Isomerism twist angle

The photosensitized isomerization of ethyler.es and stilbenes has been studied in detail by Hammond and his group and it has been found that in each case both the cis-trans and the trans-cis isomerizations occur with equal efficiency. A common (tc, jt ) triplet state is assumed to be the intermediate in each case. This triplet state is referred to as the perpendicular triplet state and was termed a phantom triplet state by Hammond. The energy of the perpendicular triplet state is expected to be low since in this configuration the overlap between the re and rc orbitals is minimized. The potential energy of S0, Tx and Sx states as a function of twist angle is given in Figure 7.7... 

Figure 7.7 (a) The potential energy of S0, Ti and Si states of ethylene and (b) its electronic configuration, as a function of twist angle during trans-cis isomerization. 

In 2,2-dinitroenamines the only possible isomerism is that around the N—C(l) bond. Only a few compounds of this type have been described204,205 and their 13C- and -NMR spectral data are included in Table 34. 1,1-Diamino-2,2-dinitroethylenes (592-599) have been prepared very recently and their structures have been studied by X-ray crystallography205. These compounds have twisted double bonds, some with twist angles (see Table 34) greater than any previously reported value for twisted ethylenes, with concomitant C(1)=C(2) bond distance distortions. The only exception is compound 596, which is essentially planar. The large chemical-shift difference between the signals of C(l) and C(2) indicates a strong polarization of the formal carbon-carbon double bond. 

Figure 7.10. Computed potential energy surfaces of the ground state and the (n,A ) excited states T, and S, for the cis-trans isomerization of diimide as a function of the twist angle 0 and the valence angle a at one of the nitrogen atoms.

Figure 11.14 Isomerization in 1,1 binaphtyl. The conformational change is the twist of the angles, right panel. The potential for the twist motion in the first excited state, as fitted to the measured isomerization rate, is shown vs. the angle. The twist angle at the barrier is taken as the zero [adapted from D. P. Millar and K. B. Eisenthal,...

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