On exo-cyclic aromaticity

The C2B2F4molecule shows the tendency of attaining the aromatic ‘sextet’, through an ‘exo-cyclic’ conjugation.

The Raman fingerprint of cyclic conjugation: the case of the stabilization of cations and dications in cycloparaphenylenes

Cyclic conjugation and Biradical formation in charged cycloparaphenylenes are described by Raman spectroscopy and DFT.

Chameleon-like behaviour of cyclo[n]paraphenylenes in complexes with C70: on their impressive electronic and structural adaptability as probed by Raman spectroscopy

A series of four 1 : 1 host–guest supramolecular complexes of [n]CPPs and C70 have been analyzed by Raman spectroscopy in the solid state and complemented with the analysis of their spectroscopic responses under mechanical and thermal stresses. By following the frequency behaviour of the G and RBM modes we have found that [10]CPP in the [10]CPP@C70 complex displays a more “ordered” structure. However, in [11]CPP@C70, the nanoring becomes oval-shaped with closer contacts with the C70 poles and less conformational restriction in the flattened region. By mechanical and thermal stresses we are able to modify the lying conformation of [10]CPP@C70 towards a standing orientation. [11]CPP@C70 resists pressure changes, although it tends to shift from the standing to the lying orientation by heating. As for the crystal cell, the [n]CPPs occupy the residual empty spaces while the main crystallographic positions are reserved for C70. These are new examples of the impressive adaptability of the [n]CPP molecules to different physico-chemical environments, a chameleon-like property which reveals the delicate equilibrium provided by cyclic conjugation and ring strain.

A test of Clar aromatic sextet theory

The Clar aromatic sextet theory predicts that the intensity of cyclic conjugation in chevron-type benzenoid hydrocarbons monotonically decreases along the central chain. This regularity has been tested by means of several independent theoretical methods (by the energy effects of the respective six-membered rings, as well as by their HOMA, NICS, and SCI values, calculated at the B3LYP/6-311G(d,p) level of DFT theory). Our results show that the predictions of Clar theory are correct only for the first few members of the chevron homologous series, and are violated at the higher members. This indicates that Clar theory is not universally applicable, even in the case of fully conjugated benzenoid molecules.

Phenyl-cyclopentadienyl rule

Within a systematic study of cyclic conjugation in the benzo-annelated derivatives of acenaphthylene and fluoranthene, a general regularity was discovered, named phenyl-cyclopentadienyl rule (PCP rule). According to this rule, six-membered rings connected to the five-membered ring by a single carbon-carbon bond increase the magnitude of cyclic conjugation in the five-membered ring. The greater the number of such six-membered rings is, the stronger the cyclic conjugation in the five-membered ring. The PCP rule was initially established by studying the energy effects of individual rings, and was eventually corroborated by a variety of other independent approaches (Wiberg bond orders, carbon-carbon bond lengths calculated by high-level ab initio DFT methods, multicenter delocalization indices, ring currents).