High-Field and High-Speed CP-MAS13C NMR Heteronuclear Dipolar-Correlation Spectroscopy of Solids with Frequency-Switched Lee–Goldburg Homonuclear Decoupling

✍ Scribed by B.-J. van Rossum; H. Förster; H.J.M. de Groot

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 147 KB
• Volume: 124
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.1996.1089

✦ Synopsis

On the other hand, the resolution in the upfield region is not In this Communication, we demonstrate that the use of optimal, due to a combination of strong homonuclear dipolar frequency-switched Lee-Goldberg irradiation during the couplings and small shift dispersion for most aliphatic proproton evolution of fast-spinning 2-D heteronuclear ( 1 Htons. To obtain good proton resolution over the entire spec-13 C) correlation spectroscopy strongly enhances the proton trum, the application of a technique to suppress the homonuresolution.

clear dipolar interactions between the abundant protons is Heteronuclear correlation spectroscopy is an important necessary. tool for resolution enhancement in multidimensional NMR It was shown by Lee and Goldburg that the application of spectroscopy. For solids, line narrowing in the proton dimension is generally achieved by application of CRAMPS techan RF field off-resonance according to DLG/v 1 Å 1 2 2 proniques to suppress the strong homonuclear dipolar interacduces an effective field in the rotating frame inclined at the tions between the abundant protons (1-11). However, in magic angle u m Å tan 01 ( 2) to the static field H 0 . With Leehigh-speed MAS NMR research of, for instance, biological Goldburg (LG) irradiation, the spins precess rapidly around systems, the use of multiple-pulse techniques to study the this magic-angle axis, which effectively results in a cancellaproton chemical-shift dispersion is limited for several reation of the first two terms of the ''dipolar alphabet'' in the sons. First, the application of multiple-pulse techniques usutilted rotating frame ( 13). It has been shown that when the ally requires cycle times that are short compared to the rotor LG irradiation is both frequency-and phase-switched (FSperiod. This effectively puts a restriction on the spinning LG) after successive periods t Å (2p/v 1 ) 2/3, correspondspeeds that can be used in practice and the efficacy of the ing with the time to complete a 2p rotation around the tilted MAS averaging. It also limits the possibilities for application axis, the efficiency of decoupling is significantly improved of CRAMPS techniques in high-field MAS, where high spin-(14, 15). ning speeds will be required to obtain sufficient resolution FS-LG can be used for homonuclear decoupling in a heterin multidimensional spectra. For instance, the WAHUHAonuclear correlation experiment, since it is not necessary to 4, which has the shortest cycle time, performs well only at observe the protons directly. The LG cycle time is the inverse moderately high spinning rates. More-elaborate CRAMPS of the precession rate along the (1, 1, 1) axis. For moderately sequences require longer cycle times, and the possibilities high RF power, the rate is typically 10 ms, which is short for application in high-speed MAS will be even less favorcompared to the MAS period, even for high spinning speeds. able. In addition, biological MAS NMR research often must Modern NMR spectrometers enable rapid coherent switching be performed at low temperatures, which makes the use of of both frequency and phase of the RF during a pulse, and the tuned-up multiple-pulse sequences impractical.

FS-LG sequence is relatively straightforward to implement. Recently, it was demonstrated that, from straightforward Since the only parameters that need to be adjusted are the 2-D high-speed MAS heteronuclear ( 1 H-13 C) CP/WISE offset frequency DLG and the duration of the successive spectra collected in a high magnetic field and without any frequency intervals t, the sequence is robust and can easily homonuclear decoupling scheme during the proton evolube readjusted if necessary. tion, 1 H-13 C correlations and proton chemical shifts in the

The pulse sequence we used in this study to collect 2D solid state can be obtained directly (12). In high field, dipoheteronuclear correlation spectra is depicted in Fig. 1. The lar interactions are truncated by the increased chemical-shift sequence starts with a (p/2 / u m ) y pulse on the protons, dispersion. The resolution enhancement is quite remarkable directly followed by a train of frequency-and phaseswitched LG pulses in the xz plane. After the evolution for protons which are subject to the largest chemical shifts.