烷基噻吩在MoS2团簇上吸附行为的密度泛函理论研究

运用密度泛函理论(DFT),采用Mo16S32团簇模型,在PW91/DNP水平上研究了噻吩(TP)及一系列烷基噻吩类硫化物如2-甲基噻吩(2-MT)、3-甲基噻吩(3-MT)、2,3-二甲基噻吩(2,3-DMT)、2,4-二甲基噻吩(2,4-DMT)、2,5-二甲基噻吩(2,5-DMT)及3,4-二甲基噻吩(3,4-DMT)等在加氢脱硫催化剂MoS2上的吸附行为.结果表明,在η1S吸附构型中,Mo16S32团簇对烷基噻吩吸附能力的顺序为2,5-DMT>2,4-DMT≈2,3-DMT>2-MT>3,4-DMT>3-MT>TP.通过键长、Mayer键级、Mulliken电荷分析可知,当噻吩环的2-或5-位不含甲基时,吸附能随硫原子电荷密度的增加而增大;2-或5-位含甲基时,甲基与团簇上相邻的Mo原子发生了弱的相互作用,使吸附能增大;虽然2,5-DMT的2-和5-位均含有甲基,但甲基离团簇上相邻的Mo较远,相互作用较小,吸附能较2,3-DMT和2,4-DMT增加的较少.文中还对各硫化物在MoS2催化剂上的加氢脱硫反应进行了讨论.     

黄嘌呤异构体质子转移异构化反应机理的理论研究

采用量子化学密度泛函B3LYP/6-31G(d,p)和M06-2X/6-311++G(d,p)方法对黄嘌呤两种酮式异构体X(1,3,7)与X(1,3,9)间质子转移引起的互变异构反应机理进行了计算研究,获得了异构化反应过程的反应焓﹑活化吉布斯自由能和质子转移反应的速率常数等参数。水相计算采用极化连续(PCM)模型。结果表明,由于可能的氢迁移顺序差异,分子内由X(1,3,7)向X(1,3,9)异构化可能共有16条反应通道,涉及11个中间体和20个过渡态,其主反应通道速控步骤的活化吉布斯自由能为183.10k J/mol,速率常数为5.17×10-20s-1,其余各通道速控步骤活化吉布斯自由能均较高,而且整体水溶剂效应不利于质子转移的发生。     

2,4-Ditellurouracil and its 5-fluoro derivative: Theoretical investigations of structural,energetics and ADME parameters

2,4-Ditellurouracil exhibits keto-enol tautomerism via different pathways resulting in seven tautomers. These pathways were studied in the gas phase using density functional theory method. The functionals used were BLYP, B3LYP and BHLYP and the basis sets were 6–311++G(d,p) for all atoms except that LanL2DZ ECP was used for tellurium atom only. The results indicate that the diketo form is more stable as observed for uracil and its sulfur and selenium analogues. The effect of introducing fluorine at position 5 was also investigated and the energy difference between the diketo and dienol forms is reduced. 2,4-Ditellurouracil and its 5-fluoro analogue are expected to exist exclusively as the diketo form due to the high interconversion energy barrier. We extended the investigation to predict ADME parameters of the most stable diketo and dienol tautomers in view of understanding their biological properties. This research enlightens keto-enol tautomerism of 2,4-ditellurouracil and its 5-fluoro derivative with additional insights to biological functions.     

Theoretical Studies on the Binding of Cd~（2＋） to Uracil and Its Thio-derivatives

The interaction of Cd2+ with uracil,2-thiouracil,4-thiouracil and 2,4-dithiouracil have been investigated by the density functional theory(DFT) calculations.For uracil and 2,4-dithiouracil,where the two basic sites are the same,Cd2+ attachment to the heteroatom at position 4 is preferred.However,for the systems where both types of basic centers,a carbonyl and a thiocarbonyl groups,are present,Cd2+ association with sulfur is favorable.The enhanced stability of these enolic and thiol forms comes from Cd2+ interaction with two basic sites simultaneously,which thereby triggers a significant aromatization of the ring.More significantly,the Cd2+ binding energy with uracil and its thio-derivatives is larger than the tautomerization barriers connecting the diketo-like forms with the corresponding enolic-like tautomers.Consequently,when associated with Cd2+,all tautomers are energetically accessible and should be observed in the gas phase.     

Ab initio study of the tautomeric forms of some quinolinediones

7,8-dihydroquinoline-4,5 (1H,6H)-dione (1) and 7,8-dihydroquinoline-2,5-(1H,6H)-dione (2) in their tautomeric oxo and hydroxy forms have been studied by ab initio Hartree-Fock calculations; tautomerization energies predict a more stable hydroxy structure having an intramolecular hydrogen bond for compound 1, whereas the oxo form is slightly-preferred for compound 2. Fourier Transform-Infra Red (FT-IR) spectra in CHCl₃ solution indicate that the predicted most stable tautomers in the vapour phase remain as such.     

Temperature effects on the phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals

The phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals have been measured as a function of temperature between 10 and 200 K. For all the guests involved, the vibrational structures of the spectra are found to be temperature dependent. This is interpreted in terms of two emissions that proceed from a triplet state having predominantly a ππ* character at low temperatures and from a thermally populated triplet state having essentially a nπ* character at higher temperatures. The energy gaps ΔE_T between ³ππ* and ³nπ* states evaluated spectroscopically are found to be 100, 70, and 340 cm^?1, respectively for 2,4-, 2,5- and 3,4-dimethylbenzaldehydes.Activation energies ΔE* determined from the Arrhenius plots of the phosphorescence decay rate constants are in good agreement with the ΔE_T for the first two guests. In contrast, the ΔE* are higher than the ΔE_T for 3,4-dimethylbenzaldehydes as well as for 2,4,5-trimethylbenzaldehyde (where ΔE_T ≈ 400 cm^?1) because of the rapid increase of radiationless transitions in the temperature range where thermal population of the upper ³nπ* state is efficient. In the low and high temperature ranges, the phosphorescence decays for all these guests are exponential. In the intermediate range, these decays are non-exponential. The origin of these non-exponential decays is discussed.     

Theoretical investigation of solvent effects on tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol

The density functional theory has been used to study the tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol(DDNP) in the gas phase and in 14 solvents at the B3LYP/6‐31G* level. The solvent effects on the tautomeric equilibria were investigated by the self‐consistent reaction field theory (SCRF) based on conductor polarized continuum model (CPCM) in apolar and polar solvents and by the hybrid continuum‐discrete model in protic solvent, respectively. Solvent effects on the computed molecular properties, such as molecular geometries, dipole moments, E_LUMO, E_HOMO, total energies for DDNP tautomers and transition state, tautomerization energies and solvation energies have been found to be evident. The tautomeric equilibrium of DDNP is solvent‐dependent to a certain extent. The tautomer I (cyclic azoxy form) is preferred in the gas phase, while in nonpolar solvents tautomer I and II (quinold form) exist in comparable amounts, and in highly polar solvents, the tautomeric equilibrium is shifted in favor of the more polar tautomer II . © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Interaction of Ca2+ with uracil and its thio derivatives in the gas phase

The structures and relative stabilities of the complexes formed by uracil and its sulfur derivatives, namely, 2-thio-, 4-thio, and 2,4-dithio-uracil when interacting with Ca(2+) in the gas phase have been analyzed by means of density functional theory (DFT) calculations carried out at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) level. For uracil and 2,4-dithiouracil, where the two basic sites are the same, Ca(2+) attachment to the heteroatom at position 4 is preferred. However, for the systems where both types of basic centers, a carbonyl or a thiocarbonyl group, are present, Ca(2+)-oxygen association is favored. The most stable complexes correspond to structures with Ca(2+) bridging between the heteroatom at position 2 of the 4-enol (or the 4-enethiol) tautomer and the dehydrogenated ring nitrogen, N3. The enhanced stability of these enolic forms is two-fold, on the one hand Ca(2+) interacts with two basic sites and on the other triggers a significant aromatization of the ring. Besides, Ca(2+) association has a clear catalytic effect on the tautomerization processes which connect the oxo-thione forms with the enol-enethiol tautomers. Hence, although the enol-enethiol tautomers of uracil and its thio derivatives should not be observed in the gas phase, the corresponding Ca(2+) complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Ca(2+) binding energies.     

Towards the comprehensive, rapid, and accurate prediction of the favorable tautomeric states of drug-like molecules in aqueous solution

Generating the appropriate protonation states of drug-like molecules in solution is important for success in both ligand- and structure-based virtual screening. Screening collections of millions of compounds requires a method for determining tautomers and their energies that is sufficiently rapid, accurate, and comprehensive. To maximise enrichment, the lowest energy tautomers must be determined from heterogeneous input, without over-enumerating unfavourable states. While computationally expensive, the density functional theory (DFT) method M06-2X/aug-cc-pVTZ(-f) [PB-SCRF] provides accurate energies for enumerated model tautomeric systems. The empirical Hammett–Taft methodology can very rapidly extrapolate substituent effects from model systems to drug-like molecules via the relationship between pK_T and pK_a. Combining the two complementary approaches transforms the tautomer problem from a scientific challenge to one of engineering scale-up, and avoids issues that arise due to the very limited number of measured pK_T values, especially for the complicated heterocycles often favoured by medicinal chemists for their novelty and versatility. Several hundreds of pre-calculated tautomer energies and substituent pK_a effects are tabulated in databases for use in structural adjustment by the program Epik, which treats tautomers as a subset of the larger problem of the protonation states in aqueous ensembles and their energy penalties. Accuracy and coverage is continually improved and expanded by parameterizing new systems of interest using DFT and experimental data. Recommendations are made for how to best incorporate tautomers in molecular design and virtual screening workflows.     

22-hydroxybenziporphyrin: switching of antiaromaticity by phenol-keto tautomerization

22-hydroxybenziporphyrin, a porphyrin analogue containing a phenol moiety, has been shown to exist as an equilibrium mixture of two distinctly different tautomers. One of them actually contains the hydroxy group and shows the local [6]annulene aromaticity in the phenol fragment. The other tautomer contains a keto group and exhibits a [20]annulenoid structure characterized by macrocyclic antiaromaticity. The tautomerization process has been investigated in detail using variable-temperature 1H NMR spectroscopy. The process is very fast, with an estimated activation energy of ca. 30 kJ/mol. Further insight into the energetics of the tautomerization is obtained from density functional (DFT) calculations. Surprisingly, the estimated energy of the antiaromatic keto species is 3-5 kcal/mol lower than the energy of the phenolic tautomer. The geometric and magnetic manifestations of aromaticity and antiaromaticity in the two tautomers are probed using a number of computational devices, including Wiberg bond indices, resonace weights derived from the harmonic oscillator model, and nucleus-independent chemical shifts. It is shown that mixing of phenolic and keto contributions in both tautomers is stronger than that in related tautomers of phenol. This effect is caused by extensive conjugation with the tripyrrolic unit of 22-hydroxybenziporphyrin and, to a lesser extent, by intramolecular hydrogen bonding.     

Adsorption Properties of Thiocontaining Schungite

Chromatography–mass spectrometry and thermodesorption mass spectrometry have been employed to study mineral schungite-III, in which various thiocompounds have been detected and identified. The influence of these compounds on the adsorption activity of schungite with respect to iodine has been investigated. It has been shown that sulfur present in schungite has no effect on the results of determining its adsorption activity and does not interact with iodine; however, it can interact with amino compounds to yield sulfides. Activation energies E_a have been experimentally determined for sulfur, iodine, and dimethyl disulfide desorption from the surfaces of schungite and a model sorbent, graphitized thermal carbon black. The E_a values of these compounds have appeared to be several times lower than the heats of their adsorption on carbon black calculated by the molecular-statistical method.     

Pd-catalyzed asymmetric hydrogenation of fluorinated aromatic pyrazol-5-ols via capture of active tautomers

An efficient palladium-catalyzed asymmetric hydrogenation of fluorinated aromatic pyrazol-5-ols has been developed via capture of the active tautomers. A wide variety of 2,5-disubstituted and 2,4,5-trisubstituted pyrazolidinones have been synthesized with up to 96% and 95% ee, respectively. The hydrogenation pathway includes Brønsted acid promoted tautomerization of pyrazol-5-ols and Pd-catalyzed asymmetric hydrogenation of the active tautomer.     

Effect of Sr2+ association on the tautomerization processes of uracil and its dithio- and diseleno-derivatives

The structures and relative stabilities of the complexes formed by uracil and its thio- and seleno-derivatives with the Sr(2+) cation, in the gas phase, have been analyzed by means of G96LYP density functional theory (DFT) calculations. The attachment of the Sr(2+) cation to the heteroatom at position 4 is preferred systematically. Although the enolic forms of uracil and its derivatives should not be observed in the gas phase, the corresponding Sr(2+) complexes are the most stable. The enhanced stability of these tautomers is two-fold, on the one hand Sr(2+) interacts with two basic sites simultaneously, and on the other hand an aromatization of the six-membered ring takes place upon Sr(2+) association. Sr(2+) attachment also has a clear catalytic effect in the tautomerization processes involving uracil and its derivatives. This catalytic effect increases when oxygen is replaced by sulfur or selenium. The Sr(2+) binding energy with uracil and its derivatives is bigger than the tautomerization barriers connecting the dioxo forms with the corresponding enolic tautomers. Consequently, when associated with Sr(2+), all tautomers are energetically accessible and should all be observed in the gas phase.     

Density functional theory of tautomerism and water-assisted proton transfer of glycoluril

Density functional theory and MP2 methods have been employed to study of proton transfer reaction in annular tautomerization of tetrahydroimidazo[4,5-d]imidazole-2,5(1H,3H)-dione (glycoluril). Ten different tautomers are possible for the tetrahydroimidazo[4,5-d]imidazole-2,5(1H,3H)-dione. For all molecules, the Gibbs free energy at 0 and 298 K was estimated. In addition variation of dipole moments and charges on atoms are studied in the gas phase and solution, the specific solvent effects with addition of one molecule of water near the electrophilic centers of tautomer and the NBO charges of atoms were investigated. NBO analysis shows that there is a strong interaction between nitrogen lone pairs and double bonds.     

Synthesis of derivatives of cyclobuteno[c]thiophen and attempts to synthesise thiophen analogues of biphenylene

Starting from 2,5-dichlorothiophen many new 3,4-disubstituted thiophens have been made including the dialdehyde. Treatment of this with phosphorus pentabromide could be made to give 2,5-dichloro-3- dibromomethylthiophen-4-aldehyde, 2,5-dichloro- or 2-bromo-5-chloro-3,4-bisdibromomethylthiophen. the latter two compounds reacted with sodium iodide to give the corresponding cis- and trans-3,4-dibromo cyclobuteno[c]thiophen derivatives (18–21). Reaction of trans-3,4-dibromo-2,5-dichlorocyclobuteno[c]thiophen with N-bromosuccinimide resulted in fission of the 4-membered ring regenerating 2,5-dichloro-3,4-bisdibromomethylthiophen.When 2,5-dichloro-3,4-di-iodothiophen was treated with butyl-lithium followed by dimethylsulphate it gave 2,5- dichloro-3,4-dimethylthiophen together with a low yield of a red compound which is considered to be a dichloro dimethyl derivative (11) of bisthiophenindigo.Several unsuccessful approaches to the synthesis of dithiophen analogues of biphenylene are outlined. These included heating various 3,4-di-iodothiophens and 4,4′-di-iodo-3,3′-bithienyls with copper as well as the flash vacuum pyrolysis of the latter compounds, of thiophen-3,4-dicar?ylic anhydrides and of 2,5-dichloro-3-iodothiophen-4-car?ylic acid.     

Tautomer interconversion of 2,4-pentanedione during gas chromatography on an oxidized cyano-modified capillary column.

The tautomerization of 2,4-pentanedione (acetylacetone) is examined on a microbore column containing an acid-modified stationary phase made by oxidizing a commercially available cyano-modified column. This stationary phase is found to provide separation of the two tautomers, which allows the kinetic and thermodynamic properties of the on-column interconversion to be investigated. The enol-to-keto tautomerization is found to occur primarily in the stationary phase, being enthalpically driven. By treating the column as a reactor, the interconversion is investigated as a function of temperature. Monitoring the loss of the more gas-stable 'enol' tautomer makes it possible to extract an energy of activation for the net tautomerization (42.7 kj/mol), because the reaction is found to obey pseudo first-order kinetics. Simple peak-shape analysis of the major component (enol), which is used commonly in treatments of peak tailing, provides insight into the nature of the retention processes of the two tautomers as well as information on chromatographic optimization.     

Synthesis,characterization and thermal decomposition of copper(II), nickel(II) and cobalt(II) complexes of 3-amino-5-methylpyrazole Schiff-bases

Copper, nickel and cobalt complexes of Schiff-bases, obtained by condensation of 3-ammo-5-methylpyrazole with salicylaldehyde; 2,3-dilhydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, and 2-hydroxynaphthaldehyde, were synthesized and characterized. Analytical data and electrical conductivity measurements indicate the formation of 1:1 and 1:2 complexes. Magnetic moments and spectral properties support O_h, T_d, penta-coordinate and square planar structures for the complexes. T.g.a., d.t.g., d.t.a. and d.s.c. studies on the formed complexes show three and four decomposition steps. Complexes obtained by direct reaction between the metal salt and the corresponding Schiff-base have magnetic moments, stereochemistry and thermal properties which are different from their analogues obtained by template synthesis. Activation energies E_a and enthalpies ΔH, associated with thermal decomposition of the complexes, were also calculated. Some of the complexes have been tested for antimicrobial activity.     

Cyclization of vicinally substituted heterocyclic analogs of 3,4-bis(indol-1-yl)maleimide under the action of protic acids: a quantum chemical study

B3LYP/6-31G(d) density functional quantum chemical calculations of vicinally substituted bis(indol-1-yl)derivatives of 1,5-dihydropyrrol-2-one, furan-2,5-dione, cyclopent-4-ene-1,3-dione, cyclobut-3-ene-1,2-dione, and pyrrolidine-2,5-dione were carried out to study the effect of modification of the maleimide moiety in 3,4-bis(indol-1-yl)maleimides on the direction of intramolecular cyclization under the action of protic acids. Geometric parameters, charge distributions, energy characteristics, and frontier orbital energies of these compounds and the corresponding indoleninium cations were determined. Alternative protonation routes of 3,4-bis(indol-1-yl)-1,5-dihydropyrrol-2-one have been studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1348–1352, July, 2008.     

Evaluation of the origin of conformational and tautomeric preferences in N‐formylformamide – A quantum chemical study

Quantum chemical study of N‐formylformamide (NFF) was carried out at various theoretical levels and the determinate equilibrium conformations were recomputed at the high level ab initio methods such as G2MP2, G2, G3, and complete basis set (CBS)‐QB3. The computational results reveal that the amide resonance and intramolecular hydrogen bonding are two superior factors in determining the most stable conformation of diamide (DA) and amide–imidic (AI) acid tautomers, respectively. The evaluation of hydrogen bond energies predicts that the hydrogen bond (HB( strength of NFF is weaker than the malonaldehyde (MA). But the results of atoms in molecules (AIM(, natural bond orbital (NBO), and geometrical parameters are given a different order, E_HB(NFF) > E_HB(MA). Although the bond average energies of tautomerization process emphasized on more stability of AI tautomer, but our theoretical calculations reveal that the DA conformers are more stable than the AI ones. The population analyses of equilibrium conformations by NBO method also predict that the origin of tautomeric preference is mainly because of the electron delocalization of amide functional group, especially LP(N)→ π*_C?O charge transfer. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Effects of the ionization in the tautomerism of uracil: A reaction electronic flux perspective

The one‐step tautomerization processes of uracil and its radical cation and radical anion have been investigated in the light of the reaction force and reaction electronic flux (REF) formalisms. The relative energies of the different tautomers as well as the corresponding tautomerization barriers have been obtained through the use of the G4 high‐level ab initio method and by means of B3LYP/6‐311+G(3df,2p)//B3LYP/6‐311+G(d, p) calculations. Systematically, the enol radical cations are more stable in relative terms than the neutral, due to the higher ionization energy of the diketo forms with respect to the enolic ones. Conversely, the enol radical anions, with the only exception of the 2‐keto‐N1 anion, are found to be less stable than the neutral. The effects of the ionization are also sizable on the tautomerization barriers although this effect also depends on the particular tautomerization process. The reaction force analysis shows that all reactions are mainly activated through structural rearrangements that initiate the electronic activity. This electronic activity is monitored along the reaction coordinate through the REF that obeys a delicate balance between the acid and basic character of the atoms involved in the hydrogen transfer. © 2015 Wiley Periodicals, Inc.