Origins of linewidth in H1 magic-angle spinning NMR

Abstract

A detailed study of the factors determining the linewidth (and hence resolution) in H1 solid-state magic-angle spinning NMR is described. Although it has been known from the early days of magic-angle spinning (MAS) that resolution of spectra from abundant nuclear spins, such as H1, increases approximately linearly with increasing sample rotation rate, the difficulty of describing the dynamics of extended networks of coupled spins has made it difficult to predict a priori the resolution expected for a given sample. Using recently developed, highly efficient methods of numerical simulation, together with experimental measurements on a variety of test systems, we propose a comprehensive picture of H1 resolution under MAS. The “homogeneous” component of the linewidth is shown to depend primarily on the ratio between an effective local coupling strength and the spin rate, modified by geometrical factors which loosely correspond to the “dimensionality” of the coupling network. The remaining “inhomogeneous” component of the natural linewidth is confirmed to have the same properties as in dilute-spin NMR. Variations in the NMR frequency due to chemical shift effects are shown to have minimal impact on H1 resolution. The implications of these results for solid-state NMR experiments involving H1 and other abundant-spin nuclei are discussed.

Unexpected effects of third-order cross-terms in heteronuclear spin systems under simultaneous radio-frequency irradiation and magic-angle spinning NMR

“…1 The ability of radio-frequency (RF) irradiation to counteract the effect of homonuclear dipolar couplings was demonstrated in the 1960s in the Lee-Goldburg 2 and WAHUHA 3 methods for static samples and in the 1970s in the combined rotation and multiple-pulse spectroscopy (CRAMPS) (Ref. 4) approach under magic-angle spinning (MAS).…”

“…8,9 For example, 13 C, 1 H heteronuclear correlation (HETCOR) experiments at moderate to fast MAS frequencies 10,11 have been widely applied to a variety of significant material types, such as proteins, 12,13 pharmaceuticals, 14,15 silica-supported catalytic complexes, 16,17 organic-templated microporous materials, 18 inorganic-organic hybrid materials, 19 as well as chemical problems, such as the characterisation of weak hydrogen bonding in sugars 20 and the solution of structures from powder diffraction data. 21 Heteronuclear 13 C{ 1 H} spin-echo experiments employing 1 H homonuclear decoupling during the a) Author to whom correspondence should be addressed. Electronic mail:…”

Longer-range distances by spinning-angle-encoding solid-state NMR spectroscopy

“…2a and 2b. Specifically, the master-curve 40 presentation in Fig. 2c shows how the probing of a smaller D for the > 4 Å effective distance in the dipeptide is enabled by using a larger  (4.2°as compared to 2.3°).…”

“…8 By comparison, distance measurements in multispin systems are challenging. Notably, "dipolar truncation", i.e., the dominant effect of the much larger one bond C-C dipolar couplings 9-12 hinders the measuring of the key longerrange distances (> 4 Å) in fully 13 C-labelled bio- 40 macromolecules. Indeed, developing new solid-state NMR approaches to overcome this problem is a current research priority for a number of groups worldwide.…”

“…sum-squared dipolar coupling, D rss , [39][40] that takes into account 10 all (intra-and intermolecular) C-C distances for the same pair of nuclei:…”