Solvatochromism of Heteroaromatic Compounds: IX. Anions of 3-Nitro-1,2,4-triazol-5-ones

Solvatochromism of the long-wave band in the electronic absorption spectra of 3-nitro-1,2,4-triazol-5-one anions was studied and quantitatively described with the Kamlet-Taft solvent parameters. Azole anions are specifically solvated in amphiprotic solvents. The strength of the hydrogen bond in H complexes with amphiprotic solvents decreases in going to the Franck-Condon excited state. The UV and IR specrta show that 3-nitro-1,2,4-triazol-5-one anions, in contrast to nitroazole anions, are specifically solvated with participation of the carbonyl group. In highly polar organic solvents the 3-nitro-1,2,4-triazol-5-one anion mainly exists in the form of the 1H tautomer.     

Tautomerism of derivatives of azines. 18. Effect of solvents on intrachelate tautomerism of the [1,5]-sigmatropic type in the acylmethylazine series

The constants of the intrachelate tautomeric equilibria of trifluoro- and trichloroacetonylpyridines in aprotic and hydroxy-containing solvents were determined by ¹H, ¹⁴N, and ¹⁷O NMR spectroscopy and UV spectrophotometry. It is shown that an increase in the polarity of the solvent and transition to hydroxy-containing solvents are accompanied by a shift of the intrachelate equilibrium to favor the ylidene tautomer.See [1] for Communication 17.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 514–520, April, 1988.     

A new aspect of azo-hydrazone tautomerism

Structures of 3,5-dimethoxy-4-hydroxyazobenzene and of a new parent compound, 2,6-dimethoxy-4-hydroxyazobenzene, are investigated. Owing to their very high solubility it was possible to proceed to a comparison of the UV, ¹H NMR and ¹³C NMR data for the same solvent. Symmetric substitution by two OMe groups in the ring bearing the OH group results in the isolation of the azo or hydrazone tautomer, using the available positions of substitution. The usefulness of the ¹³C NMR technique in the investigation of azo-hydrazone tautomerism is underlined.     

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

4(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 ,^*-state.     

Solvatochromism of anthraquinone and symmetrical dihydroxy derivatives. Local interactions

The solvent effects on the electronic absorption spectra of 9,10-anthraquinone (AQ) and its symmetric dihydroxy derivatives namely 1,5-dihydroxyanthraquinone (1,5-DHAQ) and 2,6-dihydroxyanthraquinone (2,6-DHAQ) have been studied in pure solvents and some binary solvent mixtures. The frequencies of the absorption for AQ and 2,6-DHAQ are quite solvent sensitive while those for 1,5-DHAQ are not. Due to the intramolecular hydrogen bond between the CO and OH groups, no influence of solvent hydrogen bond acceptors is observed in 1,5-DHAQ. This hydrogen bond gives a stable six member cycle which is not broken even by the strongest hydrogen bond acceptor solvents used in this work, such as DMSO and DMF. The Taft and Kamlet's solvatochromic comparison method was applied for AQ and 2,6-DHAQ. Aromatic solvents and aliphatic amines were not included in the correlations since they strongly deviate suggesting another type of interactions. All the π→π^* bands of AQ and 2,6-DHAQ show strong influence of π^* despite the fact that their dipole moment is zero. Although it would be reasonable to expect that in the absence of a solute dipole moment there is not significant orientation of solvent molecules around the solute molecules, in this case dipolar interactions between solute and solvent due to local effects might be expected. AQ may be considered as formed by two carbonyl groups weakly interacting with the benzene rings; that means that the carbonyl group can behave as an isolated dipole and independently of the other. To detect possible specific interactions between the AQ and aliphatic amines and aromatic hydrocarbons, preferential solvation in mixed solvent was investigated. It is concluded that EDA interactions are important in the solvation of AQ with these compounds as solvents.     

Synthesis,Structure and Solvatochromism Studies on Copper(II) Complexes Containing Ethylenediamine,Pyridine and Imidazol Ligands

Two copper(II) complexes type [Cu(en)X₂](ClO₄)₂, where en = ethylenediamine and X = pyridine, 1 or imidazol, 2 have been synthesized and prepared on the bases of elemental analysis, spectroscopic and molar conductance measurements. The X‐ray crystal analysis of these complexes demonstrated that the copper(II) ions are in square planar environments through coordination by two nitrogen atoms of the ethylenediamine and two nitrogen atoms of two pyridine or imidazol molecules and the ClO₄^‐ ions are bound weakly above and below of the molecular plane. The complexes show three ions behavior in all solvents. The complexes are soluble in various solvents and are solvatochromic. The solvatochromism of the complexes were investigated by UV‐Vis spectroscopy with different solvent parameters such as DN, AN, α and β using multiple linear regression (MLR) method. The results suggested that the DN parameter of the solvent has the most contribution to the shift of the d‐d absorption band of the complex 1 but in complex 2 the DN and β have almost similar importance in the observed variation in the shift of the ν_max values with solvent nature.     

Effect of water on the tautomeric equilibrium of uracil monoanions. A PCILO study

The PCILO method is used to calculate the stabilization energies of the anionic forms of uracil by water molecules forming the first hydration shell of the tautomers. The results show that the 1-HU^– tautomer is more stabilized by the solvent effect than the 3-HU^– tautomer by about 8 kcal/mole. The effect brings closer together the energies of the two tautomers which differ intrinsically by about 13 kcal/mole in favour of the 3-HU^– form. It operates in the direction inferred from experimental studies.This work was supported in part by R.C.P. 173 of the Centre National de la Recherche Scientifique of France and in part by the Polish Academy of Sciences within the project 09.3.1.     

Solvatochromism and solvation of ternary iron-diimine-cyanide complexes

Summary The solvatochromic behaviour of several complexes [Fe(LL)₂(CN)₂] with LL=Schiff base diimine has been established in a series of non-aqueous solvents, as has that of two analogues containing diazabutadiene ligands. Transfer chemical potentials have been derived from appropriate solubility measurements for several iron(II)-and iron(III)-diimine-cyanide complexes into aqueous methanol, and for [Fe(bipy)₂(CN)₂] into several binary aqueous solvent series. The usefulness of solvatochromic shifts and transfer chemical potentials as indicators of selective solvation is discussed. Kinetics of oxidation of catechol and of 4-t-butyl catechol by [Fe(bipy)(CN)₄]^– in aqueous solution are described.     

Magnetic response and its relation to the keto-enol tautomerism of 3,3′-(1,4-phenylene)bis(pentane-2,4-dione): Experimental and theoretical insights

A direct correlation between aromaticity and the thermodynamic stability in the tautomeric equilibrium of 3,3′-(1,4-phenylene)bis(pentane-2,4-dione) is studied using experimental methods and theoretical approaches. According to the results, the most abundant tautomer corresponds to the bis-β-keto-enol tautomer when conditions such as temperature, solvent polarity, and pH are deliberately changed. Theoretical results of aromaticity analysis showed good concordance with the experimental results, using rigorous computations such as induced magnetic field (B^ind) and free of in-plane component NICS (FIPC-NICS).     

Solvatochromism in Binary Solvent Mixtures by Means of a Penta‐tert‐butyl Pyridinium N‐Phenolate Betaine Dye

The solvatochromic behavior of a penta‐tert‐butyl prydinium N‐phenolate betaine dye was studied using UV‐visible spectrophotometry in several binary mixture solvents. The solvent polarity parameter, E_T (1) (kcal. mol^?1) was calculated from the position of the longest‐wavelength intramolecular charge transfer absorption band of this penta‐tert‐butyl betaine dye. For binary solvent mixtures, all plots of E_T (1) versus the mole fraction of a more polar component are nonlinear owing to preferential solvation of the probe by one component of the binary solvent mixture. In the computation of E_T (1) it was assumed that the two solvents mixed interact to form a common structure with an E_T (1) value not always intermediate between those of the two solvents mixed. The results obtained are explained by the strong synergism observed for some of the binary mixtures with strong hydrogen bond donors (HBD) solvents such as alcohols.     

Tautomeric Transformations and Reactivity of Polyfunctional Hydroxypyrimidines: IV. Effects of Tautomer Structure and Acidity and Solvent Nature on the State of Tautomeric Equilibria in the Series of 5-Nitro Derivatives of 2,4,6-Pyrimidinetrione

Tautomeric equilibria of 5-nitrobarbituric acid and 1,3-dimethyl-5-nitrobarbituric acid were studied by NMR, UV, and IR spectroscopy, as well as by quantum-chemical methods. These compounds were found to exist in the gas phase, in solutions in water and DMSO, and in crystal as the corresponding enol tautomers. In trifluoroacetic and chloroform solutions, an equilibrium mixture of the ketone and enol forms was detected. The parameters determining the state of the tautomeric equilibrium are the total energy of a tautomer, its deprotonation energy, and a component of the latter, which relates to electrostatic interaction between the mobile proton and the rest of the molecule. Proton-donor and proton-acceptor properties of the solvent and 5-nitro-2,4,6-pyrimidinetrione derivatives are the main factors responsible for displacement of the tautomeric equilibrium in solution; electrostatic solvation and tautomer stabilization via hydrogen bonding are less significant.     

Structure and IR Spectra of 3(5)-Aminopyrazoles and UV-Induced Tautomerization in Argon Matrix

The prototropic tautomerism in 3(5)-aminopyrazoles was investigated by matrix isolation infrared (IR) spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In consonance with the experimental data, the calculations predict tautomer 3-aminopyrazole (3AP) to be more stable than the 5-aminopyrazole (5AP) tautomer (calculated energy difference: 10.7 kJ mol⁻¹; Gibbs free energy difference: 9.8 kJ mol⁻¹). The obtained matrix isolation IR spectra (in both argon and xenon matrices) were interpreted, and the observed bands were assigned to the tautomeric forms with help of vibrational calculations carried out at both harmonic and anharmonic levels. The matrix-isolated compound (in argon matrix) was then subjected to in situ broadband UV irradiation (λ > 235 nm), and the UV-induced transformations were followed by IR spectroscopy. Phototautomerization of the 3AP tautomer into the 5AP form was observed as the strongly prevalent reaction.     

Excited State Proton Transfers in Hybrid Compound Based on Indoline Spiropyran of the Coumarin Type and Azomethinocoumarin in the Presence of Metal Ions

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li⁺, Ca²⁺, Zn²⁺ and Mg²⁺ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.     

Controlling Tautomerization in Pyridine-Fused Phosphorus-Nitrogen Heterocycles

Inclusion of a second nitrogen atom in the aromatic core of phosphorus-nitrogen (PN) heterocycles results in unexpected tautomerization to a nonaromatic form. This tautomerization, initially observed in the solid state through X-ray crystallography, is also explained by computational analysis. We prepared an electron deficient analogue ( 2 e ) with a fluorine on the pyridine ring and showed that the weakly basic pyridine resisted tautomerization, providing key insights to why the transformation occurs. To study the difference in solution vs. solid-state heterocycles, alkylated analogues that lock in the quinoidal tautomer were synthesized and their different ¹H NMR and UV/Vis spectra studied. Ultimately, we determined that all heterocycles are the aromatic tautomer in solution and all but 2 e switch to the quinoidal tautomer in the solid state. Better understanding of this transformation and under what circumstances it occurs suggest future use in a switchable on/off hydrogen-bond-directed receptor that can be tuned for complementary hydrogen bonding.     

Solvatochromism of Heteroaromatic Compounds: XXI. Ion Pairs of 4-Nitropyrazolide Anion

The solvatochromism of the long-wave UV absorption band of the contact ion pairs of the 4-nitropyrazolide anion with alkali metal cations in aprotic protophilic solvents with 24 was studied and quantitatively described using the Kamlet-Taft empirical solvent parameters. Of two possible types of tight ion pairs differing in the murtual arrangement of the counterions, the solvent stabilizes the pair in which the cation is located in the plane of the anion in the vicinity of its nitrogen atoms. Among the factors affecting the quantitative characterisitcs of the solvatochromism of such ion pairs, the major factors are the polarity/polarizability of the solvent and the capability of its molecules for coordination with cations. The peripheral specific solvation of the cation in an ion pair stabilizes its Franck-Condon excited state relative to the ground electronic state. In the quantitative respect, this effect is similar to that characteristic of solvation H complexes.     

Solvatochromism of the π* ← n transition of acetone by combined quantum mechanical—classical mechanical calculations

The solvent shift of the π* ← n transition of acetone in water, acetonitrile, and tetrachloromethane was calculated in a combined quantum mechanical—classical mechanical approach, using both dielectric continuum and explicit, polarizable molecular solvent models. The explicit modeling of solvent polarizability allows for a separate analysis of electrostatic, induction, and dispersion contributions to the shifts. The calculations confirm the qualitative theories about the mechanisms behind the blue shift in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state due to electrostatic interactions being preferential in the former, and favorable dispersion interaction with the excited state, in the latter case. Good quantitative agreement for the solvent shift between experiment (+1,700, +400, and −350 cm⁻¹ in water, acetonitrile, and tetrachloromethane, respectively) and the explicit solvent model (+1,821, +922, and −381 cm⁻¹) was reached through a modest Monte Carlo sampling of the solvent degrees of freedom. A consistent treatment of the solvent could only be realized in the molecular solvent model. The dielectric-only model needs reparameterization for each solvent. © 1996 John Wiley & Sons, Inc.     

Study of hydrazone-Hydrazoimine tautomerism in α-Azo-6-ketomethylphenanthridines

AB Azo-coupling of a series of 6-ketomethylphenanthridines products was performed starting from 6- ketomethylphenanthridines and benzene diazonium chloride. The products were characterised using ¹H, ¹³C and ¹⁵N NMR, IR, UV and MS spectroscopic methods. The hydrazoimine forms were the only tautomer present in chloroform solution. Two rotamers were detected in chloroform solutions of the compounds studied. Analysis of the NMR data shows that the ratio of rotamers depends on electron-donating and electron-withdrawing properties of the substituent present in the benzene ring.     

1H NMR and UV spectroscopic study of inclusion complex formation between pyridoxine and β- and γ-cyclodextrins

Inclusion complex formation between pyridoxine and - and -cyclodextrins in aqueous solution has been investigated by¹H NMR and UV spectroscopy. Both complexes exhibited a 1:1 stoichiometry and the inclusion process has been shown to perturb the equilibrium between the lactim and the lactam tautomer of pyridoxine, with a preferential inclusion of the former, less polar tautomer.     

Tautomeric behavior of 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines between the hydrazone imine and diazenyl enamine forms in acidic media. Solvent and substituent effects

The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-h and 2a-e showed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed trifluoroacetic acid/dimethyl sulfoxide media. The substituent and solvent effects on the tautomer ratios of A to B in a series of mixed media were studied for compounds 1a-h and 2a-e by the nmr spectroscopy. In compounds 1a-h and 2a-e , the ratios of the tautomer B gradually increased with elevation of acid concentration, and the tautomer B exclusively existed in trifluoroacetic acid media. The various acid concentrations (C v/v%, C' mol/1) giving the 1:1 tautomer ratios [C(A:B = 1:1), C'(A:B = 1:1)] were obtained from all compounds (Figures 1–13), and the linear correlation of the Hammett σ_p values with the log C'(A:B = 1:1) values were observed for compounds 1a-h. The larger Hammett σ_p values brought about the larger acid concentrations C(A:B = 1:1) in compounds 1a-h and 2a-e , indicating that the higher acid concentration was required for the stabilization of tautomer B possessing the electron-withdrawing p-substituents R¹, which weakened the basicity of the azo nitrogen atom. Moreover, the ester group R² of compounds 2a-e was found to decrease the electron density of the azo nitrogen atom, since the acid concentration C(A:B = 1:1) of compound 2c (R¹ = H, R² = COOMe, σ_p = O) was 52%, whose value was larger than that of compound 1b (R¹ = CN, R² = H, σ_p = 0.66) [C(A:B = 1:1) = 42%].     

Structure and spectal properties of cinnamoyl pyrones and their vinylogs

The ¹H-NMR and quantum chemical analysis of the stability of tautomers of cinnamoyl pyrone derivatives and vinylogs has been studied. The relationship between the structure of the most stable tautomer and its spectral properties has been investigated. It has been determined that the tautomer of highest stability (88–100 molar %) has an α-pyrone structure and exhibits a trans-conformation in the cinnamoyl fragment. An intense fluorescence of dyes has been observed in non-polar solvents with cinnamoyl fragments having electron-donating substituents or several double bonds in the polymethine chain. A gradient in solvent polarity resulted in fluorescence quenching which permits the use of the dyes as intensometric fluorometric probes for medium polarity examination.