Quantum chemical study on asymmetric catalysis reduction of imine

The asymmetric catalysis reaction is considered to be an important way by which chiral compounds are generated. Chiral 1,3,2-oxazaborolidine, as an effective asymmetric catalyst, is used widely in the enantioselective reduction of prochiral ketones, imines, and carbon-carbon double bonds[1—3]. Up to now, a number of quantum chemical modeling investigations of the en-antioselective reduction of prochiral ketones with borane catalyzed by chiral oxazaborolidines have been carried out[4—7]. Howe…     

Quantum chemical modelling of the effect of proline residues on peptide conformation

Semiempirical AM1 calculations were performed for quantum chemically optimized minimum-energy conformations of L -alanine oligomers (A)_n at n = 7 and their derivatives containing one, two, or three proline residues at various positions along the peptide chain. The effect of proline residues on the peptide conformation was quantified in terms of the conformational “strain energy” and also analyzed in terms of the spatial compatibility of peptides. The defined “strain energy” corresponds to the transformation of the polyalanine peptide from its minimum conformation to the conformation corresponding to that of the proline-containing peptide. The results of calculations indicate that the “strain effect” of proline residues is additive at all locations along the peptide chain, except at the first and the second positions of its N-terminal part. Also, the regular α-helical polyalanine structure was most significantly altered by the presence of some specific motifs around the proline location in the peptide. This, in turn, has its implications in the prediction of the protein secondary structure, as well as in the design of peptide inhibitors and substrates for enzymes and receptors. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 391–396, 1998     

Schiff bases of pyridoxal and polyallylamine. The formation rate determining step

The apparent rate constants of formation (k₁) and hydrolysis (k₂) of the Schiff bases formed between pyridoxal and polyallylamine has been fitted to a kinetic scheme that involve the different protonated species in the reaction medium and the individual rate constants of formation (k₁ⁱ) and hydrolysis (k₂ⁱ). The (k₁ⁱ) values precludes an acid catalyzed intramolecular process. The effects of hydrophobic medium due to the presence of the macromolecule on the reaction is also discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 1–6, 1998.     

Effect of pH on the electrochemical reduction of some heterocyclic quinones

The reduction of 1,10-phenanthroline-5,6-quinone(I), 5,8-quinolinequinone(II) and 6,7-dichloro-5,8-quinolinequinone(III) was investigated using cyclic voltammetry and coulometry at mercury electrodes and 50% dimethylsulfoxide+water solvent. Each compound is reduced to the corresponding hydroquinone in a diffusion-controlled, reversible two-electron process. The pH-dependence of the reversible potential indicated that the quinone forms were unprotonated, but the hydroquinones could be protonated at the heterocyclic nitrogen atom with pK_a = 5.3 for I and 3.5 for III. Careful analysis of the cyclic voltammetric peak shape revealed that the difference between the standard potentials for the introduction of successive electrons, E₂⁰ ? E₁⁰, was 70 ±20, >100 and 80 ± 20 mV for I–III. Investigation of the pH-dependence of E₁⁰ and E₂⁰ showed that the pK_a of the semiquinone of I was about 8.     

Quantum chemical study on the mechanism of enantioselective reduction of prochiral ketones catalyzed by oxazaborolidines

The ab initio molecular orbital study on the mechanism of enantioselective reduction of 3,3-dimethyl butanone-2 with borane catalyzed by chiral oxazaborolidine is performed. As illustrated, this enantioselective reduction is exothermic and goes mainly through the formations of the catalyst-borane adduct, the catalyst-borane-3,3-dimethyl butanone-2 adduct, and the cata-lyst-alkoxyborane adduct with a B-O-B-N 4-member ring and through the decomposition of the catalyst-alkoxyborane adduct with the regeneration of the catalyst. During the hydride transfer in the catalyst-borane-3,3-dimethyl butanone-2 adduct to form the catalyst-alkoxyborane adduct, the hydride transfer and the formation of the B-O-B-N 4-member ring in the catalyst-alkoxyborane adduct happen simultaneously. The controlling step for the reduction is the transfer of hydride from the borane moiety to the carbonyl carbon of 3,3-dimethyl butanone-2. The transition state for the hydride transfer is a twisted chair structure and the reduction leads to     

Quantum chemical study of the primary step of ozone addition at the double bond of ethylene

The mechanism of the primary step of the interaction between ozone and the double bond of ethylene has been investigated by various methods of quantum chemistry (MP2, QCISD, CCSD, MRMP2) and density functional theory (PBE0, OPTX, CPW91, B3PW91, OLYO, B3LYP, BLYP). The kinetics of two reaction pathways, namely, concerted ozone addition via a symmetric transition state (Criegee mechanism) and nonconcerted ozone addition via a biradical transition state (DeMore mechanism) has been calculated. Both mechanisms are describable well in the single-determinant approximation by the QCISD, CCSD, B3LYP, and PBE0 methods and in the multideterminant approximation by the MRMP2 method. The other methods are less suitable for solving this problem. The calculated data demonstrate that the reaction proceeds via both competing pathways. Rate constant values consistent with experimental data and plausible Criegee-to-DeMore rate constant ratios have been obtained. The concerted addition of ozone to ethylene is significantly more rapid than the nonconcerted addition.     

Quantum chemical and conventional TST calculations of rate constants for the OH + alkane reaction

Reactions of OH with methane, ethane, propane, i-butane, and n-butane have been modeled using ab initio (MP2) and hybrid DFT (BHandHLYP) methods, and the 6-311G(d,p) basis set. Furthermore, single-point calculations at the CCSD(T) level were carried out at the optimized geometries. The rate constants have been calculated using the conventional transition-state theory (CTST). Arrhenius equations are proposed in the temperature range of 250–650 K. Hindered Internal Rotation partition functions calculations were explicitly carried out and included in the total partition functions. These corrections showed to be relevant in the determination of the pre-exponential parameters, although not so important as in the NO₃ + alkane reactions [G. Bravo-Pérez, J.R. Alvarez-Idaboy, A. Cruz-Torres, M.E. Ruíz, J. Phys. Chem. A 106 (2002) 4645]. The explicit participation of the tunnel effect has been taken into account. The calculated rate coefficients provide a very good agreement with the experimental data. The best agreement for the overall alkane + OH reactions seemed to occur when the BHandHLYP geometries and partition functions are used. For propane and i-butane, in addition to the respective secondary and tertiary H-abstraction channels, the primary one has been considered. These pathways are confirmed to be significant in spite of the large differences in activation energies between primary and secondary or primary and tertiary channels, respectively of propane and i-butane reactions and should not be disregarded.     

Quantum chemical analysis of the process of reduction of nitrogen oxide by ammonia

We have carried out a quantum chemical analysis of the electronic structure of the nitroprusside ion [Fe(CN)₅NO]^2– and also the transition complex [Fe(CN)₅NO· NH₃]^2– arising in the course of the reaction of reduction of coordinated nitrogen oxide with a nucleophile (ammonia). We consider the characteristics of the redistribution of electron density in the nitroprusside accompanying the nucleophilic attack. We discuss the role of the central iron ion and the CN-ligands during nucleophilic reduction.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 4, pp. 432–439, July–August, 1989.     

Computational electrochemistry: aqueous two-electron reduction potentials for substituted quinones

The electrode potentials of some quinone derivatives in aqueous solution have been calculated. The calculations are carried out at the Hartree–Fock and B3LYP levels with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The Polarisable Continuum Model (PCM) is used to describe the solvent. The average error of calculation of electrode potentials is less than 0.03 V and is decreased compared to the average error of methods presented previously. The role of relaxation energies and frequency calculations in improving the results has been investigated.     

手性含硫恶唑硼烷催化芳酮不对称还原反应的量子化学研究

用HF方法在6-31G^*基组下,对手性含硫恶唑硼烷催化苯乙酮不对称还原反应进行了量子化学从头算研究。还原反应经历了催化剂-硼烷加合物、催化剂-硼烷-酮加合物、催化剂-烷氧基硼烷加合物的生成以及催化剂-烷氧基硼烷加合物的离解过程。催化剂-硼烷加合物、催化剂-硼烷-酮加合物和催化剂-烷氧基硼烷加合物的生成分别为放热、吸热、放热过程;催化剂-烷氧基硼烷加合物离解成催化剂烷氧基硼烷为吸热过程。催化剂-硼烷-酮加合物和催化剂-烷氧基硼烷加合物都存在四种稳定的结构。最有利于氢转移的催化剂-硼烷-酮加合物结构是次低能量结构,并且具有扭曲的船形结构。催化剂-烷氧基硼烷加合物含有一个B-O-B-N四元环,尽管四元环有较大的张力,但加合物仍有较高的稳定性。     

Quantum effects in ab-initio calculations of rate constants for chemical reactions occuring in the condensed phase

An overview of recent advances in the development of methods designed to calculate rate constants for chemical reactions obeying mass action kinetic equations in condensed phases is presented. A general framework addressing mixed quantum-classical systems is elaborated that enables quantum features such as tunneling effects, zero-point vibrations, dynamic quantum coherence, and non-adiabatic effects to be calculated. An efficient Monte Carlo sampling method for performing ab-initio calculations of rate constants and isotope effects in chemical processes in condensed phases is outlined, and the connection of isotope effects to reaction mechanism is explored     

Anomalous behavior in the two-step reduction of quinones in acetonitrile

The electrochemical reduction of eight quinones, 9,10-anthraquinone (1), duroquinone (2), 2,6-di-tert-butyl-1,4-benzoquinone (3), 2,6-dimethoxy-1,4-benzoquinone (4), 9,10-phenanthrenequinone (5), tetrachloro-1,2-benzoquinone (6), tetrabromo-1,2-benzoquinone (7) and 3,5-di-tert-butyl-1,2-benzoquinone (8), have been studied in acetonitrile. In every case it was found that cyclic voltammograms differed in significant ways from those expected for simple stepwise reduction of the quinone to its radical anion and dianion. The various types of deviations for the eight quinones have been cataloged and some speculation is offered concerning their origins.     

Quantum chemical prediction of pathways and rate constants for reaction of cyanomethylene radical with NO

High-level ab initio calculations have been performed to study the mechanism and kinetics of the reaction of the cyanomethylene radical (HCCN) with the NO. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level, and their corresponding single-point energies are improved by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) approach. From the calculated potential energy surface, we have predicted the favorable pathways for the formation of several isomers of a HCCN-NO complex. Barrierless formation of HCN + NCO (P1) is also possible. Formation of HCNO + CN (P3) is endoergic but may become significant at high temperatures. To rationalize the scenario of our calculated results, we also employ the Fukui functions and hard-and-soft acid-and-base (HSAB) theory to seek possible clues. The predicted total rate coefficient, k(total), at He pressure 760 Torr can be represented with the equation k(total) = 1.40 × 10(-7) T(-2.01) exp(3.15 kcal mol(-1)/RT) at T = 298-3000 K in units of cm(3) molecule(-1) s(-1). The predicted total rate coefficients at some available conditions (He pressures of 6, 18, and 30 Torr in the temperature of 298 K) are in reasonable agreement with experimental observation. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions.     

Quantum chemical modelling of ethene epoxidation with hydrogen peroxide: role of catalytic sites

Ethene epoxidation with hydrogen peroxide was studied on hydroxylated binuclear metal sites, using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. A decrease of the activation enthalpy of approximately 100 kJ mol(-1) was observed compared to the gas phase reaction between hydrogen peroxide and ethene. It was previously shown that micro-solvation with water reduces the activation enthalpy by approximately 77 kJ mol(-1) and only the additional 24 kJ mol(-1) can be attributed to the binuclear site. Three different metal centres were tested, Ti(iv), Si(iv) and Ge(iv), in order to investigate any specific role of the metal centre on the activation enthalpy. The results clearly show that the activation enthalpy is independent on the nature of the metal centre. This emphasises the role of the hydrogen bonded network provided by the hydroxylated metal sites, on the stabilisation of the transitions state. In ref. 1 (A. Lundin, I. Panas and E. Ahlberg, J. Phys. Chem. A, 2007, 111, 9080) it was demonstrated that, at the transition state and upon micro-solvation, the hydrogen peroxide entity becomes polarized within the hydrogen bonding network, forming a negatively-charged fragment distant from the ethene molecule and a positively-charged fragment directly involved in the oxygen insertion step. The same mechanism was found to hold also for the reaction at the binuclear catalytic site, since the required hydrogen bonding is effectively provided by the hydroxylated metal centres. This mechanism is compared to the two-step pathway which employs a metal peroxide intermediate. Both reaction channels were found to be plausible in confined environments.     

Chemiluminescence assay for quinones based on generation of reactive oxygen species through the redox cycle of quinone

A sensitive and selective chemiluminescence assay for the determination of quinones was developed. The method was based on generation of reactive oxygen species through the redox reaction between quinone and dithiothreitol as reductant, and then the generated reactive oxygen was detected by luminol chemiluminescence. The chemiluminescence was intense, long-lived, and proportional to quinone concentration. It is concluded that superoxide anion was involved in the proposed chemiluminescence reaction because the chemiluminescence intensity was decreased only in the presence of superoxide dismutase. Among the tested quinones, the chemiluminescence was observed from 9,10-phenanthrenequinone, 1,2-naphthoquinone, and 1,4-naphthoquinone, whereas it was not observed from 9,10-anthraquinone and 1,4-benzoquinone. The chemiluminescence property was greatly different according to the structure of quinones. The chemiluminescence was also observed for biologically important quinones such as ubiquinone. Therefore, a simple and rapid assay for ubiquinone in pharmaceutical preparation was developed based on the proposed chemiluminescence reaction. The detection limit (blank + 3SD) of ubiquinone was 0.05 μM (9 ng/assay) with an analysis time of 30 s per sample. The developed assay allowed the direct determination of ubiquinone in pharmaceutical preparation without any purification procedure. Figure Chemiluminescence generated through the redox cycle of quinone     

Quantum dots assisted photocatalysis for the chemiluminometric determination of chemical oxygen demand using a single interface flow system

A novel flow method for the determination of chemical oxygen demand (COD) is proposed in this work. It relies on the combination of a fully automated single interface flow system, an on-line UV photocatalytic unit and quantum dot (QD) nanotechnology. The developed approach takes advantage of CdTe nanocrystals capacity to generate strong oxidizing species upon irradiation with UV light, which fostered a fast catalytic degradation of the organic compounds. Luminol was used as a chemiluminescence (CL) probe for indirect COD assessment, since it is easily oxidized by the QD generated species yielding a strong CL emission that is quenched in the presence of the organic matter. The proposed methodology allowed the determination of COD concentrations between 1 and 35 mg L⁻¹, with good precision (R.S.D. < 1.1%, n = 3) and a sampling frequency of about 33 h⁻¹. The procedure was applied to the determination of COD in wastewater certified reference materials and the obtained results showed an excellent agreement with the certified values.     

Quantum chemical calculations on thioridazine

Quantum chemical CNDO /2 calculations for the conformational preference of the side chain of thioridazine as a function of angle indicated the crystallographically determined structure gave the lowest energy. There is also a small region of conformational flexibility within the first 90° of rotation of the side chain. This is commensurate with the results which we had obtained previously for our similar calculations for promazine and its Cl and CF₃ derivatives, perazine and its Cl and CF₃ derivatives, and for the hypothetical hitherto unknown N-piperidinopromazine and its Cl and CF₃ derivatives. The conformational profile of thioridazine resembles that of the perazines. The calculated gross atomic populations on the alkyl nitrogen in thioridazine was within the range we had previously found necessary for neuroleptic activity.