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.

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.

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

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.

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.

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.

Explanation of Tautomerism and Isomerization in Terms of the Magnetic Field

A study of the influence of magnetic field on the tautomerism, the authors proposed a new method for predicting possible new intra- and intermolecular rearrangements due to tautomerism. The authors have proposed a general mechanism of possible rearrangements and tautomerism.

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.

ab Initio Calculation on Tautomerism of Pyrazolin-5-ones

The relative stability of tautomers on pyrazolin-5-ones in gas phase and in solution is studied theoretically by STO-6-31G** +CI ab initio method and the reaciton field continuum model. the results obtained in our work are more reasonable than those by other theoretical methods and agree well with experiments.

UV-A紫外线吸收剂阿伏苯宗的研究进展

阿伏苯宗是一种安全、有效的防晒剂,也是使用最多的UV-A紫外线吸收剂,它可以阻断广谱UV-A对皮肤的伤害,被广泛应用于防晒霜、化妆品、乳液等护肤产品。但是,在太阳光(紫外光)的辐照下,阿伏苯宗很容易发生异构化和光降解反应生成多种不能吸收紫外线的产物,显著降低其防护能力,而且部分光降解产物可能对人类健康和环境造成伤害。因此,阿伏苯宗的光化学和光物理性质被研究者们进行了大量的研究。总结了近20年来阿伏苯宗的研究进展,重点阐述了阿伏苯宗的结构特征,影响阿伏苯宗稳定性的关键因素,以及阿伏苯宗对生态环境的影响。同时,指出目前阿伏苯宗研究中存在的问题,并对其未来的研究方向提出思考和展望,为后续研发高效、光稳定的防晒产品提供依据和参考。

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.

Facile Blinking Dyes with Spontaneous Enol-Keto Tautomerism for Super-Resolution Imaging of Subcellular Targets

Fluorophores with controlled on–off switching properties and subcellular targeting abilities are essential for modern super-resolution microscopy techniques.However,the effective design strategies of blinking fluorophores are still limited with scarce building blocks(e.g.,rhodamine and cyanine analogues),and sophisticated functionalization is always required for organelle recognition.Therefore,exploring live-cell compatible and organelle-targetable fluorophores with a new blinking mechanism is of broad interest to promote the emerging development of superresolution microscopy.Here,we report a type of live-cell permeable fluorophore with a facile synthesis strategy to facilitate the imaging of lipid droplets and lysosomes using single-molecule localization microscopy.The fluorophores only require a two-step synthetic route.They show signature enol-keto tautomerism by spontaneous proton transfer at the ground state without additional photoactivation,which ensures blinking performance by exciting the minor portion of molecules in keto form.This fluorescence switching mechanism provides a distinct character from that of previously reported live-cell permeable dyes for super-resolution imaging,while the specific organelle binding affinity can be tuned by simply changing the targeting moiety of fluorophores.

Transformations and Tautomeric Equilibrium among Different Intermediates in Proline-Catalyzed Reactions of Aldehydes or Ketones

The enamines, iminium ions, and oxazolidinones are thought to be the key intermediates in the proline-catalyzed reactions of aldehydes or ketones, but there is an extensive contro- versy about their roles. Here, the corresponding transition states connecting any two of the three kinds of species are located at the wB97XD/6-311＋＋G＊＊ level of theory. The calcula- tions demonstrate that the oxazolidinones are the predominant species in both the gas phase and solvents; there exists tautomeric equilibrium among these species and the equilibriums are controlled by the employed solvents and temperature in the reaction. These results demonstrate that the concentration and role of the mentioned species are controlled by the employed solvent and temperature. A new reaction pathway is presented herein for the trans- formation between iminium ions and oxazolidinones through iminium ion-water complex and oxazolidinone-water complex. The calculations demonstrate that the rate-limiting step in proline-catalyzed Mannich reaction between acetaldehyde/keteones and N-Boc imines is the formation of the C-C bond rather than the intermediates tautomerization. These calculations rationalize the available experimental observations and can be valuable in optimizing the experimental conditions of asymmetric organic-catalyzed reactions of aldehydes or ketones.

核磁共振氢谱多维度探究1,3-二羰基化合物的酮-烯醇互变异构规律

针对大学有机化学实验“核磁共振氢谱测定乙酰乙酸乙酯互变异构体”存在内容单一、教学深度与广度不足等问题进行了改进创新。实验内容由认识酮-烯醇互变异构的存在,拓展到探究时间、取代基以及溶剂三个维度对β-二羰基化合物的酮-烯醇互变异构平衡的影响。此外,在研究甲醇溶剂对平衡的影响时,可以观测到氢氘交换现象,有助于进一步理解异构过程的机理。本实验可针对化学相关专业高年级本科生开展,有助于加深学生对酮性质的理解,掌握核磁共振波谱在有机化学中的应用,从而巩固有机化学和仪器分析的理论知识,以及培养学生分析问题、解决问题的能力。

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.

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.

Potential Energy Surface of Cytosine and Tunneling Between Its Normal and Trans-imino Tautomer

The potential energy surface of H（13） proton in base cytosine of the DNA molecules is calculated at the Caussian 98 MP2/6-311C（d,p） level. Two potential wells are found. One corresponds to the normal cytosine, while the other corresponds to its trans-imino tautomer. The estimated tunneling probability of the H（13） proton from one well to another well shows that the life time of the proton staying in one of these wells is about 600 yrs. It is too long to let tautomers of cytosine be in thermodynamical equilibrium in a room temperature gas phase experiment.