Showing papers on "Transition state published in 1999"

Ru(arene)(amino alcohol)-Catalyzed Transfer Hydrogenation of Ketones: Mechanism and Origin of Enantioselectivity

Abstract: The mechanism of the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones using isopropyl alcohol as the hydrogen source has been studied by means of hybrid density functional methods (B3PW91). Three mechanistic alternatives were evaluated, and it was shown that the reaction takes place via a six-membered transition state, where a metal-bound hydride and a proton of a coordinated amine are transferred simultaneously to the ketone. Further calculations provided a general rationale for the rate of the reaction by comparison of steric effects in the ground and transition states of the ruthenium hydride complex. It was found that the TS has a strong preference for planarity, and this in turn is dependent on the conformational behavior of the O,N-linkage of the amino alcohol ligand. Finally, a general model, rationalizing the enantioselectivity of the reaction, was developed. Experimental studies of both rate and enantioselectivity were used in order to support the computational results.

Cycloaddition Reactions of 1,3-Cyclohexadiene on the Silicon(001) Surface

Abstract: [2+2] and [4+2] cycloaddition reactions of 1,3-cyclohexadiene on the Si(001) surface were studied. It is shown that not only the [4+2] cycloaddition reaction but also the [2+2] cycloadditions can occur on the Si(001) surface. Surface isomerization reactions connecting [4+2] and [2+2] are very unlikely due to a high energy barrier, implying that the surface reactions are kinetically controlled. Therefore the final surface reaction products are determined during the initial stage of the reactions in contrast with earlier assumptions that the “product distribution is thermodynamically determined”. Our interpretations are consistent with the new experimental results by the Hamers group. According to our CASSCF(6,6) calculations, the nonsymmetric π-complex transition states along the [2+2] cycloaddition mechanism, which has been suggested by many theoretical studies, seem to be an artifact. Nevertheless, the very soft Si dimer buckling motion of the Si(001) surface certainly facilitates the [2+2] reaction.

Structures and Reaction Pathways in Rhodium(I)-Catalyzed Hydrogenation of Enamides: A Model DFT Study

Abstract: The potential energy profile of Rh(I)-catalyzed hydrogenation of enamides has been studied for the simple model system [Rh(PH3)2(α-acetamidoacrylonitrile)]+ using a nonlocal density functional method (B3LYP) Intermediates and transition states along four isomeric pathways for dihydrogen activation have been located, and pathways for interconversion between isomeric reaction pathways have been explored The general sequence of the catalytic cycle involves coordination of H2 to [Rh(PH3)2(α-acetamidoacrylonitrile)]+ to form a five-coordinate molecular H2 complex, followed by oxidative addition of the coordinated molecular hydrogen to form a dihydride complex, [RhH2(PH3)2(α-acetamidoacrylonitrile)]+ This dihydride is converted into an alkyl hydride by a migratory insertion reaction Reductive elimination of the hydrogenated acetamidoacrylonitrile completes the catalytic cycle No computational support for alternate H2 activation pathways, such as direct conversion of H2 and [Rh(PH3)2(α-acetamidoacrylonitril

Reactivity of Sc+(3D,1D) and V+(5D,3F): Reaction of Sc+ and V+ with Water

Abstract: The study of the reaction of water with the early first-row transition metal ions has been completed in this work, in both high- and low-spin states. In agreement with experimental observations, the only exothermic products are the low-lying states MO+ + H2; formation of other endothermic products is also examine. An in-depth analysis of the reaction paths leading to each of the observed products is given, including various minima and several important transition states. All results have been compared with existing experimental data and our earlier work covering the Ti+ + H2O reaction in order to observe existent trends for the early first-row transition metal ions.

Ab initio analysis of the transition states on the lowest triplet H2O2 potential surface

Abstract: The lowest triplet H2O2 potential surface was analyzed for the transition and minimum-energy structures in the range from −02 to +54 eV with respect to the H2+O2 energy All the transition structures, the reaction pathways, and the local minima were found to have planar configurations for the atoms We focus on the transition structures responsible for the main bimolecular chemical reactions formally possible on this surface: H2+O2↔2HO, H+HO2, and H2O+O; H+HO2↔2HO and H2O+O; and 2HO↔H2O+O For these reactions, activation energies and rate constants in the transition state approximation were evaluated Our computed rate constants confirm the recommended values for the H+HO2→H2+O2 and HO+HO→H2O+O reactions The results obtained refute the elementary character of the H+HO2→H2O+O process and call into question the possibility of chain initiation in the H2/O2 system by means of a bimolecular reaction Most likely, the chain initiation in the gas phase is owing to trimolecular reactions H2+2O2→2HO2, 2HO+O2 S

Theoretical Study on the Origin of Enantioselectivity in the Bis(dihydroquinidine)-3,6-pyridazine·Osmium Tetroxide-Catalyzed Dihydroxylation of Styrene

Abstract: The origin of enantioselectivity in the dihydroxylation of H2CCH(Ph) catalyzed by (DHQD)2PYDZ·OsO4 ((DHQD)2PYDZ = bis(dihydroquinidine)-3,6-pyridazine) is analyzed theoretically by means of hybrid QM/MM calculations with the IMOMM(Becke3LYP:MM3) method. Twelve different possible reaction paths are defined from the three possible regions of entry of the substrate and its four possible orientations and characterized through their respective transition states. The transition state with the lowest energy leads to the R product, in agreement with experimental results. The decomposition of the interaction energy between catalyst and substrate shows how the selectivity is essentially governed by stacking interactions between aromatic rings, with a leading role for the face-to-face interaction between the substrate and one of the quinoline rings of the catalyst.

The C7H10 Potential Energy Landscape: Concerted Transition States and Diradical Intermediates for the Retro-Diels−Alder Reaction and [1,3] Sigmatropic Shifts of Norbornene

Abstract: The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and 13C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.

Quantum-chemical modeling of the hydrocarbon transformations in acid zeolite catalysts

Abstract: Results of quantum-chemical modeling of a number of elementary steps involved in the acid zeolite-catalyzed conversion of hydrocarbons are collected together and compared. The elementary steps considered are protolytic cracking, protolytic dehydrogenation, hydride transfer, skeletal isomerization, and β-scission. The hydrocarbon parts of transition states (TS) for these steps represent carbocations specific for each reaction. Geometry parameters of the TS and activation energies depend on the relative stability of these carbocations. The reactions considered can proceed via several alternative routes dependent on the species involved and on the details of the interaction of the hydrocarbon portion of the activated complex with the zeolite oxygen atoms. Variation of the acid strength of zeolite cluster models can be employed for studies of the acid strength sensitivity of the activation energies and other quantities of interest as well as for extrapolation of these quantities computed on small clusters towards zeolitic values.

Experimental Proof of the Non-Least-Motion Cycloadditions of Dichlorocarbene to Alkenes: Kinetic Isotope Effects and Quantum Mechanical Transition States

Abstract: High-precision experimental kinetic isotope effects were measured at natural abundance for the cycloaddition of dichlorocarbene to 1-pentene. These values were compared to isotope effects predicted for the addition of dichlorocarbene to propene at the B3LYP/6-31G* and B3LYP/6-311+G* levels. The results provide unambiguous evidence for a nonlinear attack in which bond formation is more advanced at the unsubstituted carbon of the terminal alkene in the transition state of the cycloaddition. This is the first experimental confirmation of the unsymmetrical, non-least-motion approach of a carbene to an alkene first proposed by Skell over 40 years ago and predicted by quantum mechanical calculations.

Toward the understanding of ethylene photodissociation: Theoretical study of energy partition in products and rate constants

Abstract: The energy partition in the products of ethylene photodissociation (including C2H4, C2D4, D2CCH2, cis- and trans-HDCCDH) at 193 and 157 nm and the rate constants of H loss channels were computed based on ab initio ethylene ground-state surfaces of which most were reported earlier. In the calculations of the energy partitions, a simple model was used in which the excess energy above the transition state is distributed statistically and the energy released by the exit barrier is described by the modified impulsive model. The rate constants of the ethylene H(D) elimination were calculated according to the variational RRKM (Rice–Ramsperger–Kassel–Marcus) theory, and the RRKM rate constants with tunneling corrections were obtained for vinyl decomposition at 193 nm. In contrast with previous conclusions drawn by LIF (laser induced fluorescence) studies, the rate constant calculations suggest that the H loss may be a nonstatistical process. However, the computed variational transition states for H loss appear re...

Density functional studies on conjugate addition of (me2culi)2 to cyclohexenone : stereoselectivity and rate-determining step

Abstract: The origin of the stereochemistry of the conjugate addition of a dimeric lithium cuprate (Me2CuLi)2 to 2-cyclohexenone has been revealed by theoretical calculations, which were compared with experimentally observed intermediates and transition states (such as the one shown here) of the rate- and stereochemistry-determining step of the reaction.

Intermittency of activated events in single molecules: The reaction diffusion description

Abstract: We generalize the reaction diffusion approach in order to calculate the higher moments of the distribution of the population which characterize intermittency and can be derived by statistical analysis of single molecule experiments. We show the way the usual Poisson statistics is asymptotically approached with increasing barrier height for a reaction dependent on diffusing up an activation free energy barrier to a critical configuration.

Thermal rearrangements of norcaradiene

Abstract: Multireference ab initio methods and density functional theory with a 6-31G* basis set have been applied to study the interconversions of norbornadiene, 1,3,5-cycloheptatriene, norcaradiene, and toluene. These include the [1,5]hydrogen shift in cycloheptatriene, the [1,5]carbon shift (walk rearrangement) in norcaradiene, the ring flip of cycloheptatriene, and the formation of norcaradiene from cycloheptatriene. Our best theoretical calculations for the walk rearrangement predict that the Woodward−Hoffmann “forbidden” suprafacial-inversion transition state and the “allowed” suprafacial-retention pathway differ in energy by <1 kcal/mol. Further, both transition states are effectively biradical-like. The transition state for formation of toluene from norcaradiene proceeds via a hydrogen transfer transition state that is formed directly from the “allowed” transition state for the walk rearrangement but not from the “forbidden” transition state. Further, the transition state for the transannular hydrogen shift...

Locating all transition states and studying the reaction pathways of potential energy surfaces

Abstract: We propose a new method for calculating all stationary states, including saddle points of all orders, of a potential energy surface based on the αBB deterministic branch and bound global optimization algorithm. This method is based on rigorous optimization methods and offers a theoretical guarantee of enclosing all solutions to the equation ∇V=0. We apply this method to Murrel–Sorbie analytic potential energy surfaces of HCN, HSiN, HBO, and CS2, and to the Empirical Conformational Energy Program for Peptides (ECEPP/3) potential energy surfaces of alanine, alanine dipeptide, and tetra-alanine. For alanine, alanine dipeptide, and tetra-alanine, we proceed to analyze the topography of the potential energy surface by calculating reaction pathways, transition rate matrices, time-evolution of occupation probabilities, and rate disconnectivity graphs.

Decomposition of beta -hydroxypropoxy radicals in the oh-initiated oxidation of propene. a theoretical and experimental study

Abstract: Environmental chamber studies of the OH-initiated oxidation of propene have been carried out in the presence of nitrogen oxides under conditions relevant to the atmosphere. The major products observed at all temperatures studied (220−300 K) are CH2O and CH3CHO, indicating that the β-hydroxypropoxy radicals formed in the oxidation process (from reaction of the corresponding β-hydroxypropylperoxy radicals with NO) predominantly undergo unimolecular decomposition. A full theoretical study of the chemistry of the dominant β-hydroxypropylperoxy, β-hydroxypropylperoxynitrite, and β-hydroxypropoxy species has been carried out. On the basis of B3LYP-DFT/6-31G** quantum chemical characterizations, the most stable conformations of the oxy radicals are found to contain intramolecular hydrogen bonds, which provide stabilizations of about 2 kcal/mol. The internal hydrogen bond in the lowest-energy oxy species is found to persist in the transition states for C−C bond rupture, which keeps the barrier to their decomposit...

Theoretical Studies of the Electrocyclic Reaction Mechanisms of Hexatriene to Cyclohexadiene

Abstract: The potential energy surfaces for electrocyclic reactions of hexa-1,3,5-triene were calculated by ab initio molecular orbital methods. The transition states of two electrocyclic reaction pathways (...

A Solvated Transition State for the Nucleophilic Attack on Cationic η3-Allylpalladium Complexes

Abstract: The observed selectivities in palladium-assisted allylic alkylation were correlated with the solvated transition-state structures obtained at high levels of theory for the reaction of cationic η3-allylpalladium complexes with different nucleophiles in the presence of a solvent.

A theoretical study on the catalysis of Cu-exchanged zeolite for the decomposition of nitric oxide

Abstract: The catalysis of Cu-exchanged zeolite for the direct decomposition of nitric oxide was investigated by hybrid density functional theory (B3LYP) using a molecular model of the active site. For reactions of two NO molecules over a Cu ion bound to the zeolite model (ZCu), the structures and energies of adsorption complexes and transition states were examined and compared with those of the corresponding reactions over an isolated Cu+ and Cu atom and also reactions of free NO. The ZCu shows an enhanced catalytic activity compared with the isolated Cu+. The ZCu and adsorbed molecules interact strongly in the transition state structures through π(d–p) bonding. Theory also suggests that the Cu atom has the potential to be a highly active catalyst for the NO decomposition reaction.

Theoretical Study of Bimolecular Reactions between Carbenium Ions and Paraffins: The Proposal of a Common Intermediate for Hydride Transfer, Disproportionation, Dehydrogenation, and Alkylation

Abstract: The mechanism of the bimolecular reactions of ethyl cation with ethane and propane and of s-propyl cation with ethane, propane, and isopentane is theoretically investigated by means of the B3PW91 density functional method. The study includes complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved, calculation of the reaction enthalpies and activation energies for the different elemental steps, and obtainment of the equilibrium constants and relative reaction rate constants by means of transition state theory. It is found that the interaction of a carbenium ion with a saturated alkane always results in formation of a stable carbonium ion intermediate and that different intramolecular rearrangements of this common intermediate can explain the mechanism of acid-catalyzed hydrocarbon reactions, such as hydride transfer, disproportionation, dehydrogenation, and alkylation.

Ab initio studies of the radical cation diels-alder reaction

Abstract: The radical cation Diels−Alder reaction of the 1,3-butadiene radical cation with ethylene, yielding the cyclohexene radical cation, was studied by B3LYP hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis set. The intermediates and transition states involved in three different mechanisms, a concerted Cs-symmetric and a stepwise unsymmetric anti [4 + 2] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyclobutane, have been considered. The synchronous Cs-symmetric pathway is prevented by a pseudo-Jahn−Teller distortion and is 19 kcal/mol higher in energy than the stepwise pathways. The stepwise anti pathway was found to be the lowest-energy pathway with an activation energy of 0.3 kcal/mol relative to the initially formed ion−molecule complex. The gauche-out pathway, leading to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pathway, leading to cyclohexene. In contrast to earlier calculations by Bauld at the MP2/6-31G*//3-21G level of theory, an i...

An ab initio and MNDO-d SCF-MO computational study of stereoelectronic control in extrusion reactions of R2I–F iodine(III) intermediates†

Abstract: MNDO-d and ab initio RHF, B3LYP and MP2 energies and geometries are reported for reactant ground and transition states for F–R′ and R–R′ extrusion and R/R′ interconversion reactions of substituted RR′I–F iodine(III) reactive intermediates. The RR′I–F reactant is predicted to form a stable asymmetric bridged dimer involving a square planar iodine centre, hitherto unconsidered as a factor in the chemistry of hypervalent iodine species. Evidence in support of this hypothesis obtained from previously reported crystal structures is discussed. The reactions of both monomer and bridged dimer are found to exhibit unusually large stereoelectronic effects at the iodine centre, deriving from electron donating and withdrawing substituents on the R groups. They are also unusual in showing transition state substituent effects which are opposite to those controlling the ground state stabilities, for which an NBO analysis is presented. Both these effects are manifest in the transition states for reaction of the dimeric species, which is stabilised by electron withdrawing groups present in the pseudo equatorial R′ group of the reacting centre and in the pseudo axial position of the unreacting R component of the dimer.

The thermodynamics and kinetics of protein folding : a lattice model analysis of multiple pathways with intermediates

Abstract: The kinetics and thermodynamics of folding of a representative sequence of a 125-residue protein model subject to Monte Carlo dynamics on a simple cubic lattice were investigated. The diverse trajectories that lead to the native state can be classified into a relatively small number of average pathways: a “fast track” in which the chain forms a stable core that folds directly to the native state and several “slow tracks” in which particular contacts form before the core is complete and direct the chain to a misfolded intermediate. Rearrangement from the intermediates to the native state is slow because it requires breaking stable contacts, which involve primarily surface residues. The transition state for folding is identified by activated dynamics simulations and consists of a reduced version of the core in the absence of other (native and nonnative) contacts which slow folding. Each track involves an ensemble of structures that can be characterized by two progress coordinates for the reaction. These coordinates are based on a comparison of folding and nonfolding trajectories: one coordinate monitors the formation of the core and the other monitors whether the chain is trapped in a long-lived intermediate. From Monte Carlo simulations, we obtain an estimate for the density of states and calculate equilibrium averages, including the free energy, energy, and entropy, as functions of the two coordinates. The thermodynamics are in good agreement with the observed kinetics; the transition states correspond to plateaus or barriers in the free energy while the intermediates are energetically stabilized local free energy minima. The complexities of the folding mechanism bear a striking similarity to those observed experimentally for lysozyme, a well-studied protein of comparable size.