Eclipsed resonances

The dithionite ion has a remarkable eclipsed stmcture of approximate C2v symmetry (Fig. 15.32b). The extraordinarily long S-S distance (239 pm) and the almost parallel SO2 planes (dihedral angle 30°) are other unusual features. Electron-spin-resonance studies have shown the presence of the S02 radical ion in solution ( 300ppm), suggesting the establishment of a monomer-dimer equilibrium 8204 ... 

The most stable conformation of a carboxyl group is a planar arrangement of the molecule. The carbon is 5p -hybridized, and the O—H bond lies in the plane, eclipsing the C=0 double bond. This unexpected geometric arrangement can he explained by resonance. The following resonance forms can be written for a carboxyl group. 

In the Algols observed outside of eclipse, however, the resonance doublet line appear in absorption and, contrary to what we have described for the rest of the close binary systems, C IV is the weakest of the ions. This fact prompted Peters and Polidan (1984) to advance the idea that in the atmosphere of the more massive components in Algol systems, C is underabundant and N overabundant, in confirmation of Parathasarathy et al s (1983) results. 

It was interesting that a few Algols, namely, U Cep (Plavec 1983), RW Tauri (Plavec and Dobias 1983) and TT Hydrae, U Sge and UX Monocerotis (Plavec et al. 1984) were observed with the IUE satellite at principal eclipse. And on these images the resonance doublets of C IV, Si IV and N V are displayed in emission and, as far as intensities and sequence of intensities go, they behave like in the rest of the close binary systems, that is, C IV is the strongest feature of the three. 

It seems surprising that an eclipsed conformation is most stable. It appears that one of the unshared electron pairs on the hydroxyl oxygen atom is delocalized into the electrophilic pi system of the carbonyl group. We can draw the following resonance forms to represent this delocalization ... 

In the previous article (1) on heteronuclear double resonance in this series it was suggested that such experiments would soon vie for importance with homonuclear ones. In fact, there has been an almost total eclipse partly as a result of the use of proton decoupling in all work, partly because of improved methods of frequency generation and control which have made experimental distinctions between the two types of experiment much less important. The present review therefore deals with both homo- and hetero-nuclear experiments and includes multiple resonance work also. The seven years up to mid-1978 are covered, although it has been impossible to mention every experiment. Emphasis is laid upon new ideas and developments of technique, with some preference for the more recent work. The Chemical Society specialist reports on NMR spectroscopy have included regular articles on multiple resonance (2,3) and a number of reviews deal with various aspects of the subject. (4-19) It has been decided to omit work on the solid state. 

Nuclear magnetic resonance experiments indicate that both staggered and eclipsed conformations of substituted arene chromium tricarbonyls exist in solution 177, 235). The preferred conformations are attributed to both steric and electronic effects. 

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