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.     

Geometric factor in prototropic reactions of neutral and charged radicals

Conclusions The difference between the rate constants of thermodynamically favorable proton-transfer reactions between NH- and OH-containing radicals and heterocyclic bases and those diffusion-controlled reactions is due to the reactivity anisotropy of the reagents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 31–34, January, 1989.     

Solvatochromism of Heteroaromatic Compounds: XIV. 5-Amino-1-methyl-4-nitropyrazole and Its Analogs

The solvatochromism of H complexes of 5-amino-1-methyl-4-nitropyrazole and 2-nitroaniline derivatives in aprotic protophilic media was described on a quantitative level with the aid of Kamlet-Taft empirical parameters. Specific solvation affects only one of the two long-wave bands, namely that corresponding to an electronic transition involving orbital electron density transfer from the H-bound nitrogen atom. When such transfer does not occur, which is typical of the second transition, specific solvatochromic effect is either weak or absent.     

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: XXVII. Configuration Isomerism of H Complexes of Methanol with Carbonyl Compounds

The energy minimum of H complexes of methanol with carbonyl compounds corresponds to the nonlinear structure in which the bridging hydrogen atom deviates from the axis of the C=O bond. The stretching vibration bands of the OH bonds in these H complexes, observed in the IR spectra of solutions (CCl₄), have a complex shape or are asymmetrical, which is due to the existence of two configuration isomers differing in the direction of the H bond. Difference in the orientation of the subunits of the complex may cause significant differences in the OH stretching frequencies.     

Solvatochromism of Heteroaromatic Compounds: VI. Comparison of the Empirical and Theoretical Approaches to Description of Solvatochromism in Nonspecific Solvation of Nitropyrazoles

The effect of nonspecific solvation on the long-wave absorption band in the UV spectra of isomeric nitropyrazoles was studied. The aprotic inert and aprotic protophilic solvents exert different spectroscopic effects. In the former solvents, the solvatochromism is mainly governed by orientation interactions, and in the latter solvents, along with the orientation interactions, the UV spectrum is appreciably influenced by the induction, polarization, and dispersion interactions, which is due to the presence of readily polarizable nonbonding electron pairs and/or a pi-electron system in molecules of such solvents.     

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. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1472–1479, August, 1997.     

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

(4-Nitrobenzoyloxymethyl)trifluorosilane containing a five-coordinate silicon atom ( = OSi) 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.     

Solvatochromism of heteroaromatic compounds: XXXI. Energetics of hydrogen bonding between <Emphasis Type="Italic">N</Emphasis>-methyltrifluoromethanesulfonamide and ethers

For a series of H-complexes of ethers with N-methyltrifluoromethanesulfonamide CF₃SO₂NHMe, a proportionality between the spectral shift (Δv_NH) 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 Δv_XH increases in the order MeOH < CF₃SO₂NHMe ～ PhOH. The absence of a single Badger-Bauer relationship Δv_XH ～ 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.