B. Ya. Simkin

The concept of aromaticity in heterocyclic chemistry

Publisher Summary This chapter reviews that the concept of aromaticity is one of the most general but also controversial constructs. The aim of this chapter is to review a critical analysis of various criteria of aromaticity, and the indices quantifying aromatic or antiaromatic character with an emphasis on the application to heterocyclic compounds. Designed as both an appropriate language for the analysis of specific features of structure, stability, and physical and chemical properties inherent in the conjugated cyclic molecular systems, and for their quantitative or semiquantitative evaluation, the concept of aromaticity has received special attention in the chemistry of heterocyclic compounds. It also emphasized that “it would be inconceivable to attempt to teach or practice heterocyclic chemistry without the use of the concept of aromaticity.” It has been long recognized that despite all its versatility and usefulness, the very idea of aromaticity lacks a secure physical basis. The aromaticity of cyclic systems is, in the first place, manifested in the stabilization (destabilization) effects of cyclic electron (bond), delocalization and non-additivity of the collective properties of a molecule.

σ-Aromaticity and σ-antiaromaticity

Abstract The concepts of σ-aromaticity and σ-antiaromaticity have been examined in the light of their connection with the type of the electronic shell of skeletal bonds. Electron count rules for σ-skeletal electrons have been formulated for both small saturated and unsaturated cyclic compounds. The electronic shells of the conjugated compounds are classified into 4 types by their properties of gp- and σ-aromaticity and/or π- and gs-antiaromaticity depending on the type of filling up of the π- and σ-skeletal subsystems. Experimental and calculational data regarding rearrangements and ways of stabilization of a variety of carbocyclic compounds have been systematized on the basis of approaches developed to the concepts of π- and σ-aromaticity and antiaromaticity.

Hartree-fock instability in connection with the structure and geometry of some conjugated molecules

Abstract The electronic-structural regularities producing instability and multiplicity of restricted Hartree—Fock (RHF) solutions have been considered. Singlet and triplet instabilities of RHF solutions for some heterocyclic conjugated molecules have been found. The significance of the starting point in the SCF iterative procedure is examined. The existence of a triplet, or probably a biradical, ground state of 1,4-dihydroquinoxalinedione-5,8 and some of its heteroanalogues is suggested.

Tunnelling effects in the double proton transfer reaction of 2,5-dihydroxy-1,4-benzoquinone

Abstract Two dynamic models were used to investigate the double proton transfer reaction of 2,5-dihydroxy-1, 4-benzoquinone. The stationary points of the potential energy surface were located using the AM1 method. A model two-dimensional potential surface based on the Cartesian coordinates of the protons is constructed. According to the first model, the synchronous and asynchronous mechanisms of the reaction are treated separately. In the second model the two-dimensional vibrational problem is solved on the corresponding potential energy surface. The unimolecular rate constants are calculated in terms of the RRKM theory and the one- and two-dimensional approximations are compared. The second model predicts the dominant role of tunnelling and the synchronous and asynchronous processes can hardly be separated.

Effect of the boundary conditions in the Monte Carlo method on energy and structure characteristics of liquids. A calculation on aqueous solutions of methanol and benzene.

Abstract A new model of boundary conditions, suggested earlier by the present authors [1], for calculating infinitely dilute solutions by the Monte Carlo method has been subjected to one more test. This model, represented by a main spherical cell surrounded by a layer of the solvent molecules with fixed positions, has permitted us to calculate fairly accurately the hydration energies of benzene and methanol, which all previous MC calculations have been incapable of achieving. The structural characteristics of solutions, too, have been shown to depend on the manner in which the boundary effects are simulated.

Quantum chemical study of the structure of tetrasilacyclobutadiene, its dication, and dianion

MINDO/3 calculations have indicated the instability of the D2h planar geometrical configuration of tetrasilacyclobutadiene relative both to pyramidalization of the silicon atoms with the formation of a nonplanar C2h structure and acoplanarization of the silicon skeleton leading to a Cs nonplanar bicyclic structure. Such distortions are also characteristic for the D4h structure of the tetrasilabicyclobutadiene dianion. The dication of tetrasilabicyclobutadiene has D2d nonplanar symmetry, similar to that for the dication of cyclobutadiene.

Photochromic and thermochromic spirans

The preparation of 1,3-dithiolanium salts from 1,2-ethanedithiol and carboxylic acid chlorides, the subsequent condensation of which with aromatic o-hydroxy aldehydes leads to spiropyrans of the dithiolane series, is described. The PMR spectra were studied, and the photochromic properties are discussed.

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.

Chemistry of 2-hetarylbenimidazoles. 7. Transformations of trans-1-methyl-2-[Β-(2′-furyl)vinyl] benzimidazole

Electrophilic substitution reactions in the furylvinylbenzimidazole series were studied. In nitration, sulfonation, bromination, acylation, formylation, and hydroxymethylation reactions, the substituent enters at the α-position of the furan ring. The presence of a vinylene group reduces the influence of the benzimidazole fragment on the furan ring, and therefore the reactions in the latter proceed considerably more rapidly and under milder conditions than in the case of furylbenzimidazole. Calculated data are given for the π-electronic density on the carbon atoms of furyl-vinylbenzimidazole, obtained by the CNDO method.

Photo- and thermochromic spirans. 18. Theoretical study of the mechanisms of the photocolorization and photodecolorization of 2H-pyrans and their structural analogs. Triplet state

The critical sections of the potential-energy surface of the excited first triplet state (T1) of the electrocyclic cyclization of compounds that contain a 2H-pyran ring were studied by means of the semiempirical MINDO/3 method. A theoretical explanation is given for the experimentally observed difference in the mechanisms of the photocolorization and photodecolorization of spiropyrans. The activation barriers of the reaction in the first triplet state range from 20 to 110 kJ/mole.