A. I. Vokin

Solvatochromism of Heteroaromatic Compounds: XX. 4(5)-Nitroimidazole

Abstract4(5)-Nitroimidazole in solution is stabilized as the 5-nitro isomer due to formation of hydrogen bond with an aprotic protophilic solvent. Amphiprotic medium favors displacement of the tautomeric equilibrium toward the 4-nitro isomer via formation of a solvate complex where 4-nitroimidazole acts as hydrogen bond acceptor. The observed specific solvatochromic effect in the UV spectrum of 4-nitroimidazole and related heterocyclic nitro compounds is determined by the electronic configuration of the excited π,π*-state.

Solvatochromism of heteroaromatic compounds: XXXI. Energetics of hydrogen bonding between N-methyltrifluoromethanesulfonamide and ethers

For a series of H-complexes of ethers with N-methyltrifluoromethanesulfonamide CF3SO2NHMe, a proportionality between the spectral shift (ΔvNH) and the enthalpy of complex formation (ΔH) was found. Comparison with hydroxyl-containing H-bond donors revealed the following contradiction: ΔH increases in the order CF3SO2NHMe < MeOH < PhOH, whereas ΔvXH increases in the order MeOH < CF3SO2NHMe ∼ PhOH. The absence of a single Badger-Bauer relationship ΔvXH ∼ f(ΔH) is due to the contribution of universal interactions into the enthalpy of H-complex formation. The account for these interactions allowed calculation of the energy of an intermolecular hydrogen bond, which varies in parallel with the spectral shift: MeOH < CF3SO2NHMe ∼ PhOH.

Solvatochromism of Heteroaromatic Compounds: XXIV. Solvation Complexes of ( = O→Si)(4-Nitrobenzoyloxymethyl)trifluorosilane in Protophilic Solvents

Abstract(4-Nitrobenzoyloxymethyl)trifluorosilane containing a five-coordinate silicon atom ( = O→Si) is specifically solvated in a protophilic medium to form a complex in which the solvent molecule donates an electron pair and the coordination number of the silicon atom increases to six. Formation of such solvation complexes gives rise to a pronounced positive specific solvatochromic effect in the UV spectra. According to dielectrometric, 19F NMR, and IR data, and also to ab initio calculations (HF/6-31G*), the solvent molecule occupies an equatorial position.

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.

Solvatochromism of heteroaromatic compounds: XXIX. Configurational isomerism of H complexes of phenols with amides and esters

Phenol and amides and esters in carbon tetrachloride form an H complex with an OH n bond, whose configurational isomers differ from each other by the direction of the hydrogen bond and, as a result, by the value of the spectral shift. Structural details of such configurational isomers and solvatochromism of their IR absorption bands are discussed.

Solvatochromism of heteroaromatic compounds: XXVIII. Factors affecting the nonspecific solvatochromic effect in the UV spectra of aromatic nitro compounds in aprotic protophilic solvents

Examination of the UV spectra of a large series of solvatochromic indicators of the general formula 1-X-4-NO2-C6H4 in aprotic solvents confirmed the proportionality between the dipole moments of these compounds in the ground (μg) and first electronically excited (1A1, μe) states: μe = rμμg. The coefficient rμ was determined by applying the equation of the Bakhshiev-Bilot-Kawski solvatochromism theory both to nonspecifically solvated molecules and to their H complexes with aprotic protophilic solvents. An anisotropy of the electron redistribution was revealed for low-symmetry 1-substituted 2,4-dinitrobenzenes. The rμ value obtained allowed the calculation of the Kamlet-Taft empirical solvatochromic parameter π* on the basis of generalized characteristics of the solvent.

Solvatochromism of Heteroaromatic Compounds: XXII. Effect of Bifurcate Hydrogen Bond on the IR Spectrum and Dipole Moment of N-(4-Methyl-2-nitrophenyl)acetamide in Solution

N-(4-Methyl-2-nitrophenyl)acetamide molecule forms a complex with a protophilic solvent, stabilized by a bifurcate H bond and existing in a labile equilibrium with the nonspecifically solvated molecule. The position of the equilibrium is affected by temperature, phase state, and protophilic properties of the medium. Three-centered solvation H complexes of N-(4-methyl-2-nitrophenyl)acetamide are characterized by nonlinear dependence of the NH stretching frequency on the energy of the complex formation and by the positive specific “dioxane” effect.

Solvatochromism of Heteroaromatic Compounds: XIX. Factors Affecting Quantitative Characteristics of Solvatochromism in Aprotic Inert Media

The effects of aprotic inert media on the electronic absorption spectra of aromatic nitro compounds p-NO2C6H4R were used as evidence for the linear correlation between the slope sa of the solvatochromism equations ηmax = η0 + saπ* and the dipole moments of the molecules in their ground electronic state μg. A linear correlation was established between η0 and the first ionization potential of subunits C6H5R. A new approach to estimating the dipole moment of electronically excited molecules (μe) for molecules like p-NO2C6H4R on the basis of the correlation μe = rμg was proposed.

Molecular Structure of Complexes with Bifurcated Hydrogen Bond: I. N-(4-Methyl-2-nitrophenyl)acetamide

According to the results of HF/6-31G* and B3LYP/6-31G* nonempirical calculations, N-(4-methyl- 2-nitrophenyl)acetamide in the synperiplanar and antiperiplanar conformations gives stable complexes with protophilic solvents, which are stabilized by bifurcated (three-center) hydrogen bond. The complexes are characterized by (1) opposite variations of the interatomic distances and angles corresponding to intra- and intermolecular components of the bifurcated hydrogen bond, (2) extension of the intramolecular component and a more pronounced shortening of the intermolecular component with increase in the strength of the three-center H-complex, and (3) nonlinear and nonmonotonic relation between the NH stretching vibration frequency and the energy of complex formation.