Study of the electronic structure of alternant radicals by the DODS method

Abstract

AbstractsThe polarization of closed shells in an alternant radical due to the field of its unpaired electron is discussed. It is shown that the Hartree-Fock solution is unstable relative to a small perturbation caused by polarization of closed shells in a long polyene radical. Its ground state is found to be antiferromagnetic with a forbidden zone in the spectra of one-particle excitations.On discute la polarisation des couches complttes dans un radical alternant due au champ de l'tlectron ctlibataire. On dtmontre que la solution des tquations de HartreeFock est instable par rapport A une perturbation causte par la polarisation des couches complttes dans le radical polytnique. L'ttat normal du radical est antiferromagnttique avec une zone dtfendue dans le spectre d'excitations d'une particule. Die vom Feld des ungepaarten Elektrons herriihrende Polarisierung der abgeschlossenenSchalen in einem alternierenden Radikal wird diskutiert. Es wird gezeigt, dass die HartreeFock-Losung beziiglich auf eine kleine, von der Polarisierung der abgeschlossenen Schalen in einem Polyenradikal, verursachte Storung nicht stabil ist. Der Grundzustand des Radikales ist antiferromagnetisch mit einer verbotenen Zone im Spektrum der Einteilchenanregungen.

Quantum-chemical studies of quasi-one-dimensional electron systems. 1. Polyenes

“…We first consider the chain without bond alternation. Substituting the function 0 () nn gE from (6) into (14),…”

“…From the analysis of the occupation numbers of natural orbitals calculated from UHF wave functions before and after projection, it was shown that the evaluation of the spin density after projection can be performed practically without loss of accuracy from non-projected values multiplied by / ( 1)  SS [57]. It was shown that this rule is asymptotically exact at  N [6,10,105].…”

“…Significant progress in understanding the properties of carbon-chain polymeric systems has been achieved due to the use of different versions of the Generalized HF approaches [106]. Thus, the relative sim-plicity of UHF equations has allowed to perform a number of analytical calculations of infinite polyene chains [16,[107][108][109][110], long polyene radicals [6,105], cumulenes, polyacenes, and graphite [17,111], long polyene chains with impurity atoms [7] and take into account the end effects in long polyenes and cumulenes [10]. These calculations have allowed, in particular, to make conclusions about the physical nature of the dielectric properties of such systems [16], which was further confirmed for polyenes by exact solutions [112].…”

“…Until recently this energy gap was supposed to arise from the instability of the equal-bond polyene configuration with respect to the bond alternation [129,130]. Nevertheless, it has recently been shown that the unrestricted Hartree-Fock (UHF) approach taking into account electron correlation can be used to describe the π-electronic spectra of large conjugated systems like polyenes, cumulenes, polyacenes, and graphite [6,16,17,107,[109][110][111]108]. Note that the papers [6,16,108] have dealt with the electronic structure of regular ideal polyene chains consisting of an even [16,108] or odd [6] number of carbon atoms.…”

“…Then will give condensed review of the Generalized Hartree-Fock method and its different versions with some demonstrative applications to atoms, molecules, and carbon polymers [5]. Further we turn to theory of electronic structure of long polyene neutral alternant radicals based on the different orbital for different spins SCF method [6]. Then we come back to local electronic states in polyene chains with an impurity atom using unrestricted Hartree-Fock approach [7].…”