Zero-quantum filter

The multiple-quantum (MQ)/MAS NMR is one of the 2D NMR methods, which is capable of averaging out the second-order quadrupolar interaction in nuclei with spin > 1/2 such as H, "B, O, etc. The "B MQ/ MAS NMR measurements on boron as contained in silyl-carborane hybrid Si-based polymer networks considered here. The molded samples are cut into small pieces to insert them into a 4-mm NMR rotor and spun at 12 kHz in a MAS probe. The observation frequency of the "B nucleus (spin number I = 3/2 and isotope natural abundance = 80.42%) is 96.3 MHz. Excitation of both the echo (—3Q) and anti echo (+3Q) coherences is achieved by using a three-pulse sequence with a zero quantum filter (z-filter). The widths of the first, second, and third pulses are 3.0 4.1 ps, 1.1-1.6 ps, and 19-28 ps, respectively. The z-filter is 20 ps. The recycle delay time is 6-15 s and the data point of FI (vertical) axis is 64 and for each the number of scans is 144. Then, the total measurement time is 15-38 h. The phase cycling used in this experiment consists of 12 phases. Boron phosphate (BPO4 3 = 0 ppm) is used as an external standard for "B. The chemical shift value of BPO4 is —3.60 ppm from BF3 O(C2H5)2 which is used as a standard reference in " B NMR in the liquid state. The transmitter frequency of " B is set on peak of BPO4 for a trustworthy chemical shift after Fourier transform." " ... 

Figure 8.37. The NOESY sequence incorporating the frequency sweep/ gradient zero-quantum filter.

Figure 8.38. Regions of 400 ms NOESY spectra recorded (a) without and (b) with the inclusion of the zero-quantum filter shown in Fig. 8.37. The ZQC suppression employed a 20 ms adiabatic smoothed CHIRP pulse with a 40 kHz frequency sweep.

As for the 2D NOESY data described above, spectra are vulnerable to interference from undesirable zero-quantum contributions between coupled spins this is apparent, for example, in trace (c) of Fig. 8.42 between H4 and H5. This can be especially problematic when shorter mixing times are employed, as in the generation of NOE build-up curves [35], and again the inclusion of the swept-frequency/gradient zero-quantum filter should prove beneficial. The complete selective ID NOESY sequence incorporating this and employing the optimised DPFGSE selection procedure is presented in Fig. 8.43, and an illustration of the improvements provided by the filter may be seen in Fig. 8.44 where the removal of unwelcome anti-phase dispersive contributions between coupled spins is apparent in (b). 

J.R. Vanderveen, M.A. Blackburn, KJ. Ooms, double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in NAFION, Can. J. Chem. 89 (2011) 1095-1104. 

Another simple and general solution to obtain heteronuclear correlation spectra that yield truly pure absorption fine shapes and IP multiplet structures for all available cross-peaks with respect to both J(CH) and aU passiveJ(HH) coupling constants along the detected dimension is the so-called Pure In-Phase HSQC (PIP-HSQC) experiment [111]. The key point is an appended adiabatic zero-quantum filter (ZQF) [112] applied just before a refocusing gradient perfect-echo element and the acquisition. Thus, after the 90°j,( H) pulse (point b in Fig. 8C) the above four components derived from Eq. (2) are converted to... 

These results suggest that the signals arise from dipole-dipole coupled protons. Kreis et al. confirmed this finding by measurements using one-dimensional zero- and double-quantum filtering, two-dimensional J-resolved spectroscopy, two-dimensional constant time COSY and longitudinal order separation... 

In addition to the selected magnetization component (e.g., 7 ), several terms in the density operator survive the application of trim pulses (or z filters). For example, if a trim pulse is applied along the x axis of the rotating frame, all terms of the density operator that commute with remain unaffected, that is, in addition to the in-phase operators and (x magnetization), antiphase combinations like (lyS - I Sy) or (I SyTy + also survive the trim pulses. In the effective field frame, these terms represent operators with coherence order p = 0. Modified z filters and spin-lock pulses that are able to suppress these zero-quantum-type terms will be discussed in Section XII.B. 

An alternative approach for the elimination of zero-quantum coherence is based on its evolution during periods of spin-locking in inhomogeneous Bq or Bj fields (Titman et al., 1990 Davis et al., 1993). This approach is particularly attractive, because it does not rely on time-consuming phasecycling schemes and variation of z-filter delays. Consider an inhomogeneous rf field with amplitude I f (r) that depends on the position r in the... 

To overcome these difficulties, the z-filter experiment was adapted to MQMAS by Amoureux et al. [24]. In this three-pulse scheme the two hard pulses (excitation of the MQ coherences and conversion into OQ coherence) are followed by a short delay during which the magnetisation is stored along the z-axis as zero-quantum coherences and then transferred into observable IQ coherences using a selective n/2 pulse (Fig. 5a). The symmetrisation of the echo and antiecho pathways during the two hard pulses (p=0—> 3—>0) forces an equal intensity of the echo and antiecho signals, leading to amplitude-modulated FIDs and, thus, to pure absorption spectra. This is a robust method, easy to optimise. 

Filter elements have been developed, not just for coupling evolution, but also for chemical shift selection [5.215 - 5.221]. An early example was the jump-return method for solvent signal suppression. Check it 5.2.3.3, whereby the resonances at a given chemical shift and related multiples were suppressed. In common with the z-filter for zero-quantum suppression, the CSSF (Chemical 5hift iSelective Filter) element uses different free precession periods to give varying degrees of chemical shift evolution for each scan in a multiple scan experiment. 

Figure 5.73. The z-filter seheme for suppressing zero-quantum contributions in ID TOCSY. The delay is randomly varied between experiments and the resulting spectra co-added. When using DIPSI-2, the two S periods on either side of the isotropic mixing scheme can act directly as independent periods and may be randomised in a similar manner.

A more direct approach to the elimination of zero-quantum interference is via the use of swept-frequency inversion pulse applied in the presence of a field gradient, which destroys the zero-quantum contributions in a single transient. The application of this has already been described in Section 5.7.3 with a view to obtaining pure lineshapes in TOCSY spectra, and the operation of the filter is itself described in Section 10.6 and so will not be explored here. The implementation of this in the 2D NOESY sequence is illustrated in Fig. 8.37 and requires the incorporation of the filter within the usual mixing time, followed by a purging gradient. The effective suppression of zero-quantum interference is illustrated in the high-resolution NOESY spectra of Fig. 8.38 in which spectrum (a) shows clear anti-phase... 

NOESY sequence incorporating zero-quantum suppression. The mixing time contains non-selective 180° nulling pulses bracketed by opposing purging gradients spaced throughout as shown, followed by the ZQ filter. 

Figure 10.38. Approaches to suppressing zero-quantum coherences (a) the basic r-filter, (b) the spin-echo z-filter and (c) the zero-quantum dephasing element employing an rf frequency sweep/ gradient pulse combination see text for details.

The presence of zero-quantum coherence during the mixing time can substantially distort NOE intensities. This coherence can not be removed with phasecycling or gradient pulses while preserving z-magnetisation. A z-filter scheme was proposed earlier for the elimination of the coherence. It utilises an adiabatic inversion pulse with linear frequency sweep applied simultaneously with a gradient. Cano et alP demonstrated that a better suppression is achieved with a z-filter cascade that combines several filter elements. The attainable suppression ration is then equal to the multiplication of the ratios for each element. 

To overcome these difficulties, the 3Q/MAS NMR experiment using -filter was developed [71] (Fig. 15). In this three-pulse experiment, two strong pulses are followed by a short delay, during which magnetization is stored into z -axis as zero-quantum coherence. It is subsequently transferred to a single-quantum, directly measurable coherence using n... 

---