Study on the Tautomerism of an Indoline Spiropyran in Compressed CO_2 and Ethanol Mixture

Thermodynamic and kinetic properties of the tautomerism of an indoline spiropyran in compressed CO2-ethanol mixture were studied by UV-Vis spectroscopy at 298.15 K, 303. 15 K, and 308.15 K. It was found that the composition of the mixture affects the equilibrium constant and the rate constant significantly.

SYNTHESIS AND TAUTOMERISM OF KETENE N,O-ACETALS WITH BENZOXAZOLINE RING

Benzoxazoline ring substituted ketene N,O-acetals 3 were synthesized by the reaction of ketene S,S-acetals 2 with 2-aminophenol.The tautomeric equilibrium of some 3 was also observed.

A reaction density functional theory study of solvent effects on keto-enol tautomerism and isomerization in pyruvic acid

It is important to study the solvent effect on keto-enol tautomerism that has applications in many areas of chemical engineering.In this work,we use a multiscale reaction density functional theory(Rx DFT)to study the keto-enol tautomerism and isomerization of pyruvic acid.The results show that both effects of solvation and water assistance could reduce the reaction barriers.The water molecule participates the reaction as a catalyst to accept/give the protons with forming a hexagonal ring in the transition state.As a result of this temporary and intermediate hexagonal ring,the solute configuration undergoes a small variation during the reaction,giving a diminished contribution to the intrinsic reaction free energy.The solvent distribution shows a local ordering behavior near the solute that also reduces the contribution of solvation effect to the reaction barrier.Water assistance plays a major role in both pre-reaction and postreaction process.In terms of the driving force for the reaction,the effects of both solvation and water assistance are important.

Ab Initio Study on the Tautomerism of Thioformamide Between Its Thione and Thiol Forms

Introduction Thioformamide plays an important role in the organic and biochemical processes. Up to now, there are some reports on the static state study of its thione and thiol form tautumors, but few papes on the mechanism of tautomerism between these two tautomors. Since this tautomerism equilibrium may model the proton shift isomerization of thionucleic acid bases approximately, the theoretical study on the mechanism and dynamics of such a process has important significance.

Cosolvent Effect on the Tautomerism of Ethyl Acetoacetate in Supercritical CO_2

The effect of cosolvent cyclohexane, chloroform, and acetonitrile on the keto-enol tautomeric equilibrium of ethyl acetoacetate in supercritical CO2 was studied by UV-Vis spectroscopy over the pressure range from 76 to 110 bar at 308.15 K. It was found that the equilibrium constant decreases with increasing polarity of the cosolvents.

Thiosemicarbazones Synthesized from Acetophenones: Tautomerism, Spectrometric Data, Reactivity and Theoretical Calculations

Tautomeric forms of Thiosemicarbazones have been investigated by spectrometric methods, their chemical reactivity and theoretical calculations of the relative tautomers stabilities. The mass spectral fragmentation of thiosemicarbazones synthesized from acetophenones has been studied by CG/MS. The analysis of the corresponding spectra shows not only the regular fragmentation mechanisms but homolytic ruptures from even-electron species. 1H NMR spectra exhibit signals for the most intense open thioketo tautomeric structure, although when using TFA a ring structure is observed in the corresponding tautomeric equilibrium. Density Functional Theory calculations (DFT) also provide evidence to support the experimental observations by GC-MS and 1H NMR. Methylation reactions give support to the occurrence of the thioenol tautomeric form which would be the second most abundant according to the Density Functional Theoretical calculations.

Covalent-organic frameworks with keto-enol tautomerism for efficient photocatalytic oxidative coupling of amines to imines under visible light

Photocatalytic oxidation of organic molecules into highly value-added products is an innovative and challenging research which has gradually attracted remarkable attention of scientists.In this work,it is demonstrated that the COF-TpPa with keto-enol tautomerism equilibrium structure shows excellent performance(yield>99%after 8 h)in the selective photocatalytic oxidative coupling of amines to imines under visible light irradiation.It is revealed that three kinds of reactive oxygen species(superoxide radical,hydroxyl radical and singlet oxygen)participate in this photocatalytic oxidation reaction.In addition,hydrogen protons cleaved from the benzyl are proven to be reduced to hydrogen in the conduction band of COF-TpPa in anaerobic atmosphere,accompanied with the formation of imines.The direct hydrogen evolution from amine provides an effective way to extract clean energy from organic molecule as well as the production of value-added chemicals.As a contrast,COF-LZU1 with similar structure and chemical composition to COF-TpPa but without keto-enol tautomerism exhibits worse optical properties and photocatalytic performance.It is also demonstrated that keto-enol tautomerism favors the adsorption of benzylamine based on the characterization results and theoretical calculations.

Mapping out the reaction network of humin formation at the initial stage of fructose dehydration in water

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 a-furanose at 130-150℃ leads to C11 path,and that of open-chain fructose at 180℃ 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 AlCl_(3) catalyst,and both the C12 and C110 paths could be effectively inhibited by adding THF as a co-solvent or accelerating heating rate via microwave heating.

Influence of Temperatures on the Tautomerism of the N-（1H- Imidazoline-2-yl）-lH-benzimidazol-2-amine and Investigation of Its Electrochemical Behaviour

Synthesis of N-（1H-imidazoline-2-yl）-lH-benzimidazol-2-amine was carried out under microwave irradiation （MWI） conditions. Dynamic 1H NMR investigation of N-（lH-imidazoline-2-yl）-lH-benzimidazol-2-amine compound was reported at temperature range of 223--333 K in DMF-d7. Some physical parameters, such as coalescence temperature （Tc）, the free energy of activation （△G＊） and rate constant （k） values were calculated from its IH NMR spectra at various temperatures. Electrochemical feature of this compound was investigated by cyclic （CV） and square wave voltammetry （SWV）.

Applying density functional theory on tautomerism in 3,4- dihydropyrimidin-2(1H)-ones

In the present study,the density functional theory(DFT)and Gibbs free energy calculations were performed to investigate the stability and tautomerism of C4-substituted-3,4-dihydropyrimidin-2(1H)-ones.Three different forms are possible for the ethyl 3,4-dihydropyrimidinones(ethyl 4-aryl-6-methyl-3,4-dihydropyrimidin-2(1H)-one-5-carboxylates,ethyl 4-aryl-2-hydroxy-6-methyl-1,4-dihydropyrimidine-5-carboxylates and ethyl 4-aryl-2-hydroxy-6-methyl-3,4-dihydropyrimidine-5-carboxylates)forms that the most stable form is ethyl 4-aryl-6-methyl-3,4-dihydropyrimidin-2(1H)-one-5-carboxylates(keto-form).The obtained data showed that the substitution on the C4-substitut position can be effective on the equilibrium constant(K_(eq)).

Outstanding Humidity Chemiresistors Based on Imine-Linked Covalent Organic Framework Films for Human Respiration Monitoring

Human metabolite moisture detection is important in health monitoring and non-invasive diagnosis.However,ultra-sensitive quantitative extraction of respiration information in real-time remains a great challenge.Herein,chemiresistors based on imine-linked covalent organic framework(COF)films with dual-active sites are fabricated to address this issue,which demonstrates an amplified humidity-sensing signal performance.By regulation of monomers and functional groups,these COF films can be pre-engineered to achieve high response,wide detection range,fast response,and recovery time.Under the condition of relative humidity ranging from 13 to 98%,the COFTAPB-DHTA film-based humidity sensor exhibits outstanding humidity sensing perfor-mance with an expanded response value of 390 times.Furthermore,the response values of the COF film-based sensor are highly linear to the relative humidity in the range below 60%,reflecting a quantitative sensing mechanism at the molecular level.Based on the dual-site adsorption of the(-C=N-)and(C-N)stretching vibrations,the revers-ible tautomerism induced by hydrogen bonding with water molecules is demonstrated to be the main intrinsic mechanism for this effective humidity detection.In addition,the synthesized COF films can be further exploited to effectively detect human nasal and oral breathing as well as fabric permeability,which will inspire novel designs for effective humidity-detection devices.

Revelation of bimolecular tautomerization induced by the concerted and radical interactions in lignin pyrolysis

Bimolecular interactions play crucial roles in lignin pyrolysis.The tautomerization of key intermediates has a significant impact on the formation of stable products,whereas bimolecular tautomerization has been rarely clarified.In the present work,the bimolecular tautomerization mechanism induced by both concerted and radical interactions was proposed and carefully confirmed.A characteristicβ-O-4 lignin dimer,2-phenoxy-1-phenylethanol(α-OH-PPE),was used as the model compound to reveal two representative keto-phenol and enol-keto tautomerism mechanisms,based on theoretical calculations combined with pyrolysis experiments.The results indicate that the unimolecular tautomerism as the rate-determining step limits product generation,due to fairly high energy barriers.While the free hydroxy compounds and radicals derived from initial pyrolysis can further initiate bimolecular tautomerism reactions through the one-step concerted hydroxyl-assisted hydrogen transfer(hydroxylAHT)and two-step radical hydrogen abstraction interactions,respectively.By alleviating and even avoiding the large ring tension of tautomerism,the unstable tautomers(2,4-cyclohexadienone and1-hydroxy styrene)can be rapidly tautomerized into stable phenol and acetophenone with the help of intermolecular interaction.Benefitting from the significant advantage of retro-ene fragmentation in breaking theβ-O-4 bond to form tautomers,a large amount of stable phenolic and ketone products can be generated following bimolecular tautomerization in the pyrolysis ofβ-O-4 linked lignin.

Computational Studies on the Ground State Tautomer,Hydrogen Conformations and Vibrational Spectroscopic Analysis of Antitumor Agents:3-Deazauracil and 6-Azauracil

The ground state hydrogen conformations and vibrational analysis of 3-deazauracil(3DAU)and 6-azauracil(6AU)tautomers(4-enol and 2,4-diol forms)have been calculated using ab initio Hartree-Fock(HF)and density functional theory(B3LYP)methods with 6-311++G(d,p)basis set level.The calculations have shown that the most probably preferential tautomer of 3DAU and 6AU are the 4-enol form,which gives best fit to the corresponding experimental data.The ground state conformer of the 2,4-diol form has two O—H bonds which are oriented externally and internally(to the N—H bond).The vibrational analyses of the ground state conformer of each tautomeric form of 3DAU and 6AU were done and their optimized geometry parameters(bond lengths and bond angles)were given.Furthermore,from the correlations values it was concluded that the B3LYP method is superior to the HF method for both the vibrational frequencies and the geometric parameters.

Irbesartan desmotropes:Solid-state characterization,thermodynamic study and dissolution properties

Irbesartan (IBS) is a tetrazole derivative and antihypertensive drug that has two interconvertible structures, 1H- and 2H-tautomers. The difference between them lies in the protonation of the tetrazole ring. In the solid-state, both tautomers can be isolated as crystal forms A (1H-tautomer) and B (2H-tautomer). Studies have reported that IBS is a polymorphic system and its forms A and B are related monotropically. These reports indicate form B as the most stable and less soluble form. Therefore, the goal of this contribution is to demonstrate through a complete solid-state characterization, thermodynamic study and dissolution properties that the IBS forms are desmotropes that are not related monotropically. However, the intention is also to call attention to the importance of conducting strict chemical and in solid-state quality controls on the IBS raw materials. Hence, powder X-ray diffraction (PXRD) and Raman spectroscopy (RS) at ambient and non-ambient conditions, differential scanning calorimetry (DSC), hot stage microscopy (HSM), Fourier transform infrared (FT-IR) and scanning electron microscopy (SEM) techniques were applied. Furthermore, intrinsic dissolution rate (IDR) and structural stability studies at 98% relative humidity (RH), 25℃ and 40 ℃ were conducted as well. The results show that in fact, form A is approximately four-fold more soluble than form B. In addition, both IBS forms are stable at ambient conditions. Nevertheless, structural and/or chemical instability was observed in form B at 40℃ and 98% RH. IBS has been confirmed as a desmotropic system rather than a polymorphic one. Consequently, forms A and B are not related monotropically.

Trans-dinitroglycoluril isomers-A DFT treatment

Isomers of trans-1,4-Dinitroglycoluril(trans-DINGU) and their 1,3-tautomers are considered within the constraints of B3LYP/6-31ttG(d,p) and B3LYP/CC-PVTZ levels of DFT calculations. Additionally, the interactions of these isomers and proton in vacuum are investigated. The data have revealed that two of the three isomers undergo C — H bond cleavage as the result of interaction with proton in vacuum. The total energies, some structural properties, the calculated IR and UV spectra are discussed.

Interaction of TATB with Cu and Cu^(+1). A DFT study

Symmetric triaminotrinitrobenzene known as TATB decomposes in contact with some heavy metals,such as copper. Presently, TATB and some of its important tautomers are subjected to B3LYP/6-31G(d,p) level of treatment. Some molecular orbital energies have been obtained and compared. Then. interaction of these species with Cu and Cu^+ are investigated. Although TATB has been found to be affected little by either Cu or Cu^+, undergoing only some bond length changes and/or distortions, the resonance-assisted tautomers having one to three aci groups decompose by the influence of Cu atom. In each case, only one N-OH bond rupture occurs.

A Quantitative Study of Solvent Effects on Keto-Enol Tautomeric Equilibria

Introduction Organic compounds usually have labile bonds. It is well known that thousands of reactions are carried out in solutions, only a few in the gas-phase. Most chemists are now familiar with the fact that solvent may have a strong influence on reaction rate and equilibrium, so in the last decade, resarching the influence of solvent effects on reaction rate and reaction equilibrium was a very important and

What Is Its Real Existing Form?——Theoretical and Experimental Studies on 2-Mercaptobenzothiazole

The geometry of 2-mercaptobenzothiazole （MBT） and its tautomeric form of benzothiazole-2-thione are optimized at B3LYP/6-311 G＊＊ and HF/6-311 G＊＊ levels, respectively. The crystal structure of benzothiazole-2-thione and its FFIR spectra are also obtained. From the calculated and experimental data, it can be concluded that in gas-phase and solid-state, the real existing form of 2-mercaptobenzothiazole is the thione-form of MBT.

Solid and liquid state characterization of tetrahydrocurcumin using XRPD, FT-IR, DSC, TGA, LC-MS, GC-MS, and NMR and its biological activities

Tetrahydrocurcumin(THC)is one of the major metabolites of curcumin(CUR),an ancient bioactive natural polyphenolic compound.This research article describes both the solid and liquid state characterization of THC using advanced spectroscopic and thermo-analytical techniques.Anti-inflammatory,anti-oxidant,and neuroprotective activities of THC were investigated using in vitro cell lines.Liquid chromatography-mass spectrometry analysis revealed that our sample comprised 95.15%THC,0.51%tetrahydrodemethoxycurcumin(THDC),3.40%hexahydrocurcumin,and 0.94%octahydrocurcumin.Gas chromatography-mass spectrometry analysis indicated the presence of 96.68%THC and 3.32%THDC.THC in solution existed as keto-enol tautomers in three different forms at different retention time,but the enol form was found to be dominant,which was also supported by nuclear magnetic resonance analysis.THC was thermally stable up to 335.55
C.THC exhibited more suppression of cytokines(TNF-a,IL-1b,and MIP-1a)than CUR in a concentration-dependent manner in mouse splenocytes,while NK-cell and phagocytosis activity was increased in macrophages.THC showed a significant reduction of free radicals(LPO)along with improved antioxidant enzymes(SOD and catalase)and increased free radical scavenging activity against ABTSt radicals in HepG2 cells.THC displayed higher protection capability than CUR from oxidative stress and neuronal damage by improving cell viability against H2O2 induced HepG2 cells and MPPt induced SH-SY5Y cells,respectively,in a concentration-dependent manner.Thus,a variation of the biological activities of THC might rely on its keto-enol form and the presence of other THC analogs as impurities.The present study could be advantageous for further research on THC for better understanding its physicochemical properties and biological variation.

Theoretical Study on the Tautomerization Reaction of Formamide to Formamidic Acid Encapsulated in a Nano-sized Molecular Container

The hybrid calculations with ONIOM（B3LYP/6-31G＊：AM1） method were carried out on the tautomerization reaction of formamide to formamidic acid in the microcontainer-encapsulated state. The free-state tautomerization process was also investigated with B3LYP/6-31G＊＊//B3LYP/6-31G＊ method for the purpose of comparison. Bare tautomerization, H20-assisted（single-H20 or multiple-HaO） and self-assisted mechanisms were all taken into consideration for the encapsulated state. The results show that only bare tautomerization and single-H2O catalysis mechanisms are possible to the encapsulated formamide tautomerization owing to the container＇s size confinement effect. Geometrical changes in the complexed container and guest molecules are discussed to deeply understand the complex＇s structural properties. The bare tautomerization barrier in the encapsulated state increases by 23.826 kJ/mol, accounting for 12% of the corresponding total energy barrier in the free state, and the increased values for the single-H2O catalysis process are 12.958 kJ/mol, accounting for 16% of the corresponding total energy barrier, respectively. This finding suggests that the encapsulation can make the tautomerization process slightly difficult.