Effects of eccentricity on nuclear magnetic relaxation by intermolecular dipole–dipole interactions: 13C relaxation of neopentane

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Abstract
In order to study the eccentricity effects on the nuclear magnetic relaxation by intermolecular dipole–dipole interactions, we measured the relaxation times T1 of the center and off‐center 13C nuclei in neopentane C(CH3)4. To eliminate a competition between several relaxation machanisms, mainly with the protons, di‐t‐butylnitroxide free radicals were included with concentrations varying up to 5.6×1020 paramagnetic centers × cm−3. Under these conditions, the dominant relaxation mechanism for the resonating 13C nuclei spins arises from their interactions with the electronic spins of the free radicals. A difference of 15% between the relaxation times of the central 13C and of the off‐center carbons is observed. These results are partially interpreted by using the spectral densities j2(ω) resulting from translational and rotational motions as established in our previous work. It is shown that pair correlation effects must also be taken into account to improve the fit with the experimental data.