Double irradiation technique

Nitrogen atoms of the azo bond are directly engaged in the azo-hydrazone tautomeric system and it can be expected that l4/l5N chemical shifts should reflect the changes in equilibrium. This basic idea was proposed by Berrie et al.,m who used l4N chemical shifts of NHQ/NQ atoms and calculated hydrazone contents (Table 15). They obtained naturally only one set of data measurable by double irradiation technique. The intramolecular hydrogen bond has a marked effect on <5N, as can be seen after comparison of l5NQ chemical shifts in trans-azobenzene (6 = 128), and compounds 87 and 4-hydroxyazobenzene (91)110 (Table 20). From these data it follows that the proper model compounds for tautomeric systems with intramolecular... 

In this section, we have only made use of chemical shift effects in studying stereostmcture of simple heterocycles. However, it needs to be pointed out that the determination of stereostmcture in complex molecules is greatly aided by special NMR experiments, such as double irradiation techniques, especially using the nuclear Overhauser effect (NOE) to detect longer range nuclear interactions, as well as by various two-dimensional techniques. The study of these techniques is beyond the level of this introductory text. 

Simultaneous determination of sulphur and phosphorus is feasible by a double irradiation technique. The theoretical background of this method was described by Op de Beeck and Hoste (13). 

To resolve hf and nuclear quadrupole interactions which are not accessible in the EPR spectra, George Feher introduced in 1956 a double resonance technique, in which the spin system is simultaneously irradiated by a microwave (MW) and a radio frequency (rf) field3. This electron nuclear double resonance (ENDOR) spectroscopy has widely been applied in physics, chemistry and biology during the last 25 years. Several monographs2,4 and review articles7 11 dealing with experimental and theoretical aspects of ENDOR have been published. 

Nohehum may he synthesized by several methods involving irradiation of isotopes of curium, plutonium, and fermium in the form of thin targets with heavy ions of boron, carbon, and oxygen using double-recod technique. The nuclear reaction in the synthesis of No-254 carried out by Ghiorso and his group is as foUows ... 

In practice, spin decoupling experiments are conducted in the following way. First, the spectrum is recorded under normal conditions. Then the spectrum is recorded while a second radiofrequency emitter irradiates at the resonance frequency of the nuclei that are to be decoupled (Fig. 9.22). This double resonance technique is used to identify nuclei which are coupled and which cause interpretation difficulties in the spectrum. 

The potential signal enhancement available from these double-resonance techniques depends on the available polarization ( H or 14N), the detection frequency, and the polarization transfer efficiency. The transfer efficiency depends not only on the coupling between the isotopes and the time spent with the frequencies matched but also on the number of 14N transitions matched and the order in which the matching conditions occur [91]. Irradiating multiple transitions, as described in Section 4.2.1., will also affect the choice of 14N transitions to polarize [94,95],... 

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. 

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). 

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... 

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. 

The dipolar coupling in the solid state can be eliminated by irradiating the protons with a strong signal at their resonant frequency. This double resonance experiment is similar to the double resonance technique used to remove similar C- H coupling in liquid samples except that much higher powers are required. 

---