NMR study of spectral densities over a large frequency range for intermolecular relaxation in liquids: Pair correlation effects

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Abstract
In order to study the dynamic effects of pair correlation functions on intermolecular translational relaxation in liquids, we have measured the relaxation times T1 of the protons in neopentane in the presence of di‐t‐butylnitroxide free radicals with concentrations varying up to 6.5×1020 paramagnetic centers × cm−3. Under these conditions, the dominant relaxation mechanism for the resonating protons arises from their interactions with the electronic spins of the free radicals. NMR experiments were performed between 2.2 and 250 MHz and these allowed us to obtain information about the behavior of the relevant spectral densities over a very wide range of frequencies 0<ωSτ≲50, where ωS is the electronic resonance frequency of the radical and where τ is the translational correlation time. It is shown that a model in which the effects of eccentricity of the spins are included and where the translational motion of the molecules is described by a Smoluchowski diffusion equation, (i.e., taking into account their impenetrability and the nonuniform relative distribution of the molecules calculated in the approximation of Lee and Levesque) gives good agreement with experiment. It is also emphasized that introducing pair correlation effects provides a model without any adjustable parameters which is an advantage compared to a theory of random jumps.