BIRD HMQC experiment

A very efficient suppression of parent resonances can be achieved using the T filter. This, however, requires a rather careful tuning of the relaxation delay T (see Figure 8). If the jump and return inversion pulse is employed, the pulse sequence can be regarded as a selective version of the BIRD experiment [57-59]. Obviously, multiple-frequency selective inversion pulses may be necessary in the case of complex proton spectra. Usually the /-BIRD HMQC experiment gives cleaner spectra as compared with equivalent heteronuclear singlequantum coherence (HSQC) experiments, presumably because of fewer 180° pulses which are frequently a source of various artefacts. 

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

This suppression scheme has been shown to work well together with HMQC experiments of small molecules at natural abundance. Even cleaner spectra are obtained, if the BIRD sequence is combined with HSQC experiments already containing a spin-lock purge pulse. Drawbacks of the BIRD pulse scheme are the fact that the relaxation delay between scans cannot be chosen freely anymore and that complete suppression of all C-bound proton signals is impossible, if they have different relaxation times. Furthermore, the BIRD pulse scheme is not applicable to molecules in the slow motional regime, since negative NOEs between the inverted proton spins and the non-inverted C-bound proton spins would reduce the magnetization of the latter. 

Selective excitation and selective detection have been recently reviewed by the recognized leaders in this field135 who viewed 29Si NMR applications from a broader perspective. In this review we focus on those selective experiments that have found wider use in studies of organosilicon compounds, selective INEPT and HEED as representatives of the two classes (excitation and detection) of selective experiments. Some other selective experiments are mentioned in other sections (e.g. SPT, selective decoupling, SPINEPTR, V>-BIRD HMQC, DQF COSY). 

BIRD-HMQC. The most difficult aspect of implementing the HMQC experiment is the suppression of signals from protons attached to C (the center-band or single quantum coherences) in favor of the protons attached to C (the satellites or double quantum coherences). The use of pulse field gradients (PFG, Section 6-6) is the most effective technique, but relatively few spectrometers are equipped with the hardware required for their generation. Fortunately, there is an effective alternative for the suppression of center bands by means of the BIRD Bilinear Rotation Decoupling) sequence, which is outlined by the vector... 

Fig. iL Pulse sequences for pseudo triple-resonance experiments with passive selection of satellites (a) z-filtered 2D-"X, H( "Y) INEPT. (b) -BIRD H,"X( "Y) HMQC. (c) H,"X( "Y) HMQC experiment with biselective excitation pulse (bis) biselective excitation may be achieved with DANTE or P-BIRD sequences, binomial pulses, or shaped pulses. (d) H, X( "Y) HEED-INEPT." ... 

Recent applications of pseudo-triple- and -quadruple resonance techniques have in particular focused on the use of indirect detection schemes to measure heteronuclear couplings with increased sensitivity. The most versatile technique for this purpose proved to be H,"X( "Y) or "Xj HK Y) HMQC experiments which use I -BIRD or bi-selective pulses for selective excitation of transitions of ""Y isotopomers and allow to determine both the magnitude of/("X, Y) and its sign relative to /( H/ B X). Experiments of this type are easily employed to spin systems where /( H/ P,"X) ( H/ B Y) which is frequently the case if "Y is a C nucleus which is directly bound to the detected H or P spin, and have been used in a number of cases to measure couplings between C and rare spins such as 109,111 57pgii2 qj. 18705 196 similar B-BIRD-HSQC experiment has also been applied to the measurement of 7( Si, N) in NH-substituted azasilaboroles. ... 

Figure 6.12. The BIRD variant of the HMQC experiment. The conventional HMQC sequence is employed, but is preceded by the BIRD inversion element and an inversion recovery delay, t. This procedure ultimately leads to saturation of the unwanted parent resonances.

A major drawback of the HMQC sequence is the H, H coupling-induced splitting of each correlation peak in the f 1 dimension. As shown for the BIRD-HETCOR experiment in section 5.6.1 this splitting can cause broader lineshapes, if the splitting is unresolved. 

These filters are usually employed to suppress strong and undesirable resonances, e.g. parent signals of isotopically diluted spin systems. In a typical experiment the undesirable magnetization is inverted and allowed to relax until the equilibrium state is reached. At this point the inverted magnetization is not observable and the basic experiment may be started. In the simplest case a jump and return inversion pulse or binomial pulses can provide the necessary selective inversion. A simple example of application of a T relaxation filter is the J/-BIRD (BI linear Rotation Decoupling) HMQC... 

Figure 13 Proton detected HMQC-COSY experiment [10] using a BIRD [36] pulse cluster...

The overall experiment thus becomes BIRD-t-HMQC-DT, in which t is chosen to null the single quantum coherences and DT is the normal delay time between pulse repetitions, which includes the time taken to acquire the signal, as well as a recycle time during which the signal is regenerated through relaxation. The details of the experiment require... 

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