Quantum Chemical Investigation of Monostanna[n]cyclacenes

Semiempirical molecular orbital treatment at the level of PM3 type calculations has been performed on the Hückel-type monostannacyclacenes having tin atom at the fusion and periposition of arenoid rings. The effect of tin substitution is found to be moderately destabilizing, but it becomes less pronounced in larger systems. The heats of formation values of fusion- and peri-type monostannacyclacenes, in some cases, are more endothermic and, in some cases, less endothermic as compared to the cyclacenes having the same n (the number of arenoid rings) value. The frontier molecular orbital energies, cryptoannulenic effects, geometries, and dipole moments of these structures have also been examined.     

Synthesis,Properties and Formation of (RCp)Co‐ and (RCp)Rh‐Stabilized[4.8]3Cyclacene Derivatives

Metal‐stabilized belts : A torus, 3 , consisting of three four‐ and three eight‐membered conjugated rings and stabilized by (RCp)Co‐ and (RCp)Rh‐ units, was generated by irradiation of [(RCp)Co(CO)₂] and [(RCp)Rh(C₂H₄)₂], respectively, and 1 .

     

Comprehensive study of the correlations that exist among the members of the [n]cyclacene series and the Möbius[n]cyclacene series

Cyclacenes are the smallest substructures of carbon nanotubes used in modelling studies. The systematics that exists between Hückel molecular orbital eigenvalues and eigenvectors of cyclacenes are delineated. This study of cyclacenes combines the interconnection of concepts of complementarity theorem, characteristic and matching polynomial recursion equations, embedding, greater than twofold symmetry and doubly degenerate eigenvalues, open-shell singlet character, and pairing theorem. Proof that cyclacenes have more open-shell (diradical) character than do polyacenes is also provided by the sum total of this work. Mirror-plane scission of even-ring cyclacenes gives linear polyacene fragments. This shows that the properties of the successor linear polyacene must be contained in the precursor cyclacene. Corresponding Möbius[n]cyclacene isomers display contrasting and unusual comparative properties. A partial list of contrasting properties include alternant (cyclacenes) versus nonalternant (Möbius[n]cyclacene) polyenes, presence of Hamiltonian circuits in Möbius[n]cyclacene and presence of oscillatory electronic properties in cyclacenes.     

From Metacyclophanes to Cyclacenes: Synthesis and Properties of [6.8]3Cyclacene

Conjugated belts : [6.8]₃cyclacene as the first hydrocarbon cyclacene was synthesized in a de novo strategy. Various [2₃]metacyclophanes are described as intermediates. The synthetic approach was extended to larger cyclacenes, and [2₄]metacyclophanes as precursors of [6.8]₄cyclacene were synthesized.

     

Energetics of Formation of Cyclacenes from 2,3-Didehydroacenes and Implications for Astrochemistry

The carriers of the diffuse interstellar bands (DIBs) are still largely unknown although polycyclic aromatic hydrocarbons, carbon chains, and fullerenes are likely candidates. A recent analysis of the properties of n-acenes of general formula C_4n+2H_2n+4 suggested that these could be potential carriers of some DIBs. Dehydrogenation reactions of n-acenes after absorption of an interstellar UV photon may result in dehydroacenes. Here the reaction energies and barriers for formation of n-cyclacenes from 2,3-didehydroacenes (n-DDA) by intramolecular Diels–Alder reaction to dihydro-etheno-cyclacenes (n-DEC) followed by ejection of ethyne by retro-Diels–Alder reactions are analyzed using thermally assisted occupation density functional theory (TAO-DFT) for n=10–20. It is found that the barriers for each of the steps depend on the ring strain of the underlying n-cyclacene, and that the ring strain of n-DEC is about 75 % of that of the corresponding n-cyclacene. In each case, ethyne extrusion is the step with the highest energy barrier, but these barriers are smaller than CH bond dissociation energies, suggesting that formation of cyclacenes is an energetically conceivable fate of n-acenes after multiple absorption of UV photons.     

A theoretical study on cyclacenes: Analytical tight‐binding approach

We present a theoretical study of cyclacene molecules performed at tight‐binding level. The orbital energies and eigenvectors have been analytically computed, and exact expressions for the axial component of the total position spread and polarizability tensors have been obtained. In absence of dimerization, the system has a D_nh symmetry, where n is the number of hexagonal units. The energy bands present no gap at the Fermi level, and to this fact it corresponds a diverging (per‐electron) polarizability for in the direction of the system symmetry axis. The two (degenerate) components of the polarizability on the σ_h symmetry plane, conversely, remain finite for . The total position spread tensor presents a qualitatively different behavior, since all the three components of the position spread per electron remain finite for . The results are analyzed and discussed for both axial and planar components separately as these are affected differently with respect to the increasing system size. Both dipole polarizability and total position spread have been computed using an ab initio approach for the smallest systems, to compare the analytical tight‐binding expressions with a higher‐level theory.