Double irradiation

The double irradiation method involving the mutual method of grafting of vinyl monomers onto preirradiated polymer recently used by Kaur et al. [85]. 

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

Two factors contribute to r K. One is the ratio of the magnetogryric ratios of the two different spins, and the other depends on relaxation mechanisms. Provided that the relaxation mechanism is purely dipole-dipole, other relaxation mechanisms affect spin I, then 4> may approach zero. Assuming that the dipolar mechanism is operational (no quadrupolar nuclei with I > 1/2 are present), r has the value ys/ 2y and is regarded as rimax. In the homonuclear case we have r max = 1/ 2. Usually one chooses nuclei where ys > y/ to ensure that the NOE is significant. For observation of 13C for instance, if the protons in the molecule are double irradiated, the ratio is 1.99 and 1 + r max equals approximately 3. To repeat a statement made above, proton broad-band irradiation enhances the intensity of the 13C nucleus, which otherwise has very low receptivity. 

Use of double-irradiation methods for the exploration of cross-relaxation phenomena. 

The nuclear Overhauser effect (NOE) is a consequence of the modulation of the dipole-dipole interactions (through space) between different nuclei and is correlated with the inverse sixth power of the internuclear distance. Experimentally, the NOE is the fractional change in intensity of one resonance when another resonance is irradiated in a double-irradiation experiment. The NOE phenomenon is intimately related to spin relaxation. The NOE varies as a function of the product of the Larmor frequency, co0, and the rotational correlation time, tc. In small molecules, tc is short relative to uo"1. In this extreme motional narrowing situation, the frequency... 

Compound A, acetylknightinol , is laevorotatory (M+ 331 empirical formula Ci9H25N04). It loses an AcO group under electron impact. The i.r. spectrum shows two ester carbonyl groups, at 1735 and 1725 cm-1. The H n.m.r. spectrum, combined with double irradiation, led to the conclusion that the alkaloid is representative of the small class of hydroxybenzyl-tropanes4 that were formerly isolated from K. deplanchei, and that structure (1) reflects all of the properties that have been observed. 

Product K, named knightalbinol is a 2ot-hydroxybenzyl-tropane-3//,7//-diol (11) that is acetylated on C-7 (or C-6). This is based on the assumption that the acetoxyvinyl radical is easily split off during mass-spectral fragmentation and gives an ion of m/z 219. The assignment of a 3//-hydroxyl is based on the large coupling constant between H-2 and H-3, and from double irradiation. 

Coupling constants which are usually difficult to measure because of peak overlap can be obtained by the addition of a shift reagent. It is necessary to keep in mind that the coupling constants obtained by addition of shift reagents are for complexed species and may not be the same for uncomplexed species. Decoupling or double irradiation experiments are feasible with addition of a shift reagent. An example where coupling constants and double irradiation were determined is the case of 7,8-cis-enrfo-diphenyl-2-oxabicyclo[4.2.0]octane.The spectrum on addition of Eu(fod)3 is shown in Fig. 10.15. 

Before we leave the discussion of these 13C spectra, there is one more qualifier to add. All three i3C spectra discussed in this chapter (and all subsequent ones we will encounter, unless specifically noted to the contrary) involve simultaneous irradiation of the hydrogens as well as the carbons, a technique known as H-spin decoupling. (It is this simultaneous, or double, irradiation that gives rise to the NOE mentioned in Section 5.4.1.) A H-spin-decoupled l3C spectrum is labeled l3C H. Spin decoupling will be described more fully in Chapter 12, but for now, suffice it to say that if we had not made use of this decoupling, these spectra would have looked far more complicated, and all intensity differences due to NOE would be absent. 

Jacobson and others(390) analyzed the H-nmr spectra at 100 MHz of codeine and isocodeine derivatives, previously reported at 60 MHz,(391) in order to establish the conformation of the spiro-oxirane derived from codeinone.<200) Double irradiation and NOE experiments confirmed the structure illustrated (91). 

N-Decoupling by double irradiation has also proved to be a very promising method for improving the resolution as well as for allowing the accurate determination of the signal of the N—H proton.30-42... 

In addition, its proton magnetic resonance spectrum unambiguously supported the 5-ketose structure (69). Rearrangements of epoxides to ketones when dicobalt octacarbonyl is used as the catalyst at temperatures above 100 , or when cobalt hydrocarbonyl is used at lower temperatures, are well known. By applying the technique of double irradiation to a sample of (68), the main component was shown to possess structure (68). Presumably, the free aldehyde group of the hydroformylation product immediately cyclized with the free hydroxyl group on C-3 to give the tricyclic structure (68). A third component (68a) (isolated in less than 5 % yield) was undoubtedly formed by subsequent reduction of the dialdose derivative (68). 

Because a deuteron has a much smaller magnetic moment than a proton, it absorbs at a much higher field and so gives no signal in the proton nmr spectrum. Furthermore, its coupling with a proton is weak and it ordinarily broadens, but does not split, a proton s signal even this effect can be eliminated by double irradiation. 

Special effects may be routinely and elegantly created by using sources of radiofrequency energy in addition to the observation frequency (uj = yBi) y = y/2i ). The technique is called multiple irradiation or multiple resonance and requires the presence of a second transmitter coil in the sample probe to provide the new irradiating frequency ( 2 = yBi). When the second frequency is applied, the experiment, which is widely available on modern spectrometers, is termed double resonance or double irradiation. Less often, a third frequency (U3 = yBf) also is provided, to create a triple-resonance experiment. We already have seen several examples of multiple-irradiation experiments, including the removal of... 

Figure 5-6 The 100 MHz H spectrum of mannosan triacetate in CDCI3 without decoupling (lower) and with double irradiation at b 4.62 (upper). (Reproduced with the permission of Varian Associates.)...

NMR spectroscopy as a routine tool. The double-irradiation field B2 was traditionally centered at about 8 5 of the H range. To cover all the H frequencies, B2 was modulated with white noise, so the technique often was called noise decoupling. 

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