NMR in rotating solids

Abstract

The NMR free induction decay from a spinning sample having inhomogeneous anisotropic interactions (chemical shifts, first order quadrupole couplings) takes the form of a train of rotational spin echoes. The Fourier transform of the echo envelope is a sharp spectrum from which the effects of anisotropy have been removed. The Fourier transform of the echo shape contains information concerning the anisotropies: This information can be extracted by a moment analysis. The effects of localized homonuclear spin–spin interactions are to convert the ’’isotropic’’ spectrum into a characteristic powder pattern. Second order quadrupole coupling produces a similar effect. It is shown in this case that the usual second-order level shifts cannot be used to calculated this pattern, which must be described by a proper average Hamiltonian theory. Finally it is shown that rotational spin echoes provide a convenient means of studying very slow random molecular rotations (τc≲1 sec).

SIMPSON: A general simulation program for solid-state NMR spectroscopy

“…This is ascribed to the presence of "homogeneous" interactions [61] which effectively call for a quite time-consuming time-ordered integration of the spin dynamics during sampling of the FID [17,62]. Several breakthroughs, among which range efficient powder averaging [45,49] and exploitation of time-translational symmetries [52,53,54], have greatly improved the conditions for such simulations and their combination with iterative fitting for extraction of accurate structural parameters.…”

Uniform-sign cross-peak double-quantum-filtered correlation spectroscopy

“…The relatively long refocusing period ($10 ms for typical 50 Hz 1 J CC couplings) may seem incompatible with solid-state NMR, where one would expect significant intensity loss due to relaxation during this time. However, the relaxation rate under echo sequences, 1=T 0 2 , is governed primarily by the homogeneous line width and is significantly slower than the relaxation rate of the directly observed signal, 1=T Ã 2 , which is governed by the inhomogeneous line width [22]. Such refocusing periods can in fact be very useful in solid-state studies [4,5].…”

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

“…41 For two dipolar-coupled spin I = 1/2 nuclei, the 15 well-known rotational-resonance phenomena arise from the non-commutation (and hence incomplete refocusing under MAS) of the homonuclear dipolar coupling and the chemical shift anisotropy (CSA). [42][43] In analogy to this, Edén and Frydman have introduced the term "spontaneous quadrupolar-20 driven recoupling" for the incomplete removal by MAS of the homonuclear dipolar coupling between two quadrupolar (I > 1/2) nuclei that arises from the non-commutation of the dipolar and quadrupolar couplings. This analysis builds upon work by Gan and Robyr 44 and Facey et al 45 for the case of a 25 dipolar coupling between two 2 H (spin I = 1) nuclei.…”

“…Note that a quadrupolardriven recoupling effect is observed for the case of parallel quadrupolar tensors (that are not colinear with the dipolar coupling); this is different to the spin I = 1/2 n=0 rotational resonance effect, where no effect is observed if the two CSA 60 tensors are parallel. [42][43] In this study, the effect of 11 B-11 B dipolar couplings on 11 B spin-echo dephasing is investigated using samples of polycrystalline lithium diborate, Li 2 O.2B 2 O 3 , with three different degrees of 11 B depletion/enrichment: 5%, 25% and 65 100%. (At natural abundance, 80% of boron nuclei are 11 B, with the remainder (20%) being 10 B.).…”