H HSQC Experiment

Fig. 7.26 J-resolved constant r C,H-HSQC experiment (a) to measure the cross-correlated relaxation rates in RNA with a geometry given in b.

For example, a direct projection of the co( N)-dimension in a multidimensional APSY experiment can be replaced by a standard [ N, H]-HSQC experiment. 

Figure 2.15. HC HSQC experiment (contour plot) of a-pinene [ CDCI3, 5 % v/v, 25 °C, 125 MHz for C, 500 MHz for h, 4 scans, 256 experiments]. This experiment gives the same information as Fig. 2.14 within 8 minutes instead of two hours required for the CH-COSY in Fig. 2.14 due to higher sensitivity because of proton detection and stronger magnetic field. Deviations of proton shifts from those in Fig. 2.14 arise from the change of the solvent. The methylene protons collapsing in Fig. 2.14 at Sh = 2.19 (200 MHz) display in this experiment an AB system with = 2.17 and Sg = 2.21 (500 MHz)...

NMR methods of diastereomeric excess determination were used in the separation of 2-methyl-l-boraadamantane into optical antipodes (see Section 12.13.2.5.1) <2003MC121>. Total assignment of H (Figure 4) and 13C NMR spectra (Table 3) of THF-2-methyl-l-boraadamantane 15 was based upon COSY and HSQC experiments <2003MC121>. 

A sequence suitable for measurement of J(H, P) and J(C, P) couplings is shown in Fig. 7.9a. The pulse sequence is a constant-time [13C, H]-HSQC (heteronuclear single-quantum correlation), in which 31P decoupling is applied in ot, in the first experiment and in co2 in the second. 

The two-bond HNC dipolar coupling is observable in a 15N-HSQC experiment in which the J coupling between the carbonyl atom C and the 15N amide is active. The doublet components in the 15 N dimension that represent the C N coupling are displaced with respect to one other in the H dimension as in an E.COSY [39] because of this two-bond coupling. 

Figure 1 shows the pulse sequence of the C HSQC experiment supplemented by a spin-lock pulse to suppress the signals from C-bound protons. The experiment is readily described in terms of Cartesian product operators [9]. For a two spin system consisting of a proton spin H coupled to a C spin C, the relevant coherence transfer pathway is... 

Fig. 1. Pulse sequence of the C HSQC experiment with a spin-lock pulse for the suppression of signals from protons not bound to C. Narrow and wide bars denote 90° and 180° pulses, respectively. The spin-lock pulse is labeled SL. r is set to 1/[2J( C, H)]. The detection period is symbolized by a triangle. Phase cycle ] = 8(y) 4>2 = 2 x,x,y,y) 03 = 4 = 4n = 8(x) 05 =4(x,—x) 05 = 4(x),4(—x) acquisition = 2(x,—x,—x,x). The phases of the C pulses before U (03 and 0.5) are subjected to the States-TPPI scheme [38].

Fig. 1. Pulse sequence for the X/Y H PFG-HSQC experiment as employed for 19F/13C correlation spectroscopy in Ref. 21. 90° and 180° hard pulses are denoted by solid and open bars, respectively groups of two solid and one open bars denote 90° 0 — 180° +9o — 90° pulse sandwiches that serve as composite 180° pulses. 2 are delays of length 1 /(2 Jx,v), and r is a short delay of the same length as the gradient pulse (typically 1 ms). Phase cycles are as in the standard HSQC experiment, and the ratio of gradient pulse strengths is set to G2/G1 = Yy/Yx- Decoupling is employed using WALTZ-16 ( H) and GARP (Y) pulse trains.

In contrast to the basic "C detected experiment, and as a consequence of the final H detection, the 2D spectra obtained with HMQC or HSQC have a projection onto the F2 axis which corresponds to the normal H spectrum and includes all chemical shifts and all Jfi, couplings. The latter may give rise to rather broad cross peaks for extensively coupled protons. The projection onto the Fl axis corresponds to a normal C spectrum but with the quaternary carbons missing. With HMQC, but not with HSQC, cross peaks are additionally split in Fl by "J couplings. The HMQC and the HSQC experiment are usually performed in phase-sensitive mode, which, after proper phasing in both dimensions, allow peaks to be displayed in pure absorption. 

Fig. 3.28 (continued) The "C/ H-COSY experiments H-detected HMQC (b), H-detected HSQC (c). 

Due to their higher sensitivity, the H detected C/ H-COSY experiments HMQC (Fig. 3.29b) and HSQC (not shown) are more suitable if only small sample amounts are available. 

Fig. 3.32 The C/ H-COSY- H/ H-TOCSY experiment (based on the HSQC pulse sequence).

Fig. 8.2. Some of the most common 2D pulse sequences that can be employed using a proper choice of parameters to record 2D spectra of paramagnetic molecules (A) NOESY, (B) ROESY, (C) COSY, (D) ISECR COSY, (E) zero-quantum (double quantum) COSY, (F) TOCSY, (G) HMQC, (H) HSQC. Sequences (A), (B) and (F) are also used to obtain EXSY spectra. SL indicates a soft spin-lock sequence, while MLEV17 indicates a train of spin-locking hard pulses that optimizes the development of J/j coupling. In the reverse heteronuclear experiment (G) the upper and lower levels refer to H and heteronucleus, respectively. The phase cycles are not indicated. For clarity of discussion, all initial pulses can be thought to be applied along the y axis, in such a way that the coherence after the first 90° pulse is always along x. ...

In every 2D experiment we have looked at, the chemical-shift evolution during the h delay produces two terms—sine and cosine—and in each case only one of them survives the mixing step to reach the FID as observable magnetization. The HSQC experiment is no exception ... 

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