Axial bonds, definition

The XY,-type molecules are veiy rare. Both IF, and ReF, arc known to be pentagonal-bipy rami dal (D ,), and their vibrational spectra have been assigned completely, as shown in Table II-9. The A, ", and , vibrations are Raman active, and the A 2 and , vibrations are infrared active. According to Eysel and Seppelt, IF, undergoes minor dynamic distortions from 0 symmetry which cause violation of the Djj, selection rules for combination bands but not for the fundamentals. Normal coordinate analysis shows that the axial bonds are definitely stronger and shorter t ian the equatorial ones. Table 11-9 also lists the frequencies and assignments of the [U02Fs] ion of Dj, symmetry. 

Another, very notable, case where the two definitions are in conflict is that of het-eroannular cisoid dienes. As we have mentioned, this was just the class of molecules that stimulated the introduction of the AAR. Here, in order to have the correct results one should refer the chirality of the axial substituent to the individual double bonds (olefin-picture), as depicted in Figure 6 and in the upper parts of Figure 7(b) and (c). The case of heteroannular dienes is anyway peculiar, because in these compounds the chromophore is unusually distorted. This case is treated in the following section. 

From the results shown in Table 1, the following conclusions can be drawn out Ci1 All the spectra correspond to non axially symmetric 31P tensors, (ii) the [dn - 033] difference ranges from 206 to 263 ppm and is definitely larger than the chemical shift difference observed in the liquid phase between a phosphonite and the corresponding thioxo-phosphonate, (iii) a linear relationship appears when the asymmetry parameter n is plotted against the intracyclic O-P-O bond angle a. 

For the equatorial C—P—C bond angles, however, larger deviations from their ideal 120° values are found. Whether these alone, without concomitant comparable alterations of the axial C—P—C angles, ran justify the definition of a gradual distortion towards a specific tetragonal-pyramidal structure might be questioned. On the basis of a proper evaluation of more latent structural features, as done in... 

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