Proton Spin Decoupling Experiments

Before the advent of 2D NMR spectroscopy, the classical procedure for determining proton-proton connectivities was by homonuclear proton spin decoupling experiments. Such experiments can still serve to determine some H/ H connectivities in simple molecules. 

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

The appearance of benzenoid signals in analyzable patterns, the coupling constants of benzenoid protons, the comparison of spectra with those of analogous compounds, the solvent and concentration dependence of the 7-proton resonance, spin decoupling experiments, and some additional factors which are summarized below, may lead to the proper assignment of the benzenoid signals, and thus to the substitution pattern. 

An ether bond could form between C-19 and C-20 as observed in compound 62 [36], the methylene protons of C-19 appeared at 8 3.78 d (12 Hz) H-19a and 3.70 dd (12 and 1 Hz) for H-19p. Spin-decoupling experiments confirmed these assignments, H-20 was seen at 8 5.20 as a singlet, A similar ether bond is observed in a group of compounds 97-101 isolated fi-om T. micropodioides [56]. 

Figure 17. Chemical shifts of Cq ring resonances of dimesityluranocene, 6, as a function of 1/T. Numbers show the ring proton assignments from spin decoupling experiments. Primed numbers show the assignments below the coalescence temperature.

Spectra were observed at 60 macrocycles/s and 100 macrocycles/s both at room temperature and at high temperatures, and spin-decoupling experiments were done. The difference in the chemical shifts of the two meso methylene protons at 60 macrocycles/s was found to be approximately 7 cycles/s for the isotactic three-unit model, whereas it was approximately 16 cycles/s for the isotactic two-unit model or heterotactic three-imit model. PVC spectra can be reasonably interpreted on the basis of this result. Observed values of vicinal coupling constants of model compounds were interpreted as the weighted means of those for several conformations, and the stable conformations of the models determined. 

Use one-dimensional spectral editing techniques such as selective spin decoupling experiments for the determination of the proton bonding of the carbons in the polymer molecule before and after photodegradation. In some cases, it is recommended to employ two-dimensional (2-D) NMR techniques [200,325,630,2222]. 

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