Catalytic effect of molecular iodine in Diels–Alder reaction: a density functional theory study

We explore here the feasibility of employing molecular iodine as Lewis acid catalyst for Diels–Alder (DA) reaction using conceptual density functional theory (DFT) based reactivity indices and transition state analysis at the DFT (B3LYP)/6-31G(d) level of theory. Catalytic effect of iodine is probed using reactivity indices considering six different substituents for the diene at the 2-position and five different substituents at the 1-position of the olefinic dienophile. Comparison of HOMO diene–LUMO dienophile gap between the catalyzed and uncatalyzed processes confirms catalytic effect of iodine in DA reaction. Mechanistic details of both the uncatalyzed and the iodine catalyzed processes is achieved through transition state analysis for four possible stereoisomeric reactive channels with respect to isoprene–acrolein model reaction. A significant cutback in activation barrier is observed in presence of iodine. Influence of iodine on regioselectivity of the reaction and asynchronicity of bond formation is analyzed using local version of the HSAB principle and philicity index.     

A density functional theory study on the Diels‐Alder reactions with vinylallenes as dienes

The Diels‐Alder (DA) reactions of vinylallenes (VA) with ethylenes were investigated using the global electrophilicity and nucleophilicity of the corresponding reactants as global reactivity indexes defined within the conceptual density functional theory. The reactivity and regioselectivity of these reactions were predicted by analysis of the energies, geometries, and electronic nature of the transition‐state structures. In general, substitution by the electron‐accepting acetyl group favors the reaction, whereas substitution by the electron‐releasing methoxy group provides the opposite effect, regardless of being on VA or ethylene. However, the substitution effect in ethylene is apparently greater than that in VA. It has also been disclosed that substitution by electron‐accepting group on both reactants accelerates the reaction, and the reaction may give different regioselectivity from that between VA and acetyl‐substituted ethylene. This has also been verified by our experiments. It seems that the DA reactions with VAs as the diene components can generally be classified as nonpolar asynchronous with the endo product formation (wherever possible) being more pronounced. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Antihydrophobic cosolvent effects for alkylation reactions in water solution, particularly oxygen versus carbon alkylations of phenoxide ions

Antihydrophobic cosolvents such as ethanol increase the solubility of hydrophobic molecules in water, and they also affect the rates of reactions involving hydrophobic surfaces. In simple reactions of hydrocarbons, such as the Diels-Alder dimerization of 1,3-cyclopentadiene, the rate and solubility data directly reflect the geometry of the transition state, in which some hydrophobic surface becomes hidden. In reactions involving polar groups, such as alkylations of phenoxide ions or S(N)1 ionizations of alkyl halides, cosolvents in water can have other effects as well. However, solvation of hydrophobic surfaces is still important. By the use of structure-reactivity relationships, and comparing the effects of ethanol and DMSO as solvents, it has been possible to sort out these effects. The conclusions are reinforced by an ab initio computer model for hydrophobic solvation. The result is a sensible transition state for phenoxide ion as a nucleophile, using its oxygen n electrons to avoid loss of conjugation. The geometry of alkylation of aniline is very different, involving packing (stacking) of the aniline ring onto the phenyl ring of a benzyl group in the benzylation reaction. The alkylation of phenoxide ions by benzylic chlorides can occur both at the phenoxide oxygen and on ortho and para positions of the ring. Carbon alkylation occurs in water, but not in nonpolar organic solvents, and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para. The effects of phenol substituents and of antihydrophobic cosolvents on the rates of the competing alkylation processes indicate that in water the carbon alkylation involves a transition state with hydrophobic packing of the benzyl group onto the phenol ring. The results also support our conclusion that oxygen alkylation uses the n electrons of the phenoxide oxygen as the nucleophile and does not have hydrophobic overlap in the transition state. The mechanisms and explanations for competing oxygen and carbon alkylations differ from previous proposals by others.     

Catalysis by polymer complexes. VIII. Enhanced nucleophilic reactivity of hydroxamate anions bound to quaternized polyethylenimines and related compounds

Water-soluble polyethylenimine derivatives composed only of the tertiary amine and quaternary ammonium groups were prepared by methylation and quaternization of commercial polyethylenimine. The reaction of the hydrophobic hydroxamate anion and p-nitrophenyl acetate (aqueous solution, 30°C) was more than 100 times accelerated by the polyethylenimine partly quaternized by stearyl bromide. The polymers quaternized by less hydrophobic groups or by the stearyl and methyl groups were much less effective. The former polymer also accelerated decomposition of the acetyl hydroxamate up to 25 times. The rate-enhancing effect of these polyethylenimine derivatives appeared to be produced by the general, hydrophobic microenvironment without regard to the peculiar structural characteristics of polyethylenimine. The effect of fully quaternized triethylenetetramine was similarly studied.     

Phase behavior and rheological properties of salt-free catanionic TTAOH/DA/H2O system in the presence of hydrophilic and hydrophobic salts

In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H(2)O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical L(α) phases with the different compositions were characterized by the transmission electron microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The L(α) phase with the positive membrane charges can be finally transferred into an L(1) phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed, and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.     

A quantum chemistry study of the Cl atom reaction with formaldehyde

The elementary vapor-phase reaction between Cl atoms and HCHO has been studied by ab initio methods. Calculations at the MP2, MP3, MP4(SDTQ), CCSD, CCSD(T), and MRD-CI levels of theory show that the reaction is characterized by a low electronic barrier; excluding the effects of spin-orbit splitting in Cl, our best estimate at the MRD-CI/aug-cc-pVTZ//RHF-RCCSD(T)/aug-cc-pVTZ level of theory predicts a Born-Oppenheimer barrier height of 0.7 kJ mol-1. The energies of the lowest six electronic states as resulting from MRD-CI calculations are presented at discrete points along the reaction path, and two avoided crossings are found in the transition state region. The spin-orbit splitting in Cl is also calculated along the reaction path; it is not negligible in the transition state region and is found to increase the barrier by only 1.4 kJ mol-1 at the RCCSD(T)/aug-cc-pVTZ transition state geometry. The minimum energy path of the reaction connects an energetically weakly stabilized adduct on the flat potential surface on the reactant side and an energetically strongly stabilized postreaction adduct. The reaction rate coefficient and the kinetic isotope effects were calculated using improved canonical variational theory with small curvature tunneling (ICVT/SCT), and the results were compared to experimental data. The experimental reaction rate coefficient is reproduced within its uncertainty limits by variational transition state theory with interpolated single-point energy corrections (ISPE) at the MP4(SDTQ) level of theory and by conventional transition state theory with interpolated optimized energies (IOE) at the MRD-CI//RCCSD(T) level of theory and interpolated optimized geometries at the RCCSD(T) level of theory on an MP2/aug-cc-pVTZ potential energy surface when employing scaled vibrational frequencies.     

A product branching ratio controlled by vibrational adiabaticity and variational effects: kinetics of the H + trans-N2H2 reactions

The abstraction and addition reactions of H with trans-N(2)H(2) are studied by high-level ab initio methods and density functional theory. Rate constants were calculated for these two reactions by multistructural variational transition state theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal∕mol lower than that of the abstraction reaction, but the addition rates are lower than the abstraction rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K.     

Proton transfer in a polar solvent from ring polymer reaction rate theory

We have used the ring polymer molecular dynamics method to study the Azzouz-Borgis model for proton transfer between phenol (AH) and trimethylamine (B) in liquid methyl chloride. When the A-H distance is used as the reaction coordinate, the ring polymer trajectories are found to exhibit multiple recrossings of the transition state dividing surface and to give a rate coefficient that is smaller than the quantum transition state theory value by an order of magnitude. This is to be expected on kinematic grounds for a heavy-light-heavy reaction when the light atom transfer coordinate is used as the reaction coordinate, and it clearly precludes the use of transition state theory with this reaction coordinate. As has been shown previously for this problem, a solvent polarization coordinate defined in terms of the expectation value of the proton transfer distance in the ground adiabatic quantum state provides a better reaction coordinate with less recrossing. These results are discussed in light of the wide body of earlier theoretical work on the Azzouz-Borgis model and the considerable range of previously reported values for its proton and deuteron transfer rate coefficients.     

CH与H2分子反应动力学及选态反应的理论研究

次甲基作为化学反应源曾引起广泛的兴趣.Schaefer 及其合作者于1977年对反应CH(~4Σ~-)+H_2→CH_2(~3B_1)+H 进行过量子化学研究,但是计算中限制了一些自由度.近年来,由于能量梯度方法的发展,反应途径哈密顿理论和变分过渡态理论的提出,有可能进一步对该反应进行分子反应动力学性质的研究.本文用从头算UHF/6-31G 方法和能量梯度方法首先优化出上述反应(原子编号为CH_a+H_bH_c→H_bCH_a+H_c)的过渡态;再用     

Ab initio study on the kinetics of hydrogen abstraction for the H+alkene-->H2+alkenyl reaction class

Kinetics of the hydrogen abstraction reaction class of the H+alkene has been studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The rate constants for the reference reaction, H+C2H4, were obtained by the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) correction in the temperature range of 300-3000 K. Combined with these data, both the RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, are found to be promising methods for predicting rate constants for a large number of reactions in this reaction class. Our analysis indicates that less than 50% systematic errors on the average exist in the predicted rate constants using the RC-TST/LER or RC-TST/BHG method while in comparison to explicit rate calculations the differences are less than 100% or a factor of 2 on the average.     

Kinetics of the hydrogen abstraction CHO + Alkane → HCHO + Alkyl reaction class: an application of the reaction class transition state theory

The kinetics of the hydrogen abstraction at alkanes by formyl radicals is investigated using the reaction class transition state theory (RC-TST) approach combined with the linear energy relationship (LER) or the barrier height grouping (BHG). The rate constants of a reaction in this class can be estimated through those of the reference reaction, CHO + C₂H₆, which are obtained from rate constants of the reaction that involves the smallest species, namely CHO + CH₄, using the explicit RC-TST scaling. The thermal rate constants of this smallest reaction are evaluated at the canonical variational transition state theory (CVT) with the corrections from the small-curvature tunneling (SCT) and hindered rotation (HR) treatments. Our analyses indicate that less than 40% systematic errors, on the average, exist in the predicted rate constants using both the LER approach, where only reaction energy is needed, and the BHG approach, where no additional information is needed; while comparing to explicit rate calculations the differences are less than 60%. Contribution to Mark S. Gordon 65th Birthday Festschrift Issue.     

Solvent effects in the fluorination of 1,2-dichloro-1,1-difluoroethane (R-132b) to 2-chloro-1,1,1-trifluoroethane (R-133a)

Trichloroethylene has been found to act as a rate enhancing co-factor in the liquid phase, tantalum (V) halide catalyzed, fluorine-for-chlorine exchange reaction of 1,2-dichloro-1,1-difluoroethane (R-132b) to 2-chloro-1,1,1-trifluorethane (R-133a). Several trifluoromethyl substituted benzenes have also been found to be rate-enhancing solvents.     

变分过渡态理论对H＋H2及其同位素选态反应的研究

将选态速度常数的计算推广到任意指定反应物、过渡态的振动激发态.用此法计算了H+H_2(v)及其同位素经不同振动激发过渡态时的速度常数,发现弯曲振动模激发所得结果与实验值更符合,并且在给定能量下,过渡态的弯曲振动模激发比其对称伸缩模激发更有利于反应进行.     

Theoretical study of DABCO-based ionic liquid: synthesis and reaction mechanism

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S_N2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S_N2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S_N2 and five-membered ring transition state mechanism. However, S_N2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.     

Theoretical insights into the mechanism for thiol/disulfide exchange

The mechanism for thiol/disulfide exchange has been studied with high-level theoretical calculations. Free energies, transition structures, charge densities, and solvent effects along the reaction pathway have been determined for the first time. Mechanistic results agree with experimental data, and support the idea that the thiolate is the reacting species and that the reaction indeed proceeds through an uncomplicated S(N)2 transition state. The transition structures have the charge density evenly concentrated in the attacking and leaving sulfur atoms. The charge densities allow us to rationalize the solvent effects. As transition structures have the charge density more widely distributed than reactants, hydrophobic environments catalyze the reaction. The effect can be so dramatic that disulfide exchange inside the active site of ribonucleotide reductase is estimated to be catalyzed 10(3) times faster than the reaction in water. It was also found that attack by thiol is much faster than previously assumed, if mediated through water chains. Although the present results, as well as experimental data, still suggest that thiolate is the main reaction species, water-mediated thiol attack is almost kinetically competitive, and can eventually become competitive under specific experimental conditions.     

Gas-phase Boudouard disproportionation reaction between highly vibrationally excited CO molecules

The gas-phase Boudouard disproportionation reaction between two highly vibrationally excited CO molecules in the ground electronic state has been studied in optically pumped CO. The gas temperature and the CO vibrational level populations in the reaction region, as well as the CO₂ concentration in the reaction products have been measured using FTIR emission and absorption spectroscopy. The results demonstrate that CO₂ formation in the optically pumped reactor is controlled by the high CO vibrational level populations, rather than by CO partial pressure or by flow temperature. The disproportionation reaction rate constant has been determined from the measured CO₂ and CO concentrations using the perfectly stirred reactor (PSR) approximation. The reaction activation energy, 11.6 ± 0.3 eV (close to the CO dissociation energy of 11.09 eV), was evaluated using the statistical transition state theory, by comparing the dependence of the measured CO₂ concentration and of the calculated reaction rate constant on helium partial pressure. The disproportionation reaction rate constant measured at the present conditions is k_f = (9 ± 4) × 10⁻¹⁸ cm³/s. The reaction rate constants obtained from the experimental measurements and from the transition state theory are in good agreement.     

H3PO→H2POH异构化反应的直接动力学研究

在QCISD(T)/6-311C++G(2df,2pd)//QCISD/6-311C++G(d,p)+ZPE水平上,对H3PO的异构化反应H3PO→(1)H2POH(trans)→(2)H2POH(cis)进行了计算研究.结果表明,H原子由P原子向O原子迁移反应(1)的能垒为250.0kJ/mol,是反应速率控制步骤,而O_H键绕P_O键旋转的构型转化反应(2)的能垒只为12.3kJ/mol.利用经典过渡态理论(TST)与变分过渡态理论(CVT)分别计算了反应(1)在200～2000K温度区间内的速率常数kTST和kCVT,获得了经小曲率隧道效应(SCT)及Eckart模型校正后的速率常数kTST/Eckart和kCVT/SCT.对只涉及H原子迁移的反应(1),量子力学隧道效应的影响在低温段非常明显,而变分效应对反应速率常数的影响很小.     

Hydrogen-Bond-Assisted Diels–Alder Kinetics or Self-Healing in Reversible Polymer Networks? A Combined Experimental and Theoretical Study

Diels–Alder (DA) cycloadditions in reversible polymer networks are important for designing sustainable materials with self-healing properties. In this study, the DA kinetics of hydroxyl-substituted bis- and tetrafunctional furans with bis- and tris-functional maleimides, both containing ether-functionalized spacers, is investigated by modelling two equilibria representing the endo and exo cycloadduct formation. Concretely, the potential catalysis of the DA reaction through hydrogen bonding between hydroxyl of the furans and carbonyl of the maleimides or ether of the spacers is experimentally and theoretically scrutinized. Initial reaction rates and forward DA rate constants are determined by microcalorimetry at 20 °C for a model series of reversible networks, extended with (i) a hydroxyl-free network and hydroxyl-free linear or branched systems, and (ii) polypropylene glycol additives, increasing the hydroxyl concentration. A computational density-functional theory study is carried out on the endo and exo cycloadditions of furan and maleimide derivatives, representative for the experimental ones, in the absence and presence of ethylene glycol as additive. Additionally, an ester-substituted furan was investigated as a hydroxyl-free system for comparison. Experiment and theory indicate that the catalytic effect of H-bonding is absent or very limited. While increased concentration of H-bonding could in theory catalyze the DA reaction, the experimental results rule out this supposition.     

H2FCS单分子分解反应的直接动力学研究

采用量子化学QCISD(T)/6-311++G(d,p)//B3LYP/6-311+G(d,p)方法研究了H2FCS单分子分解反应的微观动力学性质, 构建了反应势能剖面. 利用经典过渡态理论(TST)与变分过渡态理论(CVT)并结合小曲率隧道效应模型(SCT), 分别计算了在200～3000 K温度范围内的速率常数kTST、kCVT和kCVT/SCT. 计算结果表明, H2FCS可经过不同的反应通道生成10种小分子产物, 脱H反应和HF消去反应为标题反应的主反应通道, 其中HF消去反应产物HCS可由两条反应通道生成. 在200~3000 K温度区间内得到三条反应通道的表观反应速率常数三参数表达式分别为 , 和 . 速率常数计算结果显示, 量子力学隧道效应在低温区间对反应速率常数的影响显著, 而变分效应在计算温度范围内可以忽略.