We present a new method of performing chemical shift correlation spectroscopy in solids with magic angle spinning (MAS). Its key feature is a longitudinal mixing period of π pulses that recouples the dipolar interaction. We discuss experimental results for triply‐13C‐labeled alanine and a theory combining MAS and π pulses.

Chemical shift correlation spectroscopy in rotating solids: Radio frequency‐driven dipolar recoupling and longitudinal exchange

NMR spectra are needlessly confusing when spinning sidebands and isotropic peaks lie on the same curve. Qualitative analysis is impossible if these spinning sidebands cannot be distinguished from isotropic peaks and quantitative measurement is frustrated whenever any sideband overlaps any other peak. Resolving a spectrum into several curves, one containing only isotropic peaks, another only first higher frequency sidebands, still another only first lower, etc., eliminates these problems. These extra curves, which can be generated by an echo technique, do not require extra acquisition time. An ordinary echo spectrum refocused after two revolutions has the same S/N ratio as each curve in a separated spectrum which is made with the same total number of acquisitions.

Spinning‐sideband‐free and spinning‐sideband‐only NMR spectra in spinning samples

A new resonance effect in solid‐state nuclear magnetic resonance (NMR) is described. The effect involves a combination of magic‐angle sample rotation with irradiation of a heteronuclear spin system at the Larmor frequency of one of the spin species. If the irradiation intensity is such as to establish a match between spin nutation and sample rotation, it is shown that the heteronuclear dipolar spin interaction is selectively reintroduced into the spectrum. This allows small dipolar coupling constants to be measured in the presence of large shielding anisotropies. Applications are anticipated for determination of internuclear distances in materials lacking long‐range order, such as polycrystalline materials, polymers, and surfaces.

Rotary resonance recoupling of dipolar interactions in solid‐state nuclear magnetic resonance spectroscopy

spinning frequencies. We observe dramatic oscillations of the recoupled signal with a period ;v r/2p indicating that the 1 H reservoir is behaving partially inhomogeneously. This kind of double quantum recoupling is explored in multiple spin systems and we derive analytical forms for polarization transfer and double quantum excitation relevant for uniformly labeled systems. Finally, the wide applicability of the fivefold sequence is demonstrated with INADEQUATE type spectra of uniformly 13 C labeled sucrose and L-alanine. © 1999 American Institute of Physics. @S0021-9606~99!01615-3#

Fivefold symmetric homonuclear dipolar recoupling in rotating solids: Application to double quantum spectroscopy

We discuss several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic-angle spinning (MAS) experiments. These include the accurate measurement of weak dipole–dipole couplings between rare spins, the behavior of dipolar trajectories in multiple spin environments, and chemical shift correlation spectroscopy via polarization exchange. To describe dipolar trajectories accurately, we adopt an approach to the simulation of these experiments which includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach is outlined for the selection of decay parameters. Dipolar trajectories are shown to be dominated by the largest couplings in multiple spin systems via comparison of two and three interacting spins. Two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei is illustrated with a uniformly 15N,13C-labeled sample of the tetrapeptide achatin-II (Gly-L-Phe-L-Ala-L-Asp). In addition, a frequency-selective approach to recoupling dipolar interactions among homonuclear spins is introduced; selective approaches have possible utility in examining weak dipole–dipole couplings in the presence of strong interactions.

Homonuclear radio frequency-driven recoupling in rotating solids

Two-dimensional rotational spin-echo nmr in solids: Resolution of isotropic and anisotropic chemical shifts

Chemical shift scaling experiments which reduce the number and intensity of sidebands in high field magic angle sample spinning (MASS) spectra are described. They rely on the fact that the scaling pulse sequence reduces the effective size of the shift anisotropy, so that moderate speed spinning results in spectra devoid of sidebands of appreciable amplitude. In principle, the technique allows a realization of the full signal strength available in high field MASS spectra. In addition a number of other interesting features have emerged from the experiments. First, the problem of decoupling in the presence of a scaling sequence has been solved by arranging the rf field strengths to mismatch the Hartmann–Hahn condition. Second, it is shown that the scaling cycles used in the experiments are uniform over a wide frequency range with respect to both the scaling factor and the linewidth. Third, under certain conditions, the frequencies for scaling and spinning are about the same size and consequently new features...

D.J Ruben

Papers

Two-dimensional rotational spin-echo nmr in solids: Resolution of isotropic and anisotropic chemical shifts

Uniform chemical shift scaling: Application to 2D resolved NMR spectra of rotating powdered samples

Uniform chemical shift scaling in rotating solids