.1016/S0926-2040(03)00014-6


We studied the spin-lattice relaxation of the C-13 magnetisation, M-C, in C-13-enriched single crystal of aspirin (only methyl carbons enriched to 99%) at the carbon resonance frequency of 54.5 MHz. After the carbon saturation the recovery appears exponential except below 30 K, where it is biexponential due to the presence of the level crossing omega(t) = omega(C) + omega(H) (the symbols refer, respectively, to the tunnel frequency and the carbon and proton resonance frequencies in angular units). After the saturation of the proton magnetisation, M-H, the description of the M-C recovery needs three exponentials. The evaluation of the time constants is easiest from the data in this case, since M-C varies with time in an initial growth-subsequent decrease (or an initial decrease-subsequent growth depending on temperature) manner, instead of the monotonous growth after the carbon saturation. Experimental data agree semiquantitatively with the predictions of our recent model. According to the model the relaxation of M-C is coupled to M-H and the tunnel energy TE at temperatures below the minimum of the C-13 relaxation time. Sufficiently above this minimum M-C is coupled to M-H and the rotational polarization (but not to TE) in agreement with experiment. Also the effect of torsional oscillations of a methyl group on the magnitude of various C-13-related transition rates was considered in detail. In aspirin the rates are reduced roughly by 10% and the reduction should become larger in samples with a larger tunnel splitting. The reduction also changes somewhat the angular dependence of these rates. (C) 2003 Elsevier Science (USA). All rights reserved.



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