An experimental study of resolution of proton chemical shifts in solids: Combined multiple pulse NMR and magic-angle spinning

Abstract

High-resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic-angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic-field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′-dimethylbenzophenone, 2, 6-dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylmethacrylate), and the vitrain portion of a bituminous coal.

Proton MAS NMR Spectra at High Magnetic Fields and High Spinning Frequencies: Spectral Simulations Using Floquet Theory

“…Proton MAS spectra obtained at high spinning rates and high fields exhibit significant line narrowing (1). The combination of multiple-pulse NMR and MAS can result in high-resolution spectra (2) and in recent publications this approach has been extended and improved (3). The frequency-switched Lee-Goldburg (4) has provided an alternative approach for detecting high-resolution spectra and lately it has been implemented in two-dimensional MAS experiments (5).…”

13C NMR of Liquid Crystals with Different Proton Homonuclear Dipolar Decoupling Methods

“…The most commonly used method is perhaps the 8-pulse MREV-8 sequence (4,5). The combination of magic angle spinning and multiplepulse dipolar decoupling (CRAMPS,(15)(16)(17) has proven to be very useful in obtaining high-resolution proton spectra of solids. In 1989, Levitt and co-workers (11,12) demonstrated that the "magic-angle CW" method of Lee and Goldburg (1) can be dramatically improved by frequency switching.…”

Line Narrowing in Solid-State Proton NMR with Acquisition Delay

“…Recently it has been shown quences (6)(7)(8). Subsequently, Gerstein et al (9) confirmed that substantial narrowing of the proton linewidths of organic that single-pulse excitation with acquisition delay or spin solids can be achieved by using single-pulse excitation with acquisi-echo allows the observation of narrow peaks, and suggested tion delay or spin echo [S. Ding and C. A. McDowell, J. Magn.…”