Five polycyclic aromatic hydrocarbons of the C-26 series having similar bonding structure yield dianions upon reduction with lithium metal. Anisotropy changes, revealed from an advanced charge distribution analysis performed on these dianions, show a correlation to the bonding structure of the dianions. Electron counting and orbital considerations rationalize this correlation in terms of aromatic/anti-aromatic behaviour that is mixed into the character of the aromatic PAH upon reduction. Predictions made regarding relative stability based on this correlation were successfully tested against calculation and experiment. The anisotropy change is suggested as a valid index for the reduction-induced change in the aromatic character of PAHs, which is applicable for both aromatic and anti-aromatic changes.

Photoejection of electrons from 2,5,8,11-tetra-tert-butylcycloocta[1,2,3,4-def;5,6,7,8-d'e'f']bisbiphe nylene radical anion, BPD.-, and the respective dianion, BPD2-, are described. Photoejection of an electron from BPD2- yields a [BPD.-...e(-)] cage complex and separated BPD.- and e(-)/Li+ species. Recombination of the cage photoproducts proceeds at room temperature with a rate constant of k(rec)(1) = (7.0 +/- 0.2) x 10(5) s(-1), and the separated photoproducts recombine by a diffusional back-electron-transfer rate constant of k(rec)(2) = (1.5 +/- 0.2) x 10(9). M-1.s(-1). Photoejection of the electron from BPD.- yields the neutral pi-system, BPD, and the ejected electron reduces an unreacted BPD.- to form BPD2-. The photoejection of the electron from BPD.- thus yields the disproportionation products, BPD and BPD2-. The disproportionation products recombine by a diffusional process, k(rec) = (1.0 +/- 0.2) x 10(10) M-1.s(-1) (at room temperature).

Tetraanions of alkyl-substituted derivatives of cycloocta[1,2,3,4-def,5,6,7,8-d'e'f']bisbiphenylene (BPD) and their counter lithium cations self-assemble to form helically stacked assemblies, including a dimer, a trimer, and a tetramer. NMR self-diffusion measurements and unprecedented magnetic shielding effects for the sandwiched lithium cations support their aggregated nature. The D₂-tetramer assembly is fully characterized by NMR spectroscopy, providing unequivocal evidence for a helix of four tetraanionic BPD layers with an estimated relative twist angle of about 45degrees and interlayer spacing of ca. 4 Angstrom. The barrier for racemization through the in-plane inter-deck rotation is DG₂₀₀(double dagger) = 9.5 +/- 0.2 kcal mol^-1 in the dimer compared to >15 kcal mol^-1 in the tetramer.

Diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene (DIC) (one of the smallest symmetrical bowl-shaped fragments of C₆₀) and its tetra-tert-butyl derivative are reduced with lithium metal to yield dianions and tetraanions. Due to the high degree of symmetry (C_2v) of DIC and its derivative, their NMR spectra cannot be assigned using the standard two-dimensional NMR techniques. A novel carbon-edited NOESY method was used to complete the assignments of the neutral and dianion species, whereas the tetraanions are aided by DFT calculations for their assignment. Experimental charge-distribution patterns were obtained and match those of the calculations. An extension of the empirical approach for estimating the charge distribution from the ¹³C-NMR spectra enables a direct comparison between experimentally derived charge-distribution data and the computed electron density in each of the lowest unoccupied molecular orbitals. The overall picture evolving from the orbital structure of DIC is presented and reflects the surface reactivity of C₆₀.

The complete characterization of polycyclic aromatic hydrocarbons (PAHs) and tetrasubstituted ethylenes is critical to an understanding of their reactivity, for which NMR is an important tool. Chemical shifts can provide a direct measure of charge distribution and aromaticity. Unfortunately, COSY, NOESY and heteronuclear correlation cannot provide a complete assignment of the NMR spectra for some carbon-rich PAHs with symmetrical bay regions. The protons in the bay regions would yield NOE signals if they were not symmetrical. Natural substitution of C-12 with C-13 can be used to break the symmetry and yield these useful NOE signals. Using gradient-assisted editing, unequivocal assignments have been achieved for some previously problematic molecules. Copyright (C) 2000 John Wiley & Sons, Ltd.

The antiaromatic corannulene dianion, Cor^2-, undergoes photoejection of an electron to yield an intimate cage complex of Cor^.- and the electron. Part of the complex species undergoes cage recombination, k(rec)(1) = 1.1 x 10(6) s(-1), while the other part of complex separates, k(s) = 2.3 x 10(6) s(-1), and yields Cor(.-) and a Li+/e(-) ion pair. Diffusional recombination of the later products proceeds with a bimolecular rate constant corresponding to k(rec)(2) = 1.3 x 10(9) M-1 s(-1). Time-resolved laser flash-photolysis and FT-EPR experiments are used to characterize the transient species.

Evidence is given for the structure of aryl-substituted trimethylenemethane dianions from NMR data, and the importance of p-pi conjugation is discussed.