ABSTRACT:We have studied magnetism and aromaticity of polycyclic ring systems by analyzing ring currents for different circulations in these molecules. The technique employed for calculating ring currents uses correction vectors which implicitly includes all the eigenstates of the Hamiltonian in the space of the chosen configurations. We have employed the Pariser᎐Parr᎐Pople Hamiltonian and have carried out full configuration Ž . interaction CI calculations for small systems and approximate CI calculations for large systems. The systems studied include polyacenes, nonaromatic ring systems including the C fragments pyracylene, fluoranthene, and corannulene, and heteroatomic systems 60 with upto two six-membered rings. We find that in polyacenes, the aromaticity of the extreme phenyl rings reduces with increasing number of phenyl rings in the system, and 2 it saturates at f the benzene value. In systems containing nonaromatic rings, we find 3 paramagnetic or diamagnetic behavior for different circulations depending upon the number of atoms in the chosen ring cycle, in agreement with the 4 n q 2 rule. In corannulene, the largest fragment of C we have studied, the five-membered ring is 60 weakly diamagnetic while the six-membered ring is more diamagnetic, although much less than in isolated benzene. The ring structures with heteroatoms studied are pyridine, pyrimidine, and its isomers, s-triazine, quinoline and its isomer, and quinazoline and its isomers. All these have similar ring currents as in their purely carbon counterparts, although ions of these molecules show interesting behavior.
We report the symmetrized density matrix renormalization group (DMRG) study of neutral and doped oligomers of poly-para-phenylene (PPP) system within an extended Hubbard model. Model parameters are determined by comparing the existing results for an interacting small system. We compute a number of properties in the ground state as well as in the one-photon, two-photon and triplet states to completely characterize these states. Bond-order studies show that the lowest two-photon state corresponds to a localized excitation while one-photon and triplet excitations are extended in nature. The bipolaronic state shows clear evidence for charge separation and disproportionation into two polarons. We find that the extended nature of one-photon and triplet states of the neutral system are very similar to those of the bipolaronic ground states.
Two-band extended Hubbard model studies show that the shift in optical gap of the metal-halogen (MX) chain upon embedding in a crystalline environment depends upon alternation in the site-diagonal electron-lattice interaction parameter (eM) and the strength of electron-electron interactions at the metal site (UM). The equilibrium geometry studies on isolated chains show that the MX chains tend to distort for alternating eM and small U~ values.
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