Double quantum filtration

One way to imagine this experiment is as a carbon analogue of DQF-COSY in which both FI and F2 would be carbon axes, and theoretically this experiment is possible. For practical considerations related to obtaining complete double quantum filtration, the INADEQUATE experiment is run slightly differently. In the display of the INADEQUATE spectrum of caryophyllene oxide (Figure 5.19), we find that the F2 axis is the familiar carbon axis, which we can, of course, relate to t2 acquisition. The FI axis looks unfamiliar and requires further explanation. 

Which agrees with the result obtained using the irreducible tensor formalism.34 The triple-quantum filtration pulse sequence, applied to a spin- quadrupolar nucleus, produces almost exactly the same result as does the double-quantum filtration sequence, except for two differences. The double-quantum relaxation rate is faster than the triple-quantum relaxation rate, and the triple-quantum FID is a factor of 1.5 larger than the double-quantum FID.34 The triplequantum filter will thus require less than half the number of acquisitions to equal the signal-to-noise of the double-quantum sequence. 

Ikble 5.6. A suitable phase-cycle for double-quantum filtration... 

The potential disadvantage of using double-quantum filtration is the theoret-... 

Figure 5.43. ID double-quantum filtration of the spectrum of the peptide Leu-enkephalin 5.4 in CD3OD. The singlet resonances of the solvent, truncated in the conventional ID spectrum (a), have been filtered out in (b). The remaining peaks in (b) display the characteristic antiphase multiplet structure (which may be masked by magnitude calculation if desired).

The pulse sequence for a double-quantum filtration experiment utilizing the HORROR sequence is shown in Fig. 29. The weak on-resonance RF field leads to recoupling of the homonuclear dipolar interaction under MAS if the DQ-HORROR condition Wr = 2 is fulfilled (which may be seen as equivalent to a (homonuclear) n = 1/2 7 experiment). This resonance condition leads to a purely dipolar double-quantum average Hamiltonian which has to be calculated by averaging over two rotation periods. Similarly to... 

The one-dimensional INADEQUATE experiment was in fact the original version and was proposed as a means of measuring carbon-carbon coupling constants [81]. The use of double-quantum filtration removes the parent resonances that dominate the spectrum and mask the desired splittings, especially when smaller two- and three-bond... 

The basic components of the INADEQUATE phase cycle comprise double-quantum filtration and/i quadrature detection. The filtration may be achieved as for the DQF-COSY experiment described previously, that is, all pulses involved in the DQ excitation (those prior to t in this case) are stepped x, y, —x, —y with receiver inversion on each step (an equivalent scheme found in spectrometer pulse sequences is to step the final 90° pulse x, y, —X, —y as the receiver steps in the opposite sense x, —y, —x, y other possibilities also exist). This simple scheme may not be sufficient to fully suppress singlet contributions, which appear along/i =0 as axial peaks and are distinct from genuine C-C correlations. Extension with the EXORCYCLE sequence (Section 7.2.2) on the 180° pulse together with CYCLOPS (Section 3.2.5) may improve this. Cleaner suppression could also be achieved... 

By allowing multiple-quantum coherence to process during the evolution period of a two-dimensional experiment, Drobny et al. were able to detect its effects indirectly. This idea subsequently blossomed into the new technique of filtration through double-quantum coherence. Multiple-quantum coherence of order n possesses an n-fold sensitivity to radiofrequency phase shifts, which permits separation from the normal single-quantum coherence. This concept inspired the popular new techniques of double-quantum filtered correlation spectroscopy (DQ-COSY) and the carbon-carbon backbone experiment (INADEQUATE), both designed to extract useful connectivity information from undesirable interfering signals. 

---