4>-BIRD HSQC

As with HMQC, HSQC can also be combined with P-BIRD. The tp-BIRD HSQC... 

FIGURE 25. H—29Si I -BIRD HSQC phase-sensitive refocused and not decoupled spectrum of Si(OMe>4 (500 MHz, 5 mm ID probe, 2 mg in 450 mg of CgDg, d = 2 s, T2 = 7.0 ms, r = 68.3 ms, 16 increments of 32 transients each, spectral width 10 Hz in FI). The residual parent signal is marked with an asterisk. Reproduced by permission of John Wiley Sons, Ltd from Reference 203... 

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 16 Several pulse schemes to achieve Fl-heterocoupled HSQC spectra (A) Fl-HSQC, (B) BIRD-HSQC and (C) FI-ilNEPT experiments. Inversion and refocusing 180° pulses can be applied as adiabatic or BIP pulses and the evolution period is optional. The interpulse delays in INEPT and BIRD elements are optimized according to 4 = 1/(2 J(CH)) and a small flip angle fi=36°) generates E.COSY cross-peaks for J(HH) couplings.

An optional but very useful feature in the BIRD-HSQC experiment is the use of a small flip pulse angle (P = 36°) in the last retro-INEPT block... 

J.D. Snider, E. Troche-Pesqueira, S.R. Woodruff, C. Gayathri, N.V. Tsarevsky, R.R. Gd, New strategy for RDCs assisted diastereotopic protons assignment using a combination ofj-scaled BIRD HSQC andj-scaled BIRD HMQC/HSQC, Magn. Reson. Chem. 50 (2012) S86-S91. 

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. 

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. 

Heteronuclear coupling constants (1,b7c,h) are most commonly measured from heteronuclear 2D experiments. The 3/c H couplings can be easily extracted from /-resolved spectra as well as from f or F2 proton coupled HSQC spectra. The undesired evolution of "/CH during q can be eliminated with use of an appropriate bilinear rotation decoupling (BIRD) pulse, such as BIRDd,x in. /-resolved spectroscopy35 and 111RD in Fi-coupled HSQC.36 Spin-state selective excitation techniques, S3E and S3CT37 38 (spin-state-selective coherence transfer), can also be used for the measurement of... 

The readout of line frequencies is simpler in Fb but potentially less precise due to limited digital resolution. The digital resolution can be increased if long-range, either proton-carbon or proton—proton, interactions are removed by the action of a BIRD pulse.36,175,180 If required, overlap reduction can be achieved by separating the a//3 states into two spectra as demonstrated in S3-CT-HSQC37,147,148 or SPITZE (spin state selective zero overlap)-HSQC.170... 

A G-BIRD " modified FI coupled HSQC experiment for the accurate determination of one-bond heteronuclear residual dipolar couplings has been devised by Feher et al In this approach all the long-range couplings of the heteronucleus are decoupled and only the one-bond coupling to the directly bonded proton is retained. It is worth noting that the G-BIRD module can be easily incorporated in any other variant of HSQC sequence. 

A coupled HSQC experiment modified with a G-BIRD(r) module has been proposed for accurate determination of small one-bond heteronuclear residual dipolar couplings. The modification also has been applied in a TROSY sequence resulting in a significant sensitivity and resolution enhancement. Several sensitivity-enhanced experiments such as TROSY-anti-TROSY, E.COSY and IPAP (inphase-antiphase) experiments for measuring... 

The first proposal RESET-HSQC experiment [69] implemented the original pseudo-2D ZS element just prior to acquisition, using a BIRD element as selective inversion module to homodecouple protons from... 

Figure 3 Pulse schemes to obtain pure shift HSQC spectra. (A) HSQC-RESET experiment which uses a pseudo-3D BIRD-based ZS acquisition scheme (B) PS-HSQC experiment using real-time homodecoupling by the combination of a hard 180°( H)-BIRD element during data acquisition (C) sensitivity-improved PS-HSQC (D) HOBS-HSQC experiment using real-time homodecoupling during detection achieved by applying a pair of hard/band-selective 180° pulses (represented as solid and shaded shapes). In (B-D), the homodecoupling element is applied at the middle of 2t=AQ//i periods, where AQ is the acquisition time and n is the number of concatenated loops. See original publications for a more detailed description.

Non-refocused and refocused versions of the F2-coupled HOBS-HSQC experiment described in Section 4.2 have also been reported (see pulse diagram in Fig. 15C), and the important sensitivity and resolution enhancements as weU as the multiplet simphfication can be quickly evidenced when comparing the selected ID sHces in Fig. 15D versus E. In addition, analogs counterparts of the previously described 2D perfect-HSQC (Section 5.1.2) and PIP-HSQC (Section 5.1.4) experiments could be easdy designed by incorporating BIRD-based or HOBS broadband homodecouphng as outlined in Fig. 15B and C, respectively. 

Figure 15 F2-heterecoupled PS-HSQC experiments (A) pseudo 3D RESET-HSQC (B) PS-HSQC using a BiRD inversion eiement during acquisition (C) HOBS-HSQC using a band-seiective pulse as the inversion element (D,E) F2-coupled conventional HSQC and HOBS-HSQC spectra of (2). Figure partially adapted from Ref. [73],...

K. Feher, S. Berger, K.E. Kover, Accurate determination of small one-bond heteronuclear residual dipolar couplings by FI coupled HSQC modified with a G-BIRD module, J. Magn. Reson. 163 (2003) 340-346. 

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