Tautomeric Properties Research Articles

Computational study of nitroguanidine (NQ) tautomeric properties in aqueous solution

This study focused on investigation of the tautomeric properties of nitroguanidine (NQ), an energetic material, using theoretical approaches for NQ in both the gas phase and in aqueous solution. Density functional method (M062X) was used to estimate physical and chemical properties of NQ tautomers. Two different solvent models (CPCM and PCM) and two basis sets (6-31G(d,p) and 6-31 + G(d,p)) were utilized. A ranking of the stability order of the tautomers was predicted based on changes in Gibbs free energy values and were generally found to be in good agreement with previous theoretical and experimental studies. Electronic properties including dipole moment, electron affinity, ionization potential and charges of NQ were predicted in both gas and aqueous phases. In order to analyze NQ behavior in aqueous solution, several properties were calculated. Deprotonating as well as protonating of NQ revealed positive energies indicating that those processes are rather unfavorable and further characterized NQ as a compound with weak acid–base properties. We have calculated pKa and pKb values using data obtained from quantum chemical calculations using CPCM and COSMO-RS approaches. The COSMO-RS approach was able to reproduce experimental values with high accuracy, while the results obtained with CPCM method significantly differ from values obtained experimentally.

Adducts of manganese diketonates with redox-active ligands: Computational modeling of valence tautomeric systems

The potential valence tautomeric (VT) properties of 1:1 adducts of MnII bis-malonates and MnII bis-hexafluoroacetylacetonates with bidentate redox-active ligands – o-benzoquinone, its imine and diimine derivatives (complexes I) and 1,4-diazabuta-1,3-diene (complexes II) were computationally studied. According to the DFT B3LYP∗/6-311++G(d,p) calculations, the ground electronic states of the mixed-ligand complexes I are represented by the high-spin electromeric forms HSMnIII–L− exhibiting strong antiferromagnetic exchange coupling between unpaired electrons of the paramagnetic centers located at the metal ion and radical-anionic ligand. Same type magnetic ordering is characteristic also of the structures containing manganese center in its intermediate spin-state. Stabilization of the electromers HSMnII–L0 containing a redox-active ligand in the non-reduced form is achieved in the complexes formed by MnII hexafluoroacetylacetonate with benzoquinone imine and diimine and adducts on the basis of 1,4-diazabuta-1,3-diene (II) possessing stronger than carbonyl oxygens ligating centers (NH). The adducts I (X=Y=NH; R=CF3), (X=O, Y=NH; R=CF3) and II (R=H) characterized by the narrow energy gaps between the HSMnIII–L− and HSMnII–L0 electromeric forms and low energy barriers (estimated as minimum energy crossing points MECP at seams of intersection of the corresponding potential energy surfaces) for their interconversion may be considered as promising candidates for the observation of thermally driven valence tautomeric rearrangements.

Computational study of NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one) tautomeric properties in aqueous solution

This study focuses on elucidating the stable forms of a new energetic material that is a member of the class of insensitive munitions (IM), 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), including its tautomers, and anions. The geometry and properties of all compounds were calculated using density functional theory (M06-2X) and MP2 quantum chemical approaches. Calculations were carried out in the gas phase and in aqueous solution. Chemical stability of these compounds was evaluated in terms of the Gibbs free energy change. Two different solvation models were applied (CPCM and PCM). Calculations showed that overall differences in the results obtained using these two solvation models are negligible for all compounds considered. All possible NTO tautomers were examined and the results are in good agreement with previous studies performed in the gas phase. The stability order was revealed to be slightly dependent on the method applied. In order to estimate acidic properties of NTO, anions of several NTO tautomers were analyzed. In addition, pK a values were calculated using different approaches. As compared with available experimental data it was found that the conductor-like screening model for real solvents approach leads to more accurate estimation of the pK a value than the CPCM and PCM approaches. The pK a value calculated using PCM and CPCM data showed large errors; however, it was proven that the pattern of deprotonation energy was correctly estimated.

A comprehensive study of the structure, tautomeric properties, and conformational flexibility of 3-Hydroxy-propeneselenal

In the present work, a conformational analysis of 3-Hydroxy-propeneselenal is performed using several computational methods, including HF, DFT (B3LYP), and G2MP2 levels of theory. The relative electronic energies, the molecular structure, and intramolecular hydrogen bond energies are reported. Twenty different conformers were obtained, the most stable ones being planar. The conformational preference of this molecule was found to be mainly determined by the formation of Se...H-O and Se-H...O intramolecular hydrogen bonds, which are assisted by π-electron resonance. The atoms in molecules (AIM) theory of Bader, which is based on topological properties of the electron density, was used to analyze critical points and to study the nature of hydrogen bonds in these systems. NMR and natural bond orbital (NBO) analyses were also performed for a better understanding of the nature of intramolecular interactions in 3-Hydroxy-propeneselenal. Furthermore, calculations for all the possible conformations of the title compound in solution were also carried out at the B3LYP/6-311 ++G(d,p) level of theory (in three models such as polarizable continuum model (PCM), IEFPCM, and self −consistent isodensity polarized continuum model (SCIPCM). The aim of this work is to highlight the efficiency and suitability of silicadiphenic acid as a supported solid acid catalyst for the synthesis of bis-(indolyl)methane derivatives from a one-pot condensation reaction of various substituted aldehydes and indole in acetonitrile at 60°C.

ChemInform Abstract: Tautomerism in Nucleic Acid Bases and Base Pairs: A Brief Overview

AbstractReview: 116 refs.

Azo-8-hydroxyquinoline dyes: The synthesis, characterizations and determination of tautomeric properties of some new phenyl- and heteroarylazo-8-hydroxyquinolines

Two series of new heterocyclic and carbocyclic disperse azo dyes based on 8-hydroxyquinoline were prepared and characterized by FT-IR, 1H NMR, mass spectroscopic techniques and elemental analysis. Their solvatochromic properties in different solvents were investigated and their absorption spectra were strongly solvent dependent. The acid and base effects on this equilibrium were also examined. In addition, the colors of dyes were discussed with respect to the nature of the carbocyclic and heterocyclic rings and substituent therein. To determine the tautomeric forms of the prepared dyes in solid state, X-ray data for 5-(5-methylthiazol-2-yldiazenyl)-8-hydroxyquinoline were recorded. The X-ray results showed that the dye exists as an azo tautomer in the solid state.

Synthesis, structural characterization and tautomeric properties of some novel bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone

Nine new bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone have been synthesized in two steps using Knoevenagel condensation and diazotization-coupling reaction. The structures of the compounds were confirmed by UV–vis, IR, 1H NMR and 13C NMR spectroscopic techniques. The spectral characterizations demonstrate that there is an equilibrium between the azo (T1) and hydrazine (T2 and T3) tautomers for all prepared dyes in solutions. In addition, the solvatochromic behavior of the prepared dyes was evaluated using polarity/polarizability parameter (π*) in various solvents. The UV–vis absorption spectra of dyes show a bathochromic shift with increasing polarity and base strength of the solvents. Finally, the effects of acid and base on the UV–vis absorption spectra of the dyes with different substituent in diazo component are reported.

Structural, energetic and tautomeric properties of the T·T∗/T∗·T DNA mismatch involving mutagenic tautomer of thymine: A QM and QTAIM insight

It was revealed by thorough study of the T·T∗ (C1)↔T∗·T (C1) tautomerisation via the synchronous concerted double proton transfer (DPT) through the TS (C2v) that the T·T∗/T∗·T H-bonded mismatch is dynamically stable non-planar complex with a lifetime 1.6×10−10s. The 5 key points were firstly detected and completely investigated along the intrinsic reaction coordinate of the DPT tautomerisation. The reported data allow us to suggest that the T∗ mutagenic tautomer of the thymine (T) is shared with approximately equal probability between two DNA strands during the dissociation of the mispair by DNA polymerase.

ChemInform Abstract: Pyrazol‐5‐ones: Tautomerism, Synthesis and Reactions

AbstractReview: 244 refs.

Tautomerism in nucleic acid bases and base pairs: a brief overview

AbstractThis article provides a brief overview of current status of studies on nucleic acid bases and base pairs tautomeric properties in the different environments. Applications of high‐level theoretical calculations have been found to be extremely useful in interpreting and enlightening complex experimental results. Actually, theory and experiments are complementary to each other and therefore it is not surprising that application of theoretical calculations were necessary to assign relatively higher energy tautomers of guanine in the gas phase. Inability to observe the stable tautomers in the laser‐desorbed guanine in the jet‐cooled beam demands more experiments and rigorous analysis. The usefulness of theoretical methods in the assignment of tautomeric forms of other bases and base pairs has also been discussed. Furthermore, we would like to state that recent surge in the high‐level computation of system such as nucleic acid bases and base pairs is only possible due to the advancement in state‐of‐the‐art computer hardware and computational algorithms. Afterward, it is not unexpected to assume that future investigations would utilize high‐level quantum chemistry calculation more rigorously in explaining complex experimental data as well as in revealing new phenomena not studied by experiments. WIREs Comput Mol Sci 2013, 3:637–649. doi: 10.1002/wcms.1145This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Computational Biochemistry and Biophysics

Controlled shift in the tautomeric equilibrium of 4-((phenylimino)methyl)naphthalen-1-ol

4-((Phenylimino)methyl)naphthalen-1-ol and 4-((phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol have been synthesized and their tautomeric properties were investigated using molecular spectroscopy (UV–Vis absorption/emission and NMR), X-ray crystallographic analysis and quantum-chemical calculations. The results show that with the implementation of a flexible piperidine ring a controlled shift in the tautomeric equilibrium will be achieved upon protonation/deprotonation in acetonitrile. The addition of a metal salt also shifts the tautomeric equilibrium, but at very high concentrations, which is caused not by a complex formation, but a special effect of the salt addition.

Tautomeric properties and gas-phase structure of 3-methyl-2,4-pentanedione

The tautomeric and structural properties of 3-methyl-2,4-pentanedione, CH3C(O)CH(CH3)C(O)CH3, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). Analysis of GED intensities resulted in the presence of 100(3)% enol tautomer at 274(7)K. Quantum chemical calculations predict enol concentrations from 8% at MP2/6-31G(d,p) level to 100% at B3LYP/aug-cc-pVTZ. This would show the failure of the MP2/6-31G(d,p) method. The enol ring possesses CS symmetry with a strongly asymmetric hydrogen bond. The experimental geometric parameters are reproduced very closely by the B3LYP/aug-cc-pVTZ method.

Synthesis, Characterization, and Tautomeric Properties of Some Azo-azomethine Compounds

The primary azo compound 1-(3-formyl-4-hydroxyphenylazo)-4-nitrobenzene reacts with some aliphatic and aromatic diamines and yields the corresponding azo-azomethine compounds. These compounds were characterized by elemental analysis, IR, UV/Vis, and NMR spectroscopy. The primary azo compound exists entirely in the azo form in solution as well as in the solid phase. The tautomeric structure of azo-azomethine compounds heavily depends on the solvent and the substituents. Aliphatic diamine-based compounds favor the enol-imine tautomer while aromatic diamine-based compounds have structures that lie between the two enol-imine and keto-amine tautomers due to a relatively strong intramolecular hydrogen bond. The compounds exhibit positive solvatochromism (bathochromic shift) so that their absorption bands move toward longer wavelengths as the polarity of the solvents increases. In addition, UV/Vis spectrophotometry has shown that the studied compounds have molar extinction coefficients larger than 40000.

Synthesis, Characterization, and Tautomeric Properties of Some Azo-azomethine Compounds

Synthesis, Characterization, and Tautomeric Properties of Some Azo-azomethine Compounds

Switching azonaphthols containing a side chain with limited flexibility. Part 1. Synthesis and tautomeric properties

A series of azo dyes, possessing amide fragments with restricted flexibility tethered to 4-(phenyldiazenyl)naphthalen-1-ol, was obtained from 1-hydroxy-2-naphthoic acid by subsequent conversion to amides and diazo coupling. It was shown that the position of the tautomeric equilibrium in solution strongly depends on the solvent in both UV and NMR concentration scale. The compounds exist as pure enol forms in chloroform and hydrocarbons, while in polar solvents (acetone, acetonitrile, alcohols) a tautomeric mixture is observed. According to the quantum-chemical calculations the aggregation of the keto tautomer is the possible reason for this shift in the position of the tautomeric equilibrium. To support the theoretical predictions, it was found that from acetone the keto form crystallizes as a dimer with hydrogen bonding between N 1– H in the one molecule and amide C O in the other forming a three-dimensional structure. The importance of the side-chain nitrogen atom on the dimer formation was confirmed by solution and solid state study of 4-(phenyldiazenyl)-2-acetylnaphthalen-1-ol. The results indicate that the new azo-dyes obtained could be suitable candidates for switching and sensing applications in non-polar solvents.

Bridging the Gap between Porphyrins and Porphycenes: Substituent‐Position‐Sensitive Tautomerism and Photophysics in meso‐Diphenyloctaethylporphyrins

2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin (2), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen-bonding strength as that in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen-bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.

The effect of benzoannulation on the transition state and the proton transfer equilibrium in di(2-pyridyl)methane derivatives

The tautomeric properties of di(2-pyridyl)methane and its benzoannulated derivatives were studied using a computational approach (M05/6-31G(2d,p)). Our analysis showed that the degree of cyclic π-electron delocalization in benzene and pyridine rings is directly connected to the effect of resonance present in quasi-rings formed by intramolecular hydrogen bonds of the N–H⋯N type. This direct relation can be explained using two concepts, namely, the concept of Clar's aromatic sextet and the Leffler–Hammond concept originally developed for the explanation of the energy relation between ground-state and transition-state structures corresponding to proton transfer in H-bridges. Application of these two different concepts allows us to explain in detail the role of intramolecular hydrogen bonding in polycyclic aromatic hydrocarbons containing N heteroatoms.

Synthesis and characterization of novel color chemosensors based on azo dyes for possible application in opioid pharmacology

Abstract magnified imageAn applicable strategy of chemical labeling of morphine (M) and 6‐acetyl morphine (6‐AM) using diazonium salts is described. M or 6‐AM was coupled with aryl diazoinum salts to give morphine azo dyes. The coupling of the diazotized 4,4′‐diaminostilbene with M or 6‐AM in ratio 1:2 gave stilbene‐based azo dyes containing two M or 6‐AM units, respectively. Diazotization of 5‐(p‐aminophenyl)‐10,15,20‐triphenylporphyrin and subsequent azo coupling of the diazoniun salt with M and with 6‐AM was our route to get highly fluorescent morphine dyes. The resulting dyes can exist in two possible tautomeric structures, azo and hydrazone, stabilized to a significant extent by intramolecular H‐bonding. It was shown that these dyes exists predominantly or exclusively in their hydrazone form. This conclusion is drawn from the lack of a distinct band in the 380–420 nm region of the absorption spectra (a region in which the long wavelength absorption band for the azo‐form is observed), together with results of NMR studies in deuterated DMSO. The tautomeric properties of the compounds were judged by density functional calculations at the B3LYP/6‐31G* and B3LYP/6‐31G** levels. The analysis of spiked compounds in human urine samples was studied by capillary electrophoresis (CE) with UV‐fluorescence photo‐diode array detectors. J. Heterocyclic Chem., (2010).

The keto/enol tautomerism in acetoacetyl fluoride: properties, spectroscopy, and gas-phase and crystal structures of the enol form

Tautomeric properties of acetoacetyl fluoride, CH(3)-C(O)-CH(2)-C(O)-F, were studied by IR (gas phase), Raman (liquid and solid), and NMR spectroscopy (neat liquid), gas electron diffraction (GED), X-ray crystallography, and quantum chemical calculations. The keto-enol tautomer possesses a much higher vapour pressure than the diketo form and therefore the keto-enol form strongly predominates in the gas phase. In the neat liquid state the thermally unstable compound tautomerizes at low temperatures slowly to yield the diketo form. Raman and NMR spectra show equilibrium with strong predominance of the diketo tautomer (>90%) in the liquid phase. Single crystals, grown at -90 °C from the gas phase, contain exclusively acetoacetyl fluoride in the keto-enol form and the molecules possess a planar skeleton with overall C(S) symmetry and with the O-H bond adjacent to the methyl group.

Unique Tautomeric and Recognition Properties of Thioketothymines?

The tautomeric and recognition properties of thymine, 2- and 4-thioketothymines have been studied by means of accurate ab initio methods combined with molecular dynamics simulations and free energy calculations. In contrast to previous suggestions in the literature, the replacement of carbonyl oxygens by sulfur atoms does not lead to dramatic changes in tautomeric properties of the pyrimidine derivatives neither in vacuum nor in aqueous solution. Moreover, the presence of thioketothymines induces only mild changes in DNA structure, stability and fidelity. Despite the fact that mismatching can largely stabilize minor tautomeric forms, thioketothymines are found in the canonical thioketo-form irrespective of the paired base. Our theoretical results, confirmed by new experimental studies, describe the complete tautomeric and recognition characteristics of thioketothymines and demonstrate that both 2-thioketo and 4-thioketothymine are excellent molecules to introduce special chemical properties in modified DNA.