Delocalized Electronic Systems

Whereas the pyrrolo[l,2-a]azepinium cation, a 107t-aromatic system, is unknown, 5-(cyano-methyl)pyrrolo[1,2-a]azepinium perchlorate (7) is available by protonation of the cyanomethy-lene derivative 6 in perchloric acid.7 H NMR studies reveal that the cation has a completely delocalized -electron system. 

Further oxidation of the nine-atom clusters to formal [Ge9] leads to linear polymers oi[-Ge9-] with two covalent intercluster bonds (Fig. 2i). Trimers [Ge9=Ge9=Ge9] (Fig. 6a) and tetramers [Ge9=Ge9=Ge9=Ge9] (Fig. 6b) occur via nonclassical bond formation between two neighboring atoms of the triangular prism basis planes of the c/oio-shaped clusters, which results in Ge-Ge-Ge bond angles of 90° and in considerably longer Ge-Ge contacts between the cluster units. Quantum-chemical calculations have shown that the exo-bonds participate in a delocalized electronic system that comprises the whole anion [204]. 

The discovery of electrical conductivity and superconductivity in crystalline materials derived from conventional molecular species effectively introduces a new area of synthetic chemistry. The conductivity is associated with specific molecular arrays and synthesis of the materials requires the ability to conceive and implement the preparation of a particular crystalline state. Molecular conductors are derived from pairs of redox reagents, an area where heterocyclic systems are well established. Placement of heteroatoms of selected electronegativity at chosen positions in a delocalized electron system offers a subtle and effective means for altering the orbital energies which ultimately control the electron transfer properties of redox reagents. 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

In this paper, I propose a promising new electron acceptor of cyclobutenedione for nonlinear optical materials to prevent centrosymmetric crystal structures by the introduction of chirality and hydrogen bonding property into the acceptor itself. Compared with electron donative groups, electron acceptor is not yet well studied for nonlinear optical materials. The most commonly used electron acceptor is nitro (NO2) group. Therefore, we evaluated the possibility of cyclobutenedione as a new electron acceptor for nonlinear optical materials. One of the most simple cyclobutenediones is squaric acid. Squaric acid is known to be soluble in water and show very strong acidityQ2), as squarylium anion formed in water has a stable 2n delocalized electron system as shown below. 

The oxidation potentials can be finely tuned by the attachment of substituents. Delocalized -electron systems of the dithiafulvenes show low oxidation potentials however, non-conjugated dithiafulvenes are typically oxidized at between Fox+0.9 and +1.4 V. Electron-sufficient and deficient substituents on the dithiafulvenes also affect the potentials. 

The interpretations of the bond-types and the interplay between them as well as the concept of delocalized electron systems are described below. 

The fourth paper in this volume is devoted to some extensions and generalizations of the algebra of the be- and r-matrices. The latter is only valid for the chemistry of molecular systems that are representable by integer bond orders and thus is not applicable to the great variety of molecules with multi-center bonds and delocalized electron systems. This deficiency is overcome by the introduction of the so-called extended be- and r-matrices the xbe- and xr-matrices. They contain additional rows/columns which refer to the delocalized electron systems. Some corresponding data structures are presented that also account for stereochemical aspects. 

N.L. Allinger and J.T. Sprague, Calculation of the structures of hydrocarbons containing delocalized electronic systems by the molecular mechanics method, J. Am. Chem. Soc., 95 (1973) 3893-3907. 

Figure 3 is an attempt to synthesize and highlight the key molecular features of the edges and the basal planes of graphene layers. Only some of the most prevalent surface functional groups are depicted [37]. Special attention should be paid to the delocalized % electron system that acts as a Lewis base in aqueous. solution [71-73] ... 

Allinger, N.L. and Sprague, J.T. (1973) Calculations of the Structures of Hydrocarbons Containing Delocalized Electronic Systems by the Molecular Mechanics Method, J. Am. Chem. Soc. 95, 3893-3907. 

The usage of the molecular orbital version of the Lewis definitions allows to discuss donor and acceptor interactions, involving delocalized electron systems and locahzed but multicentered bonds. Moreover, one can take into account all degrees of electron donation ranging from nearly zero in the case of weak intermolecular attractions and idealized ion association, to complete transfer of one or more electrons (redox). 

The phosphorus chemical shift of Mes-P=CPh2 is < p=233.1 ppm upfield from the resonance observed for molecules shown in Table 5.6, where the extended delocalized -electron system of the fluorenyl moiety can distribute partial charges significantly better than the isolated phenyl rings in Mes-P=CPhj. 

Hiberty, P.C. The Distortive Tendencies of Delocalized % Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

With their extensively delocalized-electron systems along their backbones, PBX polymers have emerged as ideal candidates for NLO applications. We are particularly interested in their third-ordered susceptibility characteristics. Indeed, it has been shown " that PBZT film processed from polyphosphoric acid (PPA) solution had a relatively high value of about 3 x 10"" e.s.u. Further improvement in the optical quality of the PBZT film by coagulation of a methanesulfonic acid (MSA) solution in water had increased the x (3) value by an order of magnitude (3.3-4.5 x 10 e.s.u.). Since the three-fused-ring... 

---