Experiment selective decoupling

Fig. 14. SeqHNCA-TROSY experiment for establishing sequential 1HN(i), 15N(i), 13Ca(i— 1) correlations in 13C/15N/2H enriched proteins. Durations of transfer delays A = 1/(4/Hn) 2Ta = 20-27 ms, depending on rotational correlation time of protein 2Tc = 5-7 ms S = gradient + field recovery delay 0 < k < Ta/t2,max- Phase cycling i = y (j>2 = y, — y + States-TPPI 0 = x 0ret. = x, — x. Semi-selective decoupling of 13C spins is attained using a SEDUCE-1 decoupling sequence.95...

The procedure works when each detected spin B can be selectively decoupled from a spin C, although other couplings may be present. If this is the case, the spectra obtained in the coupled experiment and the reference experiment are identical except for the additional splitting due to the coupling of interest J(l>, C). This splitting can be determined by convolution ( in Fig. 7.8) of the decoupled spectrum with a stick doublet to yield the coupled spectrum. 

Some carbon-13 chemical shifts of benzotriazoles polysubstituted at both N- and C-atoms of the benzotriazole ring are listed in Table 8. They are generally assigned by comparison to literature values and by careful selective decoupling experiments. The contribution of C-substituents to the chemical shifts of benzotriazoles is much large than that of V-substituents. 

The structural assignments of 40, as based on H-NMR spectra, have been confirmed by l3C-NMR spectroscopy.90 In the formation of adducts the carbon atoms undergo upfield shifts in the order C6 C5 > C4 > C2. In accordance with this behavior, indicating a change in hybridization of C-6 from sp2 to sp3, the 7(C6-H) value decreases from 180 Hz of the initial substrate to 150 Hz of the adduct. In decoupling experiments, selective irradiation at vH6 and vH5 causes the C-6 and C-5 doublets, respectively, to collapse into singlets. 

In a 13C XH experiment, a 13C— 1H multiplet can be selectively decoupled by an B2 field whose frequency matches the Larmor frequency of the coupling protons. In routine 13C NMR, however, usually all 13C H multiplets are decoupled for reasons of sensitivity and simplicity. This is achieved when the decoupling field B2 covers the range of all proton Larmor frequencies. For the protons in organic compounds in a magnetic field Bq of 2.3 Tesla, this is at least 1 kHz at about 90 MHz. Decoupling fields with large... 

Selective population or polarization transfer (abbreviation SPT) was first achieved for chloroform [51] by brief irradiation of one of both 13C satellites in the proton NMR spectrum before recording the 13C—1H doublet as FID signal. In this experiment, the decoupling pulse inverts the population of the proton precession states connected by the irradiated satellite transition (Fig. 2.43(b, c)). The two SPT experiments shown for the 13C —H doublet of chloroform in Fig. 2.43 (b, c) give an impression of the signal enhancement achievable by population transfer in comparison to a normal 13C NMR spectrum... 

For an effective measurement of "/(SiH) (1 < n < 4) values in silylated silyl enol ethers, a selective population transfer experiment was modified so that selective decoupling was applied during acquisition. 1H-29Si heteronuclear COSY confirmed the presence of silylated groups in the molecule6. 

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). 

This traditional ID NMR experiment involves selectively decoupling a proton from its J coupling partners. This is accomplished by low-power irradiation at the frequency of the... 

Shaped pulses are created from text files that have a line-by-line description of the amplitude and phase of each of the component rectangular pulses. These files are created by software that calculates from a mathematical shape and a frequency shift (to create the phase ramp). There are hundreds of shapes available, with names like Wurst , Sneeze , Iburp , and so on, specialized for all sorts of applications (inversion, excitation, broadband, selective, decoupling, peak suppression, band selective, etc.). The software sets the maximum RF power level of the shape at the top of the curve, so that the area under the curve will correspond to the approximately correct pulse rotation desired (90°, 180°, etc.). When an experiment is started, this list is loaded into the memory of the waveform generator (Varian) or amplitude setting unit (Bruker), and when a shaped pulse is called for in the pulse sequence, the amplitudes and phases are set in real time as the individual rectangular pulses are executed. 

Selective decoupling experiments are sometimes still useful in correlating specific peaks via spin coupling and deducing bonding networks, and continuous wave NOE can occasionally be helpful in establishing spatial relationships. Generally, however, correlations and their implications for molecular structure are best carried out via 2D techniques. 

Selective Hj NOE of quaternary carbons have been observed in selective decoupling experiments. (296,297) However, a study (517) of the frequency dependence of the NOE in [28] shows that, although differential NOE are observed for C-1 and C-3 when H-2 is irradiated with a field of strength yB2/2n 20 Hz, the frequency selectivity is not very good. The utility of the experiment as an assignment technique may therefore be somewhat restricted if the protons responsible for the selective NOE cannot be identified. 

Figure 3. H-NMR (400 MHz) of compound X isolated from a K329A reaction mixture by chromatography on MonoQ. The chemical shifts and proton-proton coupling assignments, based on selective decoupling and 2D experiments (not shown), are consistent with the structure of 2-carboxytetritol 1,4-bisphosphate (inset). Two phosphorous resonances were observed by 3 P-NMR proton-phosphorous couplings, assigned by broad-band decoupling and 2D heteronuclear COSY experiments (not shown), are also consistent with the proposed structure.

Complex (I) also loses CO when heated at 110°C or irradiated (A = 366 nm) under Nj. This product (II) also adds H, and a tetrahydride complex (III) is formed. The H NMR spectrum ( —40°C) shows four distinct hydride resonances (1 1 1 1). Selective decoupling NMR experiments show that each of the terminal PPh, ligand couples to two inequivalent hydrides, suggesting structure (III). 

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