Selective irradiation overlapped

It is also possible to simplify two overlapping multiplets by selectively irradiating a neighboring proton, thus disclosing multiplicity and coupling constants in the residual multiplet. 

In modem NMR, in order to obtain data on through-bond, scalar connectivities or through-space, dipolar connectivities between individual spins, double or multiple irradiation experiments are used. These rely on selective irradiation of a particular resonance line with a radio frequency field and observation of the resulting effects in the rest of the spectrum. With 2D ESR techniques as well as with 2D NMR techniques, limitations of one-dimentional methods connected with overlapping resonance have been overcome (Fig. 1.7). 

Figure 4. ID Selective INADEQUATE spectra obtained by selective irradiation at 27.89 ppm, overlapped to spectra of a syndiotactic polypropylene sample.

Morawetz et al. [105,106] were the first to use non-radiative energy transfer (NRET) fluorospectroscopy for exploring polymer-polymer miscibility. The basic principle is as follows. In a system containing two kinds of fluorescence chromophore, if the emission spectrum of one (donor D) overlaps the absorption spectrum of the other (acceptor A), a non-radiative energy transfer from the former to the latter may occur when the system is excited by irradiation that the former selectively absorbs. The efflciency of energy transfer (E) inversely proportional to Icj/Ia> where Id and la denote the emission intensities of D and A, respectively, depends on the average distance r between D and A according to the relationship ... 

It sometimes happens that the most likely interference is of a very similar structure to the gas of interest. Consequently, their spectra are also very similar and there is considerable overlap of absorption bands. In such cases, high selectivity can still be obtained by irradiating at two wavelengths at which the gases have different ratios of absorption. 

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