Ground state solvent arrangement

Atkins et al. [130] reported in 1977 that irradiation of mixtures of benzene and methyl acrylate or methyl methacrylate, both acceptors, yields mixtures of endo and exo adducts. A subsequent report from the same groups [120] describes the results of the irradiations of benzene in the presence of ethyl vinyl ether, //-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene. In all these cases, the major products were exo-ortho photocycloadducts. The orientations of these vinyl ethers with respect to benzene, in their loose ground-state associations, were inferred from NMR spectra. For ethyl vinyl ether, n-butyl vinyl ether, 2,3-dihydropyran, and 1,4-dioxene, the vinyl proton resonances were either unaffected by a solvent change from carbon tetrachloride to hexadeuterobenzene or appeared 4-10 Flz downfield, whereas the methyl and/or methylene signals all moved up-field by 10-25 Hz. This implies an endo arrangement of the molecules in the ground state. Thus, the ortho photocycloadducts of vinyl ethers with benzene show exo stereochemistry, even when the ground-state orientation is endo. 

Figure 11-15. Electronic excitation of a solute with zero dipole moment in its ground state, So, but a large dipole in its excited electronic state, Si. Since the electronic transition is faster than nuclear motions the initial arrangement of solvent molecules in the exited state is the same as in the ground state. Then the solvent molecules reorganize in order to lower the solvation energy in the excited state. The figure is taken from Stratt and Maroncelli [110] with permission...

Franck-Condon ground state after emission, which persists momentarily until the solvent molecules reorganize to the equilibrium arrangement for the ground state. 

Fig. 12.45 The solvent can shift the fluorescence spectrum relative to the absorption spectrum. On the left we see that the absorption occru with the solvent (the ellipses) in the arrangement characteristic of the ground electronic state of the molecule (the sphere). However, before fluorescence occurs, the solvent molecules relax into a new arrangement, and that arrangement is preserved during the subsequent radiative transition.

---