V. K. Turchaninov

Strong intramolecular hydrogen bond N—H...O in 2-(2-acyl-1-phenylethenyl)-5-phenylpyrroles

The molecular structure of 2-(2-benzoyl-1-phenylethenyl)-5-phenylpyrrole and 2-(2-furoyl-1-phenylethenyl)-5-phenylpyrrole was studied by X-ray diffraction analysis at 110 K and quantum chemistry methods (B3LYP/6-31G*). In the crystalline state, both compounds have cyclic structures closed by strong intramolecular hydrogen bond N—H...O. Canonic zwitterionic structure contributes largely to the ground state of the molecules. This is probably due to synergism of the H-bonding and π-electron interactions.

Dipole moment and tautomeric form of 3(5)-nitropyrazole in dioxane solution

The dipole moments of 3(5)-nitropyrazole, its methyl-substituted derivatives, and H-complexes with dioxane were measured experimentally and estimated byab initio calculations (6-31G* basis set). Comparison of the experimental and calculated dipole moments suggests a shift of the tautomeric equilibrium toward the 3-nitroisomer.

THE EFFECT OF THE PHYSICAL STATE ON THE EQUILIBRIUM CONFORMATION OF 2-ARYLPYRROLES

The equilibrium torsion angles of 2-arylpyrroles in the liquid and solid phases were estimated by UV spectroscopy. In solution, compounds containing no substituents in positions 1, 3, and 2′ possess an average torsion angle of 24°, those containing one substituent have an angle of 29°, and in the case of two and three substituents, the angles are 53° and 65°, respectively. Phase transitions lead to flattening of the molecules in almost all cases. The average torsion angles in the compounds with no substituents in positions 1, 3, and 2′ decrease by ≈5° on passing from the gas to the liquid state and by ≈25° on going from the liquid to the solid state. The geometric parameters of 2-arylpyrroles with one or two substituents in positions 1, 3, and 2′ are less sensitive to phase transitions, while trisubstituted derivatives even retain their equilibrium conformations upon phase transitions.

The study of benzimidazoles

The nature of the first two long-wave bands in the UV spectra of 2-substituted benzimidazoles has been analyzed on the basis of photoelectron spectra (PES). Three basic types of spectroscopic situations are typical of these compounds: 1) a change in the sequence of the occupied MO of the ground state does not influence the order of the excited energy levels; 2) when the π-levels are in the same order, inversion of the excited states occurs; 3) reversal of the occupied π-MO of the ground state causes the inversion of the excited states.

Acid properties of short-lived radicals and their appearance in photoinduced hydrogen atom transfer

Conclusions1.The acid properties of short-lived radicals from the pyrrole, indole, benzazole, and benzoquinone series were studied by nanosecond laser spectroscopy in acetonitrile.2.The linear dependence of the change in the standard free energy of formation of neutral radicals as a result of transfer of an H atom from the heterocyclic molecules to the quinone molecules and a change in the standard free energy of electron transfer in these systems was demonstrated.3.The role of intermediate states with electron transfer in the process of photoinduced cleavage of the hydrogen atom was investigated. It was found that the efficiency of formation of products of atom transfer is determined by the acid-base properties of the components of the radical ion pairs (exiplexes).

AM1 studies of photoelectron spectra

Deep-lying π- and σ-orbitals of 10-alkylphenothiazines were studied by photoelectron spectroscopy and quantum-chemical AM1 calculations. It was demonstrated that in 10-ethylphenothiazine the lone electron pair of the S atom interacts with the π-system of the aromatic fragments. The π-MOs, whose energies are a function of the dihedral angle between the planes of the benzene rings of phenothiazines and are independent of the degree of pyramidality of the N atom, were found. The differences in the energy of these MOs were used for estimating equilibrium dihedral angles of tricyclic molecules in the gaseous phase. These values differ only slightly from those observed in the solid phase. The replacement of the hydrogen atom at position 10 by the methyl group leads to a decrease in the dihedral angle, leaving the orientation of the substituent unchanged. The orbital energies of phenothiazines, which were calculated by the AM1 method, adequately reflect the order of changes in the ionization potential. However, contributions of the two highest occupied π-MOs to the total charges on the N and S atoms are inconsistent with the experimental data.

Tautomeric transformations of arylidenebarbiturates in solutions

IR and UV spectroscopy and quantum chemistry were employed to investigate the structure of arylidenebarbiturates able to undergo tautomeric transformations. The composition of tautomeric mixtures depends on the properties of the solvent and on the concentrations of the solutions.

Interpretation of photoelectron spectra in the AM1 semiempirical method. 3. 2-Substituted benzimidazoles

Analysis of photoelectron spectra of 2-substituted benzimidazoles showed that their orbital structure is determined by the form and energy position of the fragment orbital (π-symmetry) of the substituent.

SOLVATION OF 4-NITROPYRAZOLES ACCORDING TO UV SPECTROSCOPY DATA

The dependence of the energies of intramolecular charge-transfer electron transitions in 1,3,5-trimethyl-4-nitropyrazole, 4-nitropyrazole, and its anion on the dielectric and acidbase properties of solvents was analyzed. The dipole moments of these species in the excited state are larger than those in the ground state. The nonspecific solvation is accomplished through dipole-dipole interactions. 4-Nitropyrazole, which possesses acidic properties, is also solvated by the mechanism of hydrogen bonding with the solvent, which acts as a base. The strength of the hydrogen bond increases on going from the ground to the electron-excited state. The 4-nitropyrazole anion is a base, and its hydrogen bonds with the solvent are stronger in the ground state. The mechanism of specific solvation of the molecule and the anion of 4-nitropyrazole during electronic excitation is discussed.

AM1 study of photoelectron spectra

The π-orbital structure of the monomeric form of the 2,2,4,6-tetrachloro-2,2-dihydro-1,5,2-diazaphosphorinine has been studied by photoelectron spectroscopy and using quantum-chemical calculations by the semiempirical AM1 method. It has been concluded that the electronic and energy characteristics of four higher π-MOs (frontier and three next orbitals) of this compound may be interpreted in terms of semipolar bonds formed by three atoms (C, P, and N). For describing two low-lying π-MOs of the σ4,λ5-phosphorine studied, it is necessary to take into account the π-σ-interaction.