Comparative evaluation of quantum-chemical methods for calculating aromatic and antiaromatic nitrogen heterocycles (review)

The possibilities of quantum-chemical methods in the determination of characteristics of aromatic and antiaromatic nitrogen heterocycles such as the thermodynamic stabilities, geometries of the molecules, electron distributions, dipole moments, ionization potentials, electron and proton affinities, energies and geometries of the molecules in the excited states, electronic spectra, and reactivities are discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1587–1603, December, 1989.     

Electronic structure of perimidine

The electronic configurations and dipole moments of the ground and excited states of perimidine, imidazole, and benzimidazole were calculated by means of the self-consistent-field molecular orbital (SC F MO) method using the Pariser-Parr-Pople (PPP) approximation. The reactivity of perimidine is discussed. The results of the calculations by the PPP method are compared with earlier calculations based on the simple MO method. An interpretation of the electronic spectrum of perimidine is given.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 678–681, May, 1971.     

Molecular structure of the boron(III) oxide molecule in the SCF approximation

The potential surface for the boron(III) oxide (B₂O₃) ground state has been calculated in restricted HF approximation with a minimal STO—3G basis set. The equilibrium geometry has C_2v symmetry; the corresponding structural parameters are as follows: r(O_i—B₁)= 1.241 Å; r(B_i—O)= 1.341 Å ; ∠B₁QB₂ = 142° and ǒO₁B₁O = 177°. Calculations were also carried out using the SCF-Xα approach for two configurations of B₂O₃ with C_2v and D_∞h symmetry.     

On the threshold problem for Latin boxes

Let m ≤ n ≤ k. An m × n × k 0‐1 array is a Latin box if it contains exactly m n ones, and has at most one 1 in each line. As a special case, Latin boxes in which m = n = k are equivalent to Latin squares. Let be the distribution on m × n × k 0‐1 arrays where each entry is 1 with probability p, independently of the other entries. The threshold question for Latin squares asks when contains a Latin square with high probability. More generally, when does support a Latin box with high probability? Let ε > 0. We give an asymptotically tight answer to this question in the special cases where n = k and , and where n = m and . In both cases, the threshold probability is . This implies threshold results for Latin rectangles and proper edge‐colorings of K_n,n.     

Photochromic and thermochromic spirans

By means of the SCF MO LCAO method in the π-electronic (PPP) and all-valence (CNDO) approximations the electronic structures and spectral characteristics of the valence isomers of 2H-chromenes, thiochromenes, and 1,2-dihydroquinolines have been calculated. On the basis of a consideration of the electronic configuration of various excited states, an explanation has been given of the mechanism of the photochromism of the chromenes and their analogs. A hypothesis has been put forward concerning the possibility of thermochromism in 1,2-dihydronaphthalene. It has been shown that the diffuseness of the long-wave absorption band of the open forms of the compounds investigated is not connected with the existence of several stereoisomers. The influence of the heteroatom and of benzo-fusion on the long-wave absorption has been analyzed. The nature of the electronic transition responsible for the long-wave absorption of the photo-colored forms has been explained.