Search

Guanidine-containing molecules are an interesting class of compounds within both medicinal and material sciences. Having knowledge of their tautomerism is key in designing guanidines that interact with biological and chemical receptors. However, there are limited data on the solid-phase structure of N,N′-substituted guanidines. Thus, eleven guanidines bearing a 4,6-dimethylpyrimidyl at one end and substituents of varying sizes and electronic properties at the other side, were synthesised, crystallised, and analysed by X-ray crystallography. Calculations of isolated molecules of tautomer energies and bond lengths were performed for comparison. One class of guanidines crystallised as a cis–trans tautomer with the shorter bond directed towards the pyrimidyl unit. When more electron-deficient aniline substituents were inserted, the crystallised tautomer changed to a cis–cis form where the shorter bond was directed towards the aniline. The switch in the tautomer structure is concluded to be due to both the electronic properties of the substituents and the intermolecular hydrogen bonding in the crystal lattice. [ABSTRACT FROM AUTHOR]

The formation of humins hampers the large-scale production of 5-hydroxymethylfurfural (HMF) in biorefinery. Here, a detailed reaction network of humin formation at the initial stage of fructose-to-HMF dehydration in water is delineated by combined experimental, spectroscopic, and theoretical studies. Three bimolecular reaction pathways to build up soluble humins are demonstrated. That is, the intermolecular etherification of β-furanose at room temperature initiates the C12 path, whereas the C–C cleavage of α-furanose at 130–150 °C leads to C11 path, and that of open-chain fructose at 180 °C to C11′ path. The successive intramolecular dehydrations and condensations of the as-formed bimolecular intermediates lead to three types of soluble humins. We show that the C12 path could be restrained by using HCl or AlCl3 catalyst, and both the C12 and C11′ paths could be effectively inhibited by adding THF as a co-solvent or accelerating heating rate via microwave heating.

Guanidine-containing molecules are an interesting class of compounds within both medicinal and material sciences. Having knowledge of their tautomerism is key in designing guanidines that interact with biological and chemical receptors. However, there are limited data on the solid-phase structure of N,N′-substituted guanidines. Thus, eleven guanidines bearing a 4,6-dimethylpyrimidyl at one end and substituents of varying sizes and electronic properties at the other side, were synthesised, crystallised, and analysed by X-ray crystallography. Calculations of isolated molecules of tautomer energies and bond lengths were performed for comparison. One class of guanidines crystallised as a cis–trans tautomer with the shorter bond directed towards the pyrimidyl unit. When more electron-deficient aniline substituents were inserted, the crystallised tautomer changed to a cis–cis form where the shorter bond was directed towards the aniline. The switch in the tautomer structure is concluded to be due to both the electronic properties of the substituents and the intermolecular hydrogen bonding in the crystal lattice.

The quinolone–quinoline tautomerization is harnessed to effect the regioselective C8‐borylation of biologically important 4‐quinolones by using [Ir(OMe)(cod)]2 as the catalyst precursor, the silica‐supported monodentate phosphine Si‐SMAP as the ligand, and B2pin2 as the boron source. Initially, O‐borylation of the quinoline tautomer takes place. Critically, the newly formed 4‐(pinBO)‐quinolines then undergo N‐directed selective Ir‐catalyzed borylation at C8. Hydrolysis of the OBpin moiety on workup returns the system to the quinolone tautomer. The C8‐borylated quinolines were converted to their corresponding potassium trifluoroborate (BF3K) salts and to their C8‐chlorinated quinolone derivatives. The two‐step C‐H borylation‐chlorination reaction sequence resulted in various C8‐Cl quinolones in good yields. Conversion to C8‐OH‐, C8‐NH2‐, and C8‐Ar‐substituted quinolones was also feasible by using this methodology. [ABSTRACT FROM AUTHOR]

Nitrogen heterocycles are a class of organic compounds with extremely versatile functionality. Imidines, HN[C(NH)R]2, are a rare class of heterocycles related to imides, HN[C(O)R]2, in which the O atoms of the carbonyl groups are replaced by N—H groups. The useful synthesis of the imidine compounds succinimidine and glutarimidine, as well as their partially hydrolyzed imino–imide congeners, was first described in the mid‐1950s, though structural characterization is presented for the first time in this article. In the solid state, these structures are different from the proposed imidine form: succinimidine crystallizes as an imino–amine, 2‐imino‐3,4‐dihydro‐2H‐pyrrol‐5‐amine, C4H7N2 (1), glutarimidine as 6‐imino‐3,4,5,6‐tetrahydropyridin‐2‐amine methanol monosolvate, C5H9N3·CH3OH (2), and the corresponding hydrolyzed imino–imide compounds as amino–amides 5‐amino‐3,4‐dihydro‐2H‐pyrrol‐2‐one, C4H6N2O (3), and 6‐amino‐4,5‐dihydropyridin‐2(3H)‐one, C5H8N2O (4). Imidine 1 was also determined as the hydrochloride salt solvate 5‐amino‐3,4‐dihydro‐2H‐pyrrol‐2‐iminium chloride–2‐imino‐3,4‐dihydro‐2H‐pyrrol‐5‐amine–water (1/1/1), C4H8N3+·Cl−·C4H7N3·H2O (1·HCl). As such, 1 and 2 show alternating short and long C—N bonds across the molecule, revealing distinct imino (C=NH) and amine (C—NH2) groups throughout the C—N backbone. These structures provide definitive evidence for the predominant imino–amine tautomer in the solid state, which serves to enrich the previously proposed imidine‐focused structures that have appeared in organic chemistry textbooks since the discovery of this class of compounds in 1883. [ABSTRACT FROM AUTHOR]

[Display omitted] • Complete vibrational assignments of all species and its harmonic scaled force constants are reported. • NBO and AIM calculations support the higher stability of monohydrate dimer due to the six H bonds interactions. • The monohydrated dimer reveals a higher solvation energy in aqueous solution. • The monohydrate dimer is the most reactive species, as revealed by the lowest gap value. • Anhydrous and monohydrate species evidence by NBO calculations a very important delocalization of electrons. Experimental FT-IR, FT-Raman and UV spectra have been combined with hybrid B3LYP/6–311++G** calculations and the scaled quantum mechanical force field (SQMFF) methodology to study structural and vibrational properties of monohydrated dimer (MD) of violuric acid in gas and aqueous media. Complete vibrational assignments and its scaled force constants are reported together with the corresponding to anhydrous and monohydrate monomer. From four anhydrous C1, C2, C3, C4 monomers, C4 is the most stable in both media. In solution, the initial structure of C2 change to the tautomeric species most stable C4. The MD reveals a higher solvation energy while natural bond orbital (NBO) and atoms in molecules (AIM) calculations support the higher stability of this species due to the six H bonds interactions and to its higher expansion of volume in solution. The MD is the most reactive species, as revealed by the lowest gap value and by high global electrophilicity and most negative global nucleophilicity indexes. Very good concordances are observed among the predicted IR, Raman, 13C NMR and UV spectra and the corresponding experimental ones. Comparisons of predicted 13C NMR and electronic spectra with the experimental one show that those three species of violuric acid could be present in aqueous solution. Similar f(νC=O) and f(νC-O) force constants for the three species are justified by the important delocalization of electrons evidenced in anhydrous and monohydrate species by NBO calculations. [ABSTRACT FROM AUTHOR]

In this study, 12 pairs of tautomeric flavonol derivatives containing quinoxaline were synthesized. The results of antifungal activity showed that in the enol-keto tautomerism, the target compounds containing keto ( Y B series) had better inhibitory activity against Sclerotinia sclerotiorum ( S.s .) than compounds containing enol ( Y A series). Y B9 showed the strongest antifungal activity against S.s., and the median effective concentration (EC 50 ) value was 1.0 μg/mL, which was better than azoxystrobin ( Az , 35.3 μg/mL). In vivo fungal inhibition experiments showed that the protective activity of Y B9 against rape leaves was 83.4% at 200 μg/mL, which was superior to that of Az (70.2%). The activity of succinate dehydrogenase and molecular docking results showed that Y B9 had a stronger antifungal effect than Y A9 . The results of oxalic acid content determination showed that Y B9 could reduce the pathogenic ability of S.s. Then, the inhibitory effect of Y B9 against S.s. was further verified by scanning electron microscopy, fluorescence microscopy, cell membrane permeability, cell content leakage, and malondialdehyde content.

Tautomers can be defined as isomers of single molecules existing in solutions or cells. Tautomers have the ability to interchange due to numerous spontaneous arrangements of chemical bonds, unlike chirality, whose molecules represent mirror images of enantiomers of one another. Tautomerization of the carmustine mechanism as a potential anti-cancer medication was examined using the DFT method. Two conformational tautomers were identified in the structure of carmustine, and the structure of both tautomers was shown to consider the contribution of atom changes to carmustine conformation. It was possible to obtain the relative energies B3LYP/6-311G++ (d,p), Aug-cc-pVDZ, and 6-311++g(2d,2p) basis sets. Calculations of the highest occupied molecular orbital (HOMO), the lowest unoccupied orbital (LUMO), and bandgap energies of structures were performed while also obtaining the electronics parameters, electrophilicity, electronegativity, softness, and hardness in order to determine the compounds’ reactivity within the biological medium. Based on the results, the carmustine structure and both tautomer conformations showed stability, but T1 had greater stability than T2.

• Both aspects of proximity effects are shown: through-bond and through-space interactions. • Quantum-chemistry methods enable detailed characterization of proximity effects. • Electron-donation/withdrawal by substituents depends on proximity effects. • Through-space effects determine the conformation of substituents. • NO 2 can form NO∙∙∙HN hydrogen bonds (2–3 kcal/mol) with ortho endocyclic NH groups. Proximity effects occur between vicinal functional groups or between a group and a specific molecular fragment in close proximity, for example an endocyclic N atom in the ortho position relative to the group. They are a combination of through-bond inductive and resonance effects, and through-space interactions, which can be either attractive or repulsive. In this review, we discuss quantum-chemical tools and examples from the literature that explore the structural and physicochemical implications of proximity effects. We focus on derivatives of small molecules: benzene, small polycyclic aromatic hydrocarbons, simple heterocycles, and lastly, purine derivatives. [Display omitted] [ABSTRACT FROM AUTHOR]

Structural analysis of dexrazoxane, as a cardioprotective agent, was done in this work by exploring formations of tautomeric conformations and investigating the corresponding effects. Density functional theory (DFT) calculations were performed to optimize the structures to evaluate their molecular and atomic descriptors. In addition to the original structure of dexrazoxane, eight tautomers were obtained with lower stability than the original compound. Movements of two hydrogen atoms in between nitrogen and oxygen atoms of heterocyclic ring put such significant effects. Moreover, electronic molecular orbital features showed effects of such tautomerism processes on distribution patterns and surfaces, in which evaluating the quadrupole coupling constants helped to show the role of atomic sites for resulting the features. As a consequence, the results indicated that the tautomeric formations could significantly change the features of dexrazoxane reminding the importance of carful medication of this drug for patients. [ABSTRACT FROM AUTHOR]

In this manuscript, the influence of solvent on stability and electronic properties of enolimine, enaminone, and iminone forms of 6‐ketomethylphenanthridine is theoretically investigated using polarizable continuum model (PCM). Here, PCM is used during optimization of structures of tautomeric forms in solution. Water, ethanol, acetone, acetonitrile, chloroform, diethyl ether, dichloromethane, and dimethyl sulfoxide are chosen as solvent. The calculations are performed at M06‐2X/6‐311++G(d,p) level. In gas phase and in the solvents, the stability of tautomers increases as enaminone > enolimine > iminone. The topological properties of electron charge density are also studied for better understanding of stability of tautomers. It is observed that the stability of tautomers is diminished by increasing dielectric constant of solvent. Also, the increase in dielectric constant of solvent kinetically and thermodynamically facilitates the enolimine ⇌ enaminone tautomerization. To obtain more valuable information on stabilities of tautomers in water phase, the PCM calculations have also been performed with one water molecule. At M06‐2X/6‐311++G(d,p) level of theory, the stability of complexes is amplified as enaminone–water > iminone–water (V) > iminone–water (IV) > enolimine–water. The relationships between dielectric constant of solvent and some electronic properties of tautomers such as band gap, electron affinity, electrophilicity index, first ionization energy, electronic chemical potential, chemical softness, and hardness are also investigated. [ABSTRACT FROM AUTHOR]

This work was performed regarding the importance of iron (Fe) chelation for biological systems. This goal was investigated by assistance of a model of thiocytosine (TC) for participating in Fe-chelation processes. First, formations of tautomeric conformations were investigated to explore existence of possible structures of TC. Next, Fe-chelation processes were examined for all four obtained tautomers of TC. The results indicated that thiol tautomers could be seen at higher stability than thio tautomers, in which one of such thiol tautomers yielded the strongest Fe-chelation process to build FeTC3 model. As a consequence, parallel to the results of original TC tautomers, Fe-chelated models were found to be achievable for meaningful chelation processes or sensing the existence of Fe in media. Examining molecular orbital features could help for sensing purposes. The results of this work were obtained by performing density functional theory (DFT) calculations proposing TC compounds suitable for Fe-chelation purposes. [ABSTRACT FROM AUTHOR]

This study is devoted to the investigation of the G·C*tO2(WC)↔G*NH3·C*t(WC), G·C*O2(WC)↔G*NH3·C*(WC) and G*·C*O2(WC)↔G*NH3·C(wWC)↓ tautomerization reactions occurring through the proton transfer, obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of theory in gas phase under normal conditions ('WC' means base pair in Watson-Crick configuration, T=298.15 K). These reactions lead to the formation of the G*NH3·C*t(WC), G*NH3·C*(WC) and G*NH3·C(wWC)↓ base pairs by the participation of the G*NH3 base with NH3 group. Gibbs free energies of activation for these reactions are 6.43, 11.00 and 1.63 kcal·mol-1, respectively. All of these tautomerization reactions are dipole active. Finally, we believe that these non-dissociative processes, which are tightly connected with the tautomeric transformations of the G·C base pairs, play an outstanding role in supporting of the spatial structure of the DNA and RNA molecules with various functional purposes. [ABSTRACT FROM AUTHOR]

The aim of this study was to examine dacarbazine tautomerization with the density functional theory, where two tautomer structures have been indicated. The B3LYP/ 6-311G++ (d, p) , 6-311(d, p), and 6-311 quantum methods were considered to calculate the relative energies transferred between two structures. Also, calculations of the HOMO, LUMO, as well as the structures’ band gap energy were performed. Estimation of electronic parameters, such as electrophilicity, electronegativity, softness, and hardness, were also investigated to determine the compound’s reactivity in a biological media.

In this work, tautomeric preference of fenobam in solution was investigated by homonuclear and heteronuclear solution nuclear magnetic resonance (NMR) spectroscopy. 1H–1H nuclear Overhauser effect spectroscopy (NOESY) spectrum revealed that fenobam in liquid state exists exclusively in one of the two possible tautomeric structures, which was confirmed by 1H–13C HSQC and heteronuclear multiple bond correlation (HMBC) spectra. Moreover, difference between the two tautomeric structures was studied by theoretical calculations, which further proved the result obtained by the NMR experiments. [ABSTRACT FROM AUTHOR]

As our ongoing searching for the bioactive natural terpenoids, nine ent -kauranoids (1 – 9), including three previously undescribed ones (1 , 2 , and 9), were isolated from the aerial parts of Isodon amethystoides. Their structures were elucidated on the basis of spectroscopic data analysis, including NMR, MS, and ECD. Compounds 1 and 2 were a pair of tautomeric compounds, which was confirmed by the HPLC analysis and low temperature NMR testing. The underlying mechanism of the tautomer was proposed as an intramolecular S N 2 reaction, which was explained by quantum chemical calculation. The HOMO-LUMO gap and the free energy revealed the spontaneous of the tautomeric of the 1 and 2. Additionally, the similar phenomena were also found in the two groups of known compounds 3 and 4 and 6 and 7 , respectively. Apart from the tautomer, compounds 3 and 4 can be hydrolyzed into 5 through ester hydrolysis in CDCl 3 , while compounds 6 , 7 can be hydrolyzed into 8 through ester hydrolysis. These phenomena were also confirmed through HPLC analysis and low temperature nuclear magnetic resonance tests and the mechanism was studied using quantum chemical calculation. [Display omitted] [ABSTRACT FROM AUTHOR]

The title compound, 2C14H14N4O·H2O, comprises a neutral molecule containing a central pyrazol-3-one ring flanked by an N-bound phenyl group and a C-bound 5-methyl-1H-pyrazol-3-yl group (at positions adjacent to the carbonyl substituent), its zwitterionic tautomer, whereby the N-bound proton of the central ring is now resident on the pendant ring, and a water molecule of crystallization. Besides systematic variations in geometric parameters, the two independent organic molecules have broadly similar conformations, as seen in the dihedral angle between the five-membered rings [9.72 (9)° for the neutral molecule and 3.32 (9)° for the zwitterionic tautomer] and in the dihedral angles between the central and pendant five-membered rings [28.19 (8) and 20.96 (8)° (neutral molecule); 11.33 (9) and 11.81 (9)°]. In the crystal, pyrazolyl-N—H...O(carbonyl) and pyrazolium-N—H...N(pyrazolyl) hydrogen bonds between the independent organic molecules give rise to non-symmetric nine-membered {...HNNH...NC3O} and {...HNN...HNC3O} synthons, which differ in the positions of the N-bound H atoms. These aggregates are connected into a supramolecular layer in the bc plane by water-O—H...N(pyrazolide), water-O—H...O(carbonyl) and pyrazolyl-N—H...O(water) hydrogen bonding. The layers are linked into a three-dimensional architecture by methyl-C—H...π(phenyl) interactions. The different interactions, in particular the weaker contacts, formed by the organic molecules are clearly evident in the calculated Hirshfeld surfaces, and the calculated electrostatic potentials differentiate the tautomers.

1-Phenyl and 1-methyl derivative of 3-methyl-5-pyrazolone were prepared quantitatively via a scalable solvent-free reaction of corresponding hydrazine derivative with ethyl acetoacetate. Different mechanisms have been proposed for the reaction of hydrazine derivatives (methyl or phenyl) with ethyl acetoacetate and also the tautomeric aspects of the targeted compounds have been discussed. Synopsis: 13C NMR and quantitative proportional amount of different tautomeric forms of 1,3-dimethyl-5-pyrazolone in DMSO-d6. [ABSTRACT FROM AUTHOR]

Finding the most stable tautomer or a set of low-energy tautomers of molecules is critical in many aspects of molecular modelling or virtual screening experiments. Enumeration of low-energy tautomers of neutral molecules in the gas-phase or typical solvents can be performed by applying available organic chemistry knowledge. This kind of enumeration is implemented in a number of software packages and it is relatively reliable. However, in esoteric cases such as charged molecules in uncommon, non-aqueous solvents there is simply not enough available knowledge to make reliable predictions of low energy tautomers. Over the last few years we have been developing an approach to address the latter problem and we successfully applied it to discover the most stable anionic tautomers of nucleic acid bases that might be involved in the process of DNA damage by low-energy electrons and in charge transfer through DNA. The approach involves three steps: (1) combinatorial generation of a library of tautomers, (2) energy-based screening of the library using electronic structure methods, and (3) analysis of the information generated in step (2). In steps 1–3 we employ combinatorial, computational and chemoinformatics techniques, respectively. Therefore, this hybrid approach is named “Combinatorial*Computational*Chemoinformatics”, or just abbreviated as C3 (or C-cube) approach. This article summarizes our developments and most interesting methodological aspects of the C3 approach. It can serve as an example how to identify the most stable tautomers of molecular systems for which common chemical knowledge had not been sufficient to make definite predictions.

Quantum chemical caculations of the most stable conformers of methylamine-pyridoxal-5-phosphate Schiff base three tautomeric (enol, keto, and zwitterionic) structures and their monohydrates and dihydrates have been carried out at the B3LYP/6-31+G**, M062X/6-31+G**, and BVP86/TZVP levels of theory. The optimized structures, harmonic force fields, and vibrational frequencies of tautomers have been obtained. The influence of macrohydration on the conformational composition of tautomers have been studied using the polarized continuum model. Vertical ionization potentials of the most stable monohydrates and dihydrates of methylamine-pyridoxal-5'-phosphate tautomers have been calculated. The influence of macrohydration and microhydration on the structure, spectra, and vertical ionization energies of the monohydrates and dihydrates of methylamine-pyridoxal-5-phosphate tautomers have been discussed. [ABSTRACT FROM AUTHOR]

Curcumin and related compounds have been validated to remove even well-developed human β-amyloid plaques from the brain of transgenic mice, in vivo. However, their molecular mechanism of the plaque buster activity is rather unknown. Computational chemistry was employed here to better understand the β-amyloid protein elimination. According to our docking studies, a tautomeric "keto-enol" flip-flop mechanism is proposed that may chop up β-amyloid plaques in Alzheimer's due to removing each hairpin-foldamers one by one from both ends of aggregated fibrils. According to the experimented models, other bi-stable "keto-enol" pharmacophores might be identified to break up amyloid plaques and enhance rapid clearance of toxic aggregates in Alzheimer's disease. • Curcumine and its derivatives diminish Alzheimer's β-plaques utilizing „keto-enol" tautomerization. • Computational chemistry was employed here. • Protein binding assay for identification of additional molecules. [ABSTRACT FROM AUTHOR]

The pKa is the standard measure used to describe the aqueous proton affinity of a compound, indicating the proton concentration (pH) at which two protonation states (e.g. A− and AH) have equal free energy. However, compounds can have additional protonation states (e.g. AH2+), and may assume multiple tautomeric forms, with the protons in different positions (microstates). Macroscopic pKas give the pH where the molecule changes its total number of protons, while microscopic pKas identify the tautomeric states involved. As tautomers have the same number of protons, the free energy difference between them and their relative probability is pH independent so there is no pKa connecting them. The question arises: What is the best way to describe protonation equilibria of a complex molecule in any pH range? Knowing the number of protons and the relative free energy of all microstates at a single pH, ∆G°, provides all the information needed to determine the free energy, and thus the probability of each microstate at each pH. Microstate probabilities as a function of pH generate titration curves that highlight the low energy, observable microstates, which can then be compared with experiment. A network description connecting microstates as nodes makes it straightforward to test thermodynamic consistency of microstate free energies. The utility of this analysis is illustrated by a description of one molecule from the SAMPL6 Blind pKa Prediction Challenge. Analysis of microstate ∆G°s also makes a more compact way to archive and compare the pH dependent behavior of compounds with multiple protonatable sites. [ABSTRACT FROM AUTHOR]

The present study compared the interactions among Na +, K +, Mg2+ and Ca2+, thymine and its tautomers in the gas and solvent phase, an interaction dependent upon the electronic construction of the tautomers. Three types of cation interaction with thymine and its tautomers were observed. In the first one, the metal cations interacted with a lone pair of nitrogen or oxygen of the tautomers. In the second type, there was an interaction among the cations, nitrogen and oxygen at the same time; the last one was that of cations with the electron density of thymine π-system, where the cations were perpendicular to the ring of thymine. The interaction of metals cation with tautomers was studied in the gas and solvent phases; a comparison was then made between interactions in two phases. The interaction energy for all complexes indicated the stability of complexes, an energy which was higher in Ca2+ and Mg2+ compared with Na+ and K+. Concerning K+ and Na+, the stability of all complexes of tautomers was greater than that of thymine complexes; however, the stability of certain Ca2+ and Mg2+ complexes was lower than the complexes of thymine. [ABSTRACT FROM AUTHOR]

In this work, chemical computations were carried out to study of tautomeric conversion of 5-fluorouracil (A → products) at CBS-QB3, M06-2x, and B3LYP levels of theory. These 11 tautomers connected to each other via 10 transition state molecules and four pathways. In addition, the stationary points (including stable structures and transition states), in which playing a significant role on ground potential energy surfaces of isomerization processes, were found. Multichannel RRKM steady-state approximation calculations have been performed to calculate the rate constants at 1 atm pressure and over a range of temperature from 268.15 to 310.15 K to cover the in vivo and atmospheric conditions. The stability ranking of tautomers obtained and demonstrated that di-keto form of 5-fluorouracil is the most stable tautomer. The energy barriers for tautomeric conversions were calculated. The information reported in this study may help with the identification of drug in vivo and in the laboratory. [ABSTRACT FROM AUTHOR]

In this study, the synthesis, characterization, and tautomeric properties of 2,2'-bis(phenylazo)-5,5'- biindane-1,1',3,3'-tetrone, a new bisindandione derivative, were investigated. 2,2'-Bis(phenylazo)-5,5'- biindane-1,1',3,3'-tetrone was obtained after the diazo coupling reactions between [5,5'-biindane]- 1,1',3,3'-tetrone and aniline, and characterized by Fourier Transform-Infrared, proton nuclear magnetic resonance, ultraviolet-visible and mass spectroscopy. The structural and spectroscopic analyses of azo dye were examined using Density Function Theory, and vibration frequencies calculated with the optimized structure were compared with experimental data. The obtained data showed that 2,2'- bis(phenylazo)-5,5'-biindane-1,1',3,3'-tetrone could be found in two tautomers, and the molar ratio of azo/hydrazo was found to be 0.86. The obtained data showed that the hydrazo form was dominant for 2,2'-bis(phenylazo)-5,5'-biindane-1,1',3,3'-tetrone. [ABSTRACT FROM AUTHOR]

Butin and butein are significant bioactive flavanones derived from plants, existing as tautomers of each other. However, their physicochemical attributes, such as their spectral profiles under varying experimental conditions in aqueous solutions and established chromatographic methods for distinguishing between them, remain undetermined. In this study, we determined the basic properties of butin and butein using conventional spectroscopic, reversed-phase, and chiral HPLC analyses. The spectra of the synthesized butin and butein were analyzed using a UV-Vis spectrophotometer in several solvents with different polarities as well as in aqueous solutions at various pH values. Furthermore, the behavior of the measured spectra was reproduced by calculations to reveal the effects of the solvent and pH on the spectra of butin and butein in organic and aqueous solutions. Subsequently, we assessed the structural stability of butin and butein using reversed-phase HPLC, which revealed that butein is unstable compared with butin in a general culture medium. The synthesized butin was effectively separated into R- and S-isomers with positive and negative Cotton effects, respectively, via HPLC using a chiral column. These findings will aid in uncovering the individual properties of both butin and butein that may have been concealed by their tautomerism and enable the synthesis of S-butin, which is typically challenging and time-consuming to isolate.

3-(Piperidin-4-yl)-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazole was synthesized through a four-step process including etherification, hydrazonation, cyclization, and reduction with an overall yield of 39%. The final product was characterized by 1H NMR and ESI-MS/MS. The molecular structures of benzyl (Z)-4-(2-(pyrrolidin-2-ylidene)hydrazine-1-carbonyl)piperidine-1-carboxylate and related compounds were analyzed using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. The results indicated a higher stability of the hydrazone tautomers. [ABSTRACT FROM AUTHOR]

The title compound, 2C14H14N4O·H2O, comprises a neutral mol­ecule containing a central pyrazol-3-one ring flanked by an N-bound phenyl group and a C-bound 5-methyl-1H-pyrazol-3-yl group (at positions adjacent to the carbonyl substituent), its zwitterionic tautomer, whereby the N-bound proton of the central ring is now resident on the pendant ring, and a water mol­ecule of crystallization. Besides systematic variations in geometric parameters, the two independent organic mol­ecules have broadly similar conformations, as seen in the dihedral angle between the five-membered rings [9.72 (9)° for the neutral mol­ecule and 3.32 (9)° for the zwitterionic tautomer] and in the dihedral angles between the central and pendant five-membered rings [28.19 (8) and 20.96 (8)° (neutral mol­ecule); 11.33 (9) and 11.81 (9)°]. In the crystal, pyrazolyl-N--H...O(carbon­yl) and pyrazolium-N--H...N(pyrazol­yl) hydrogen bonds between the independent organic mol­ecules give rise to non-symmetric nine-membered {...HNNH...NC3O} and {...HNN...HNC3O} synthons, which differ in the positions of the N-bound H atoms. These aggregates are connected into a supra­molecular layer in the bc plane by water-O--H...N(pyrazolide), water-O--H...O(carbon­yl) and pyrazolyl-N--H...O(water) hydrogen bonding. The layers are linked into a three-dimensional architecture by methyl-C--H...π(phen­yl) inter­actions. The different inter­actions, in particular the weaker contacts, formed by the organic mol­ecules are clearly evident in the calculated Hirshfeld surfaces, and the calculated electrostatic potentials differentiate the tautomers. [ABSTRACT FROM AUTHOR]

Dithio biuret conformers were optimised and frequency calculations were performed at MP2/6-311++G(d,p) level of theory. Classification of conformers was done to five different tautomers and the most stable conformer of each tautomer was selected. Tautomer 3 having trans conformation of amino and thiocarbonyl functional group was the most stable tautomer in the gas phase. Optimisation and frequency calculations were also done in four different solvents as follows: benzene, acetonitrile, DMSO and water. Stability of tautomers were compared in these solvents to explore effect of polarity on stabilisation of tautomers. Inter-conversion of selected tautomers was investigated both in the gas phase and in four different media. Results showed that when solvent is used as an implicit media, it has negligible effect on proton transfer. Water-explicit effect was also investigated and the results exhibited high dependency of proton transfer mechanism on water molecules when they participate in breaking and formation of bonds. [ABSTRACT FROM AUTHOR]