Double-frequency sweeps

Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

A. P. M. Kentgens and R. Verhagen, Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spin I = 3/2 nuclei. Chem. Phys. Lett., 1999,300, 435-443. 

D. luga, H. Schafer, R. Verhagen and A. P. M. Kentgens, Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems. /. Magn. Reson., 2000,147,192-209. 

Two signal enhancement schemes capable of providing large signal-to-noise gains in static and MAS NMR spectra of the central transition of half-integer quadrupolar nuclei have been presented by Schurko et al. Amplitude-modulated double-frequency sweeps (AM-DFS) and rotor-assisted population... 

The double frequency sweep (DFS) technique was developed to simultaneously invert the population of the energy levels associated with the STs and thus to enhance the CT of half-integer quadrupolar nuclei. The... 

Mg solid-state MAS and static NMR spectra of several representative layered magnesium phosphates were acquired at 21.1 and 9.4 T by using quadrupolar echo and double-frequency sweep quadrupolar... 

Several convenient routes for signal enhancement in quadrupolar SSNMR experiments involve manipulation of the ST populations [46], which results in an enhanced CT population difierence, thereby boosting experimental S/N and reducing experimental times. The apphcation of double-frequency sweeps (DFS) before a Hahn-echo or QCPMG experiment results in the DFS-echo [61,62] and DFS—QCPMG [50,63] pulse sequences, respectively, which Rossini et al. showed are very usefijl for the acquisition of challenging SSNMR spectra [64]. Utilization of... 

Potassium ( K) (I = 3j2). The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) and double frequency sweep (DFS)/QCPMG pulse sequences were applied to acquire solid-state NMR spectra of organometallic complexes. The results show there are significant quadrupolar interactions, which depend on temperature, in the polymeric main group metallocenes cyclopenta-dienyl potassium and pentamethylcyclopentadienyl potassium. Sodium and potassium tetraphenylborates were examined by solid-state Na and NMR spectroscopy as mentioned in 2.1.5 Sodium. ... 

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