Solution behavior from dioxane

Compounds in which the carbons are sp3 hybridized display the same confused situation. Erickson 78> reports that the trans vicinal H—H coupling constant of c//-dibromosuccinic anhydride (which is reasonably rigid) varies from 2.5 Hz in CHC13 to 6.0 Hz in acetone and dioxane. The same paper reports that the meso dibromosuccinic anhydride and the two corresponding dichloro compounds do not display any solvent dependence of their coupling constants. (Erickson also reports that lJc.H of the dl- dibromide decreases from 172 Hz in chloroform to 166 Hz in acetone and 165 Hz in dioxane exactly the opposite behavior from that observed for any other 1JC H coupling ever studied). It is at least possible that these data result from chemical degradation of the solute rather than from true solvent effects as discussed here. 

Similar solution behavior was reported(9-11) for sulfonate ionomers. Rochas eit al. (9) observed a polyelectrolyte effect for acrylonitrile-methallylsulfonate copolymers in DMF. Lundberg and Phillips(10) studied the effect of solvents, with dielectric constants ranging from c 2.2 to e 46.7, on the dilute solution viscosity of the sulfonic acid and Na-salt derivatives of sul-fonated polystyrene (SPS). For highly polar solvents such as DMF and dlmethylsulfoxide (DMSO, e 46.7) they observed a polyelectrolyte effect, but for relatively non-polar solvents such as THF and dioxane (c = 2.2) no polyelectrolyte effect was observed. Like Schade and Gartner, these authors concluded that polar solvents favor ionization of the metal sulfonate group while non-polar solvents favor ion-pair interactions. 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

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