Fig. 2. Relaxation approaches in liquids and solids.
(A) Spectral density function, and the frequencies sampled by 13C relaxation experiments in three different static magnetic field strength, indicated with colors/vertical lines. (B)-(C) Principle of high-resolution relaxometry; (B) the magnetic field as a function of the vertical distance from the magnetic center of the magnet. (C) Schematic overview of the experiment design. The Boltzmann polarization at high field is excited, and for the relaxation delay the sample is shuttled to different locations in the low-field region of the magnet. For high-resolution detection of the signal the sample is shuttled back to the magnetic center. (D) Longitudinal and transverse 15N relaxation rate constants in a spinning solid, induced by the 1H-15N dipolar coupling and the 15N CSA tensor. As revealed by these data, each relaxation parameter has a range of correlation times to which it is sensitive, while it is insensitive outside. Therefore, motions on time scales outside these sensitive “windows” are difficult to probe. In all simulations the NH order parameter was assumed to be S2 = 0.9. For the R2 data, the magnetic field strength was 14.1 T. The MAS frequency is important for all transverse relaxation experiments, but it is negligible for R1 experiments. (E) R1ρ relaxation dispersion experiments in solids. Isotropic chemical-shift fluctuations (left) and angular fluctuations both contribute to different regimes of the relaxation dispersion profile. Both effects are relevant, and therefore the experimentalist can choose to probe chemical-shift or angular fluctuations, depending on the MAS and spin-lock frequencies. Details are reported in reference [57]. (F) Differential relaxation of ZQ and DQ coherences due to fluctuations of anisotropic and isotropic chemical shifts in a 1H-15N spin pair. While the differential relaxation due to cross-correlated modulation of the two isotropic chemical shifts is present in liquids and solids, the CSA/CSA CCR is present only in solids. Details and more simulations are reported in reference [56]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)