NOE enhancement factor

Fig. 2.24. Measurement of NOE enhancement factors by pulsed proton decoupling sample menthol, 200 mg in 1 mL of deuteriochloroform, 25 DC, 20 MHz ...

The authors believe that in the case of organogermanium compounds, the relaxation mechanism in the solid state is the same as that in solution where the quadrupolar mechanism was proved predominant. They could confirm the absence of a dipolar mechanism by determining that the NOE enhancement factor is zero for 38. 

Keywords T Relaxation Time / NOE Enhancement Factors / Silanes... 

To get an impression of the influence of the sample concentration on T, viscosity-dependent spectra of the trisilane PhaMeSi-SiMePh SiMePh2 were recorded and the NOE enhancement factors T determined. T decreases with increasing viscosity, as expected from literature [4]. But the expected minimum and the subsequent increase in Tt did not occur in the range we worked in. Table 3 lists the determined and calculated relaxation rates for the different silicon atoms in (Ph2MeSi )2Si MePh. 

Little difference was noted when peak heights were used. The error in the T data is less than + 10%. Nuclear Overhauser enhancement factors (q) were obtained by measuring the integrated intensity of peaks in a difference spectrum from one with enhancement minus one with no enhancement and dividing that value by the integral from the one with no enhancement i.e. n ( nOe no nOe / (I nOe" Accuracy should be 10% or better. Linewidtns were measured at half heights, and chemical shifts are relative to TMS. 

Broadband XH decoupling, in which the entire proton spectral window is irradiated, collapses all of the 13C multiplets to singlets, vastly simplifying the 13C spectrum. An added benefit of broadband proton decoupling is NOE enhancement of protonated 13C signals by as much as a factor of three. 

The terms pc and py correspond to 1/Tic and 1/Tih, respectively, and CTCH is the cross-relaxation rate. It should be stressed that the simplicity of the above equation is a consequence of the rareness of the I spins and of the dominant strength of the dipolar interaction between directly bonded nuclei. The situation for homonuclear proton spin systems is often more complicated, since the protons usually constitute a much larger spin system, and a separation into distinct two-spin systems may be not valid in this case. The broadband irradiation of the protons yields, in a steady state, Mhz = 0 and M z = Mj (1 rj). The factor 1 + 77 is called, as introduced above, the nuclear Overhauser enhancement factor. The NOE factor is related in a simple way to the equilibrium magnetizations of the I- and S-spins (which are proportional to the magnetogyric ratios 71 and 7s), the cross-relaxation rate and the relaxation rate of the I-spin ... 

In summary, NOE sensitivity enhancements are attained in heteronuclear NMDR when the nucleus with low y is observed while the nucleus with high y is decoupled. On the other hand, negligible enhancement of H NMR sensitivity can be expected from H 13C) experiments, the enhancement factor being only >jiH(13C) = 0.216 [34]. 

Fig. 2.24(a). For long decoupling times, the signal intensity /, benefits from the full NOE enhancement. In the second experiment with short decoupling time, practically no NOE enhancement builds up and a weaker signal intensity I2 < /, is recorded. The intensity ratio fjl2 is then the NOE factor ric. The measurement can be averaged from a series of experiments as is shown in Fig. 2.24 (b) for the carbons of menthol with an r]c of less than 2. 

Next, one should consider the line width of the one-dimensional spectra of both nuclei. The detected nucleus should be the one with the smaller linewidth, since a short T2 leads to signal losses during the pulse sequence. Compared with metal nuclei usually has the smaller linewidth. Because of the chemical shift anisotropy and the temperature dependence of the chemical shift of heavy nuclei, the measurement of is therefore favoured in these cases. For detection, in addition, the large NOE enhancement caused by proton irradiation has to be taken into account. Heteronuclei usually have no directly bound proton their NOE factors are therefore lower. To facilitate further discussion, the y values and the natural abundances of the nuclei used for C-X correlation are given in Table 1. 

A study of the NOE in -emiched plant composts revealed that secondary and primary amides in their organic material produces an enhancement factor of approximately -2.5 (at 30.4 MHz). The band at -306 ppm is assigned to NH2 groups in amino acids, citrulline or uric acid and nucleotide derivatives and shows enhancement factors of approximately -3.5.(64)... 

H NOE can result in a completely nulled resonance. Silicon-29 also has a negative gyro-magnetic ratio, so similar complications ensue. For the Si H experiment, = -2.52. The maximum enhancement factor (1 + T]jnax) is ben — 1.52, which results in an inverted intensity with an increase of 52% over the unirradiated case. For " Sn H) = - 1.34),... 

Figure 6-31 The enhancement factor t as a function of the effective correlation lime Tc for the standard nuclear Overhauser experiment (NOE) and for the spin-lock, or rotating-coordinate, (ROE) variant. The curves were calculated for an interproton distance of 2.0 A and a spectrometer frequency of 500 MHz, (Reproduced from F. J. M. van de Ven, Multidimensional NMR in Liquids, VCH, New York,...

Enhancement, q. Syn. NOE enhancement. The numerical factor by which the integrated intensity of a resonance increases as the result of irradiation of a spin that is nearby in space. For the irradiation of nuclear spins, the upper limit for the observation of a nearby spin is on the order of five angstroms. 

For the common situation of carbon-13 observation in the presence of proton saturation (broadband decoupling), 7h/7c 4 and NOE enhancements can be as much as 200%, equating to a three-fold intensity increase. Since the relaxation of carbon nuclei is largely dominated by proton dipolar interactions, this maximum is almost met in practice. This is clearly a valuable route to sensitivity enhancement and, at least for the case of C, compares favourably for routine acquisitions with the factor of four attainable with the H to polarisation transfer sequences, given in Chapter 4. The maximum NOE enhancements for a variety of nuclei in the presence of proton saturation, denoted X H), are summarised in Table 8.2. 

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