Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 91

confirmed by the good agreement between the observed chemical shifts at low temperature and those
obtained by quantum chemical calculations. These experiments were the first solution NMR observations
of a face-to-face radical

dimer, and prove that NMR spectroscopy is an effective and fascinating method
to observe a dynamic behavior of the open shell molecular systems. 15
The formation of the

π

-dimeric pair at low temperature raises the issue of the aromaticity of the radical
aggregates from the viewpoint of fundamental chemistry. 15

π

NICS (nucleus-independent chemical shifts)

dimer of TBPLY have been examined by quantum chemical calculations. 16
As shown in Figure 3.10a, the ring center of the

values of the monomer and the π

π dimer becomes more aromatic ( − 7.1 ppm) than that of

the monomer ( dimer.

These results indicate a great enhancement of the aromaticity due to the dimerization. Since the contribution
of spin paramagnetism is already subtracted in the radical NICS values, this change reflects a susceptible
probe for the

− 3.8 ppm). The trend of the enhancement is more pronounced in the interior of the π

dimerization of TBPLY . This “aromaticity generation” is for the first time demonstrated in
the TBPLY system, and is inherent in spin delocalized organic open shell molecules, unlike closed shell
molecules and open shell molecules with spin localized nature. In the electrostatic potential surface calculated
for the

π

dimer of TBPLY , a negative electron density is largely populated at the internal region, contrasting
with the external region, despite the whole neutrality of the dimer (Figure 3.10b). 15 The imbalanced electron
distribution is totally consistent with the anomalous NICS distribution, suggesting a share of unpaired
electrons and an existence of a 12-center-2-electron long C-C bond 15 or a 12-centered covalent bond. 14a
TBPLY possesses reversible two-stage amphoteric redox ability. 13a Generation and elucidation of the
electronic structures of the corresponding cation and anion species of TBPLY have been carried out.
The BF 4 −

π

salt of the cation can be prepared from the phenalene derivative (precursor of TBPLY ) and is a

(b)

Figure3.10 (a)NICS valuesand (b)electrostaticpotentialsurfaces of the monomer and the π dimer of TBPLY

calculated by quantum chemical calculations, which indicate ''aromaticity generation'' by the dimerization.

(Reprinted with permission from [15]. Copyright 2006 American Chemical Society.) A full-colour version of
part(b)ofthisfigureappearsintheColourPlatesection of this topic.

π