Selective saturation

Selective saturation (using shaped pulses) of all the aromatic ring CH resonances together since they show up generally in a short window of 6.5-8.5 ppm. The Q-matrix is easily set up for this case. 

A short introduction in the application of MT techniques has been given in Section 3.3. Mechanisms of MT are described by interactions between two separated pools of protons, one of them consisting of free protons with long transverse relaxation time (pool A, 7 2 > 10 ms) and the other of protons bound to macromolecules with an inherent short transverse relaxation time (pool B, T2< 10 ps). By selective saturation of the magnetization of the protons in pool B it is possible to reduce signal intensity of pool A and therefore to draw conclusions about the amount of protons present in pool B. 

If the molecules are placed in a magnetic field, the different spatial orientations correspond to different Zeeman levels and the selective saturation explained above leads to a nonthermal population of the Zeeman niveaus. By exposing the sample simultaneously to an rf field with the proper frequency, magnetic dipole transitions between the Zeeman levels can occur and the thermal population can be restored. This will again increase the polarization of the fluorescence at most up to its presaturation value. The fluorescence polarization is thus used as detector for the rf transitions. 

Although these two methods have found widespread application for the synthesis of free carbenes, they failed for selected saturated imidazolidin-2-ylidenes and especially in the preparation of triazolin-5-ylidenes. In these cases the free carbene species 7 can be obtained from 2-alkoxyimidazolidines 6 [44] or 5-aUcoxytriazoles [36] by thermally induced ot-elimination of an alcohol (Fig. 5). In addition to 2-aUcoxyimidazolidmes, 2-(pentafluorophenyl)imidazolidines [45, 46] have also been used for the generation of NHCs by cx-elimination. The adduct 8 eliminates acetonitrile upon heating [47] to yield the benzimidazolin-2-ylidene 9. In a more exotic procedure, imidazolium salts have been reduced electrochemically to give the free imidazolin-2-ylidenes [48]. 

Selective saturation experiments were performed with the HX270 employing the homonuclear decoupling mode availcible with the instrument but substituting the usual situation by... 

On the other hand, Schaefer ( ) has shown from selective saturation experiments of amorphous cis polyisoprene, crystalline trans polyisoprene, as well as carbon black filled cis polyisoprene, that the resonant lines are homogeneous. The linewidths in these cases are thus not caused by inhomogeneous broadening resulting from equivalent nuclei being subject to differing local magnetic fields. The results for these systems are thus contrary in part to what has been found here. 

If ESR spectra are recorded at different microwave powers and a plot of spectrum area versus the square root of power constructed, then the spectrum shows increasing deviation from linearity above a critical "saturation" power. This is normally the maximum microwave power which should be used to record spectra. However, observation of the ESR spectra from irradiated aromatic polymers at higher power levels has shown selective saturation of the singlet produced preferentially at low doses. Thus the dose saturation behavior is mirrored by the power saturation. 

Fig. 3.9 Expan.sion of the ID NOE Spectra of peracetylated glucose Top trace -Unperturbed reference spectrum. Bottom trace - Difference spectrum. Proton H-C(6) with resonance at 4.3 ppm has been selectively saturated.

Table A2.2.1 Water Activity of Selected Saturated Salt Standards"...

Table A2.3.1 Water Activities (aw) of Selected Saturated Salt Solutions at Various Temperatures0 3...

Table A2.3.2 Regression Equations for aw of Selected Saturated Salt Solutions at Different Temperatures0...

We have chosen to dedicate a full chapter to the nuclear Overhauser effect [1,2] (NOE) because its comprehension is of fundamental importance in dealing with nuclear relaxation [3,4], Especially in paramagnetic compounds, it still represents a most powerful technique to detect dipolar connectivities finally it allows the comprehension of two-dimensional spectroscopies based on dipolar coupling. The NOE is the fractional variation of the intensity of a signal when another signal is selectively saturated. 

It has been mentioned in Section 7.3, and it was implicit all over Chapter 7, that a finite time is required to achieve selective saturation or inversion of a signal by a soft pulse, during which time polarization starts to be exchanged, causing non-linearity of the response (see also Section 9.3). It should be stressed that this is not the case in all common 2D experiments based on non-selective pulses, which have durations of the order of microseconds instead of milliseconds, as required for selectivity. Selectivity in 2D experiments is intrinsic because of the double frequency labeling along f and /2. 

Results similar to those shown in the slice of Fig. 8.22 can be obtained with the so-called NOE-NOESY sequence [36]. Here a hyperfine shifted signal, e.g. I2-CH3 of the above compound, is selectively saturated, and then the NOESY pulse sequence is applied. The NOESY difference spectrum obtained by subtracting a NOESY spectrum without presaturation of the I2-CH3 signal is shown in Fig. 8.23. Here, some more cross peaks are evident with respect to the 3D NOESY-NOESY experiment because secondary NOEs develop much more when the primary NOEs from the I2-CH3 signal evolve in a steady state experiment like the NOE-NOESY rather than in a transient-type experiment like the NOESY-NOESY. In Fig. 8.23, dipolar connectivity patterns are apparent among protons... 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

Since differences in the hydrogenation rates of variously substituted monoalkenes were found over Pt-zeolite catalysts modified by CVD (see Section IV.A.l) these catalysts were anticipated to exhibit regioselectivities in the hydrogenation of dienes. Selective saturation of the less substituted double bonds of linear and cyclic dienes was indeed achieved over Pt-zeolite A and Rh-zeolite A catalysts treated with tetraethoxysilane78 or diethoxydiphenylsilane79,80 (Table 13). 

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