Structure of contact ion pairs in the ground and first excited states. Aromatic closed-shell anions containing a five-membered ring

Abstract

The absorption and fluorescence spectra of the carbanion of indene have been recorded as a function of temperature, solvent and counter ion. In ethereal solvents indenyl-lithium forms contact and solvent-separated ion pairs, depending on the temperature, whereas only contact ion pairs occur when Na+ and K+ are the counter ions. The absorption spectra exhibit a blue shift on formation of contact ion pairs, but in the fluorescence spectra a red shift is observed.The results for the indenyl anion are compared with those for the carbanion of fluorene and the anion of carbazole. The differences in cation-anion attraction and fluorescence shift are discussed with the aid of energy level diagrams, and it is concluded that these differences are ultimately determined by the r-electron charge distribution of the anion in the ground and first excited states. Theoretically calculated charge densities conflrm this conclusion ; an exception is the carbazolyl anion, where the nitrogen lone pair electrons play an important role, leading to the formation of a o-complex.The structures of the contact ion pairs of the two carbanions indenyl and Auorenyl in the ground and first excited states are inferred from the calculations. An apparent discrepancy with proton n.m.r. results is resolved by a theoretical examination of the effects of the cationic field on the proton chemical shifts.

Optical and NMR Investigations on Alkali Ion Pairs of Carbanions and Nitranions

“…The absorption and fluorescence ion pair shifts of Cb and Im are approximately equal in magnitude (9). The nitranion Idl also forms in the first excited state only contact ion pairs.…”

“…However, the fluorescence ion pair shifts, just as the absorption ion pair shifts, increase in the order K , Na , Li . Moreover, for Flu the fluorescence ion pair shifts are smaller than the absorption ion pair shifts, whereas there is only a small difference for InH (9).…”

“…Their values are determined by the depths of the minima in the corresponding potential energy curves which depend, on its turn, on the extent of charge localization. VESCF calculations supply the following data (9) . For InH the minima for the ground and first excited singlet states amount,respectively,to -4.92 and -4.90 eV; for F1H the values are respectively -4.68 and -4.56 eV.…”

“…This seems to be in disagreement with the theoretical result, that in the ground state the negative charge is localized in.the: five -membered ring and thus the cation is located over this ring. The discrepancy may be resolved by taking into account a so-called "indirect" effect of the cation, which causes a redistribution of the excess negative charge in the anion and modifies the C-H bond polarization (9). If the alkali ion resides above the five-membered ring the negative charge in this ring will be enhanced and an upfield shift for the protons 1, 2 and 3 is expected.…”

“…Hogen-Esch et al (11 & 12) The fluorescence spectra of mB (see Fig. 6), on the contrary, are temperature dependent (9). mB can form contact and solvent separated ion pairs in the first0excited singlet state.…”