Aprotic solvents Aromatic solvent shifts

Point 2 above is understandable (Eq. I) since in apolar solvents the energy for charge separation is large and makes electron transfer slow. In the case of aromatic nitriles as acceptors, with which we are particularly concerned, there is obviously also a strong n-interaction that can occur between donor and acceptor, and irradiation in aprotic solvents usually causes no reaction but only quenching of the nitrile monomolecular fluorescence and appearance of a new red-shifted emission attributable to the exciplex (see Section 2.2). Independent solvation of the radical ions is also important. For example, the bichro-mophoric molecules 5 and 6 show strong exciplex emission and... 

The evidence for fluorine hyperconjugation derived from nmr measurements available in 1970 was not deflnitive (3,4). Taft and his associates had originally argued that the fluorine substituent chemical shifts (SCS) of p-fluorine atoms in molecules with substituents which are capable of resonance interactions, for example the nitro group, exhibited a special solvent dependence and shifted to lower field as the polar character of the solvent was increased (38,39). The fluorine substituent chemical shift for the aromatic fluorine atom in 4-fluo-robenzotrifluoride, indeed, exhibits a small downfleld shift from 4.95 in 3-methylpentane to 5.75 in nitromethane to 6.05 in 75% aqueous methanol. For comparison, the substituent chemical shift for the fluorine atom in 4-fluoroni-trobenzene exhibits a shift from 9.00 in 3-methylpentane to 10.55 in nitromethane to 11.20 in 75% aqueous methanol (39). Holtz concluded on these grounds that fluorine hyperconjugation, structure XVC, might contribute to the determination of the substituent chemical shift. However, Brownlee, Dayal, and Taft subsequently showed that the effects of dipolar aprotic solvents on the... 

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