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.     

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. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 867–869, April, 1997.     

Self-association of N-methyltrifluoromethanesulfonamide in the gas phase and in inert solvents

IR spectroscopy and quantum-chemical calculations (B3LYP/6-31G*) were used to show that N-methyltrifluoromethanesulfonamide in the gas phase and in a low-polarity inert solvent (CCl₄) exists as an equilibrium mixture of monomers and cyclic dimers. In a more polar solvent (CH₂Cl₂), highly polar open-chain dimers are stabilized.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 10, 2004, pp. 1659–1664.Original Russian Text Copyright © 2004 by Chipanina, Sherstyannikova, Turchaninov, Shainyan.This revised version was published online in April 2005 with a corrected cover date.     

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 (μ_g) and first electronically excited (¹ A ₁, μ_e) states: μ_e = r _μμ_g. The coefficient r _μ 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 r _μ value obtained allowed the calculation of the Kamlet-Taft empirical solvatochromic parameter π* on the basis of generalized characteristics of the solvent.     

Directivity of the intramolecular O→Si bond in pentacoordinate organosilicone compounds

According to the data of X-ray diffraction analysis and quantum chemical calculations, in the hypervalent silicon compounds where the coordination cycle is closed by the C=O→Si-F fragment, the O→Si interatomic distance is governed by the orientation of the silicon atom determined by the C=O→Si angle. This is an indication of a directivity of the coordination bond O→Si; a regular variation in the nature of the latter is reflected in the observed dependence of the O→Si distance on the C=O→Si angle.