Showing papers on "Transition state published in 1986"
TL;DR: In this paper, the authors applied the theory for unimolecular reactions described in part 1 is applied to the recombination of methyl radicals in the high pressure limit, and the model potential energy surface and the methodology are briefly described.
Abstract: The theory for unimolecular reactions described in part 1 is applied to the recombination of methyl radicals in the high-pressure limit. The model potential energy surface and the methodology are briefly described. Results are presented for the recombination rate constant k_ ∞ at T = 300, 500, 1000, and 2000 K. Canonical and Boltzmann averaged microcanonical values of k_ ∞ are compared, and the influence of a potential energy interpolation parameter and a separation-dependent symmetry correction on k_ ∞ are examined. Earlier theoretical models and extensive experimental results are compared with the present results which are found to have a negative temperature dependence. The present results agree well with some of the available but presently incomplete experimental determinations of the high-pressure recombination rate constant for this reaction over the 300-2000 K temperature range. There is also agreement with a decomposition rate constant for a vibrationally excited ethane molecule produced by chemical activation.
132 citations
TL;DR: The utilization of enzyme-substrate binding energy in catalysis has been investigated by experiments on mutant tyrosyl-tRNA synthetases that have been generated by site-directed mutagenesis and the apparent binding energies of certain side chains with the tyrosine and nucleotide moieties increase.
Abstract: The utilization of enzyme-substrate binding energy in catalysis has been investigated by experiments on mutant tyrosyl-tRNA synthetases that have been generated by site-directed mutagenesis. The mutants are poorer enzymes because they lack side chains that form hydrogen bonds with ATP and tyrosine during stages of the reaction. The hydrogen bonds are not directly involved in the chemical processes but are at some distance from the seat of reaction. The free energy profiles for the formation of enzyme-bound tyrosyl adenylate and the equilibria between the substrates and products were determined from a combination of pre-steady-state kinetics and equilibrium binding methods. By comparison of the profile of each mutant with wild-type enzyme, a picture is built up of how the course of reaction is affected by the influence of each side chain on the energies of the complexes of the enzyme with substrates, transition states, and intermediates (tyrosyl adenylate). As the activation reaction proceeds, the apparent binding energies of certain side chains with the tyrosine and nucleotide moieties increase, being weakest in the enzyme-substrate complex, stronger in the transition state, and strongest in the enzyme-intermediate complex. Most marked is the interaction of Cys-35 with the 3'-hydroxyl of the ribose. Removal of the side chain of Cys-35 leads to no change in the dissociation constant of ATP but causes a 10-fold lowering of the catalytic rate constant. It contributes no net apparent binding energy in the E X Tyr X ATP complex and stabilizes the transition state by 1.2 kcal/mol and the E X Tyr-AMP complex by 1.6 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)
111 citations
TL;DR: In this paper, a variational TST calculation on the reaction CH+4'→'CH+3+H in the zero angular momentum limit using a simple model to account for the smooth conversion of the transitional modes from vibrations of the reagent to rotational-orbital modes of the products as a function of energy and position along the reaction coordinate is presented.
Abstract: We report the results of a variational TST calculation on the reaction CH+4 → CH+3+H in the zero angular momentum limit using a simple model to account for the smooth conversion of the transitional modes from vibrations of the reagent to rotational‐orbital modes of the products as a function of energy and position along the reaction coordinate. The calculations show that the simultaneous existence of multiple transition states depends strongly on the behavior of the transitional modes. Specifically, if the transitional modes loosen early in the decomposition process the orbiting transition state is the dominant transition state, and in extreme cases the only transition state, until energies well above threshold are reached. If, on the other hand, the loosening occurs late in the decomposition, the oribiting transition state, although present, may always be dominated by the ‘‘free energy bottleneck’’ tight transition state. Since the long range potential only mildly affects the orbiting transition state in...
65 citations
TL;DR: In this paper, the authors investigated the alternate thermal decomposition pathways for ethane in its ground state using ab initio electronic structure calculations, using MP4/6-311 G∗∗ level, using 6-31 G ∗ geometries for reactant, products, and transition states.
32 citations
TL;DR: In this paper, the (1,5)-H shift in cis-1,3-pentadiene has been investigated in the presence of B/sub 1/ symmetry and the energy of this structure is favored by 5.2 kcal/mol over that of C/sub s/ symmetry.
Abstract: Ab initio 3-21 G calculations have been performed for the (1,5)-H shift in cis-1,3-pentadiene. A transition state of C/sub s/ symmetry has been compared with one of C/sub 2nu/ symmetry. The lowest energy configuration of this latter structure has B/sub 1/ symmetry and must therefore be described by an open-shell calculation. The energy of this structure is favored by 5.2 kcal/mol over the one of C/sub s/ symmetry. Both structures are found to be real transition states. Both the calculated reaction rates and the kinetic isotope effects are found to be considerably smaller than the observed ones. A mechanism is suggested in which tunneling takes place between high-vibrational states of the reactant and the product. It is shown that this mechanism is most likely for the transition state of C/sub 2nu/ symmetry. The calculated tunneling rates indicate that the (1,5)-H shift in cis-1,3-pentadiene mainly takes place via this mechanism.
27 citations
TL;DR: In this article, an expression for the distribution of quantum states of the reaction products of unimolecular dissociations is obtained, based on statistical theory, by introducing an adiabatic approximation for motion from transition state to products.
Abstract: An expression for the distribution of quantum states of the reaction products of unimolecular dissociations is obtained, based on statistical theory. A recently formulated RRKM-type treatment of unimolecular reactions with highly flexible transition states is used to obtain a distribution of quantum states of the products, by introducing an adiabatic approximation for motion from transition state to products. Any impulsive (nonadiabatic) exit channel effects are neglected thereby. Both the final yields of the quantum states of the products and the time evolution of these states are considered. The time evolution of the yield of the products can permit a direct test of non-RRKM effects and, additionally via the long-time component, of other aspects of RRKM theory. The long-time component of the yield of individual quantum states of the products then provides a test of the additional (here, adiabatic) approximation. Such tests are the more definitive the narrower the distribution of initial E's and J's of the dissociating molecule.
25 citations
TL;DR: In this article, a variational transition state theory for Li++H2O→Li+(H 2O) recombination was determined by finding the maxima in the free energy along the reaction path.
Abstract: Canonical variational transition state theory rate constants are calculated for Li++H2O→Li+(H2O) recombination. Temperature dependence transition states are determined by finding the maxima in the free energy along the reaction path. Only one maximum is found at each temperature. The transition state theory rate constants are larger than those determined in a previous quasiclassical trajectory study of Li++H2O recombination. This results from a dynamical recrossing of the transition state dividing surface in the trajectory calculations.
24 citations
TL;DR: Sigmoidicity and high catalytic efficiency are to be observed when this half-liganded state is destabilized relative to the corresponding unstrained state.
23 citations
TL;DR: The role ofhydrogen bonding in specificity, binding and catalysis by the tyrosyl-tRNA synthetase from Bacillus stearothermophilus has been investigated by systematic mutation of residues which form hydrogen bonds with substrates during the reaction between ATP and tyrosine to form tyroSyl adenylate.
Abstract: The role of hydrogen bonding in specificity, binding and catalysis by the tyrosyl-tRNA synthetase from Bacillus stearothermophilus has been investigated by systematic mutation of residues which form hydrogen bonds with substrates during the reaction between ATP and tyrosine to form tyrosyl adenylate. Data on hydrogen bonding as a determinant of biological specificity are summarized thus: deletion of an hydrogen-bond donor or acceptor between the enzyme and substrate to leave an unpaired but uncharged acceptor or donor weakens binding by only 2-7 kJ mol$^{-1}$; but deletion to leave an unpaired but charged acceptor or donor weakens binding by some 17 kJ mol$^{-1}$ or so. Hydrogen bonding is found to have a profound role in catalysis by mediating the differential binding of substrates, transition states and products. The formation of tyrosyl adenylate is not catalysed by classical mechanisms of acid-base or nucleophilic catalysis but the enhancement of rate is solely a result of a combination of hydrogen bonding and electrostatic interactions which stabilize the transition state of the substrates relative to their ground states. The binding energy of ATP increases by more than 29 kJ mol$^{-1}$ as it passes through the transition state, enhancing the rate by more than a factor of 10$^{5}$. The residues involved in differential binding are spread over the molecule, away from the seat of reaction. The catalysis is delocalized over the whole binding site and not restricted to one or two specific residues. Some regions of the binding site are complementary in structure to the intermediate, tyrosyl adenylate. The apparent binding energies of certain side chains increase as the reaction proceeds, being weakest in the enzyme-substrate complex, stronger in the enzyme-transition-state complex and strongest in the enzyme-intermediate complex. This converts the unfavourable equilibrium constant for the formation of tyrosyl adenylate in solution to a favourable value for the enzyme-bound reagents and helps sequester the reactive tyrosyl adenylate.
20 citations
TL;DR: A kinetic analysis of the first-order rate equations for the three-component system in both protio and deuterio species gives a bond making kinetic isotope effect of 1/1.04 (0.04) so that the extent of bond formation in this boat-like bicyclo-1,4-diyl transition state is roughly 25%, a value to be compared with ca.
Abstract: In order to examine the extent of bond making in the boat-like 3,3-sigmatropic shift transition states, trans-2,3-dimethyl-1,4-dimethylenecyclohexane (T) and its exomethylene tetradeuteria derivative (TXD) were prepared. The 3,3-shift of TXD at 305/sup 0/C results in interconversion of starting material, 5,5,6,6-tetradeuterio-trans-2,3-dimethyl-1,4-dimethylene-cyclohexane (TND), and 2,2,3,3-tetradeuterio-anti-1,4-diethylidenecyclohexane (AD). A kinetic analysis of the first-order rate equations for the three-component system in both protio and deuterio species by numerical integration of the data and simplex minimization of the rate constants with symmetry and the assumption of no equilibrium or kinetic isotope effect on the TND-AD reaction gives a bond making kinetic isotope effect of 1/1.04 (0.04). The equilibrium isotope effects observed are 1/1.16 (0.04) so that the extent of bond formation in this boat-like bicyclo(2.2.2)octyl transition state is roughly 25%, a value to be compared with ca. 67% in chair-like acyclic 3,3-shift transition states. This rules out significant intervention of a bicyclo(2.2.2)octane-1,4-diyl intermediate or transition state. 30 references, 6 figures, 4 tables.
20 citations
TL;DR: In this article, a linear transition state of two protons between three heavy entities of equal mass is assumed, where the reaction coordinate involves heavy-atom motion rather than a protonic vibration.
Abstract: Kinetic isotope effects are calculated for a reaction involving the simultaneous transfer of two protons. A linear transition state of two protons between three heavy entities of equal mass is assumed. Expressions are obtained for the four frequencies associated with motion along the line of centers. The variation in the size of these frequencies and their associated amplitudes as a function of the coupling of the protonic motion is explored. As the coupling is increased, one finds possible transition states where the reaction coordinate involves heavy-atom motion rather than a protonic vibration: such a transition state could exhibit a kinetic isotope effect of 2 to 3. Kinetic isotope effects larger than 20 are unlikely. The breakdown of the rule of the geometric mean for successive deuterium substitutions is explored; the largest breakdown is found for a concerted symmetrical transition state and may be as large as 15%. The breakdown of the Swain-Schaad relation is smaller; for a symmetrical transition state it may be 5%.
TL;DR: In this article, the authors assume an icosahedral arrangement for Pt particles consisting of 13 atoms and located inside the supercage of a Y-zeolite, and they show a pronounced crystal face dependence of adsorption which can be rationalized in terms of the coordinative unsaturation of the surface atoms, and the ensemble size of the adsorbing site.
TL;DR: In this article, it was shown that methanol can react with protonated methanoline to form two initial intermediates. But the major route involves formation of a hydrogen bond between the methanoil oxygen and an oxonium hydrogen to produce an unsymmetrical bismethanolhydrogen (1+) ion.
Abstract: Ab initio calculations (GAUSSIAN 82, SCF 4-31G) indicate that methanol can react with protonated methanol to form two initial intermediates. The minor route produces a species where the methanol oxygen is hydrogen-bonded to the methyl hydrogen atoms of Me[graphic omitted]H2; the major route involves formation of a hydrogen bond between the methanol oxygen and an oxonium hydrogen to produce an unsymmetrical bismethanolhydrogen (1+) ion. Both initial intermediates react further to form the dimethyl oxonium ion plus water: the former reaction proceeds efficiently through an SN2 transition state, the latter by a less efficient process involving the same SN2 transition state.
TL;DR: In this article, the authors investigated the thermal and photochemical [1,3]-H shift in propene and [ 1,5]-h shift in 1,3-pentadiene by means of ab initio calculations.
Abstract: The thermal and photochemical [1,3]-H shift in propene and [1,5]-H shift in 1,3-pentadiene have been investigated by means of ab initio calculations. The suprafacial and antarafacial transition states are compared with a transition state (TS) of C2v symmetry, which results from a shift of the migrating hydrogen atom in the plane of the carbon skeleton (planar shift). Full geometry optimizations were performed at the ab initio level of computation using the restricted and unrestricted Hartree—Fock formalism with a 3-21G basis set. All structures were characterized by a complete vibrational analysis. Final state energies were calculated from a configuration interaction among the leading configurations. The effect of all other configurations was treated through a second-order perturbation. The thermal shifts are found to proceed according to the rules of Woodward and Hoffmann, i.e. an antarafacial [1,3]-H shift in propene and a suprafacial [1,5]-H shift in pentadiene. In both cases a second TS of C2v symmetry is found close in energy to the first. These TSs must be described with an open-shell configuration. For the photochemical process, both shifts are found to proceed through a TS of C2v symmetry. This TS correlates with a twisted conformation of the reactant via a planar shift.
TL;DR: In this article, the ground-state structure of triazene (1a and 1b) at various levels of theory was analyzed at the HF/3-21G, MP2/6-31G/MP2-6-33G, and MP2-7.2 kJ/mol.
Abstract: Ab initio calculations of structure, properties, and tautomerization reactions of triazene (1) at the HF/3-21G//3-21G, HF/6-31G*//6-31G*, HF/6-31G**//6-31G*, and MP2/6-31G*//6-31G* levels led to the following conclusions and predictions: (a) Calculations of the ground-state structure of (E)- and (Z)-triazene (1a and 1b, respectively) at various levels of theory show for both isomers C1 geometry with a rather flat pyramidal configuration at N(3), and small energy differences (0.2–7.2 kJ/mol) between C1 and Cs geometry, i.e. inversion at N(3) is a quasi-free process. With all levels of calculations, 1a is found to be of lower energy than 1b by 23-30 kJ/mol. (b) Comparison of vibrational frequencies of (E)-diazene (3) calculated at the HF/3-21G level with experimental values reveals that HF/3-21G calculations are reliable for the prediction of vibrational frequencies of polyaza compounds, if corrected by a factor of 0.91. On this basis, the harmonic vibrational frequencies of 1a and 1b were predicted. (c) For the rotation around the N(2)—N(3) bond of 1a two conceivable transition states, 5a (syn) and 5b (anti) were located (HF/3-21G). The energy differences between 5a or 5b, and 1a are in the order of magnitude of 50-56kJ/mol and show a slight preference for the anti-mode, i.e. energy barriers for the N(2)—N(3) rotation are obtained comparable to those observed experimentally with substituted (E)-triazenes (4). (d) Protonation of 1a at N(1), N(2), or N(3) leads to 6a, 6b, and 6c, respectively –the last one resembling an intermediate of formation of 1 from hydrogendiazonium ion (7) and ammonia (8). Energetically, the conjugate acids of 1a follow the sequence 6a < 6c < 6b. (e) The preference of N(1) protonation of 1a is also reflected in the relatively high gain of energy in the formation of H-bonded dimers of 1a with H-bonds from N(3)—H to N(1). Calculations of three different H-bonded dimers 9a–c of 1a with the 3-21G basis show that an eight-membered cyclic dimer 9c with two H-bonds from N(3)H to N(1) is energetically most favoured (67.5 kJ/mol below two separate molecules of 1a). This dimer might well be the starting situation of double intermolecular H-transfer leading to an automeric dimer 9cvia an energetically low-lying transition state 12, thus offering a low-energy pathway for the known easy tautomerization of mono- or disubstituted (E)-triazenes. For 9c⇄9c, the activation energy including correction for polarization and correlation effects as well as for vibration zero-point energy is estimated to be ca. 54kJ/mol. (f) A six-membered cyclic dimer 9b of 1a with two H-bonds from N(3)H to N(2) might be involved for double H-transfer via a transition state 11 to a dimer 10 of (E, Z)-azimine (2). This process, however, turns out to be energetically highly disfavoured (estimated energy barrier for 9b→10: 232 kJ/mol) in contrast to the reverse reaction (10→9bvia11: 4 kJ/mol). This leads to the prediction that azimines bearing an H-atom at N(2) might be kinetically too instable for isolation, being, instead, easily tautomerized to triazenes by bimolecular H-transfer.
TL;DR: In this article, a preliminary survey of the energy hypersurface for the addition reaction of the singlet oxygen molecule and ethene to give dioxetane has been performed at the MC-SCF level of theory using an active space of six orbitals and a minimal basis set.
Abstract: A preliminary survey of the energy hypersurface for the addition reaction of the singlet oxygen molecule and ethene to give dioxetane has been performed at the MC-SCF level of theory using an active space of six orbitals and a minimal basis set. Four modes of approach have been studied by complete optimization of the geometries followed by diagonalization of the Hessian matrices to characterize the critical points as transition states or higher-order saddle points. None of the approaches examined (supra-supra, supra-antara, syn diradicaloid and peroxirane-like) corresponds to a true transition state: there is a strong indication that the only path left to the system is diradical in nature, presumably in correspondence of a gauche geometry of approach.
TL;DR: In this article, the primary carbanion competitively eliminates C2H4 and C4H8, and forms C2HO-with MeCOEt2 and CH2COCHEt 2.
Abstract: Reaction of HO– with MeCOCHEt2 produces two enolate ions, MeCOEt2 and –CH2COCHEt2. The primary carbanion competitively eliminates C2H4 and C4H8, and forms C2HO–. The elimination of C2H4 is a stepwise reaction proceeding through a six-membered transition state; the first step (deprotonation) is rate-determining. The loss of C4H8 is a rearrangement reaction –CH2COCHEt2 [graphic omitted] –CH2COMe + EtCHCH2. The tertiary carbanion competitively eliminates H2, CH4, and C3H8. The losses of CH4 and C3H8 are stepwise processes occurring through six- and five-membered transition states, respectively. A double isotope fractionation experiment (2H, 13C) shows that both steps of the CH4 elimination are rate-determining.
Journal Article•
TL;DR: In this article, the MNDO MO method was used to study the intramolecular proton transfer equilibria of acetamide and methyl carbamate and found that the carbonyl-O protonated tautomer was the most stable form, the next most stable one being N-protonated form.
Abstract: Intramolecular proton transfer equilibria of acetamide and methyl carbamate have been studied theoretically by MNDO MO method. For both substrates, carbonyl-O protonated tautomer was found to be the most stable form, the next most stable one being N-protonated form. Gas phase proton transfers take place by the 1,3-proton rearrangement process and in all cases have prohibitively high activation barriers. When however one solvate water molecule participates in the process, the barriers are lowered substantially and the process proceeds in an intermolecular manner through the intermediacy of the water molecule via a triple-well type potential energy surface; three wells correspond to reactant(RC), intermediate(IC) and product complex(PC) of proton donor-acceptor pairs whereas two transition states(TS) have proton-bridge structure. General scheme of the process can be represented for a substrate with two basic centers(heteroatoms) of A and B as, Involvement of a second solvate water had negligible effect on the relative stabilities of the tautomers but lowered barrier heights by 5∼6 Kcal/mol. It was calculated that the abundance of the methoxy-O protonated tautomer of the methyl carbamate will be negligible, since the tautomer is unfavorable thermodynamically as well as kinetically. Fully optimized stationary points are reported.
01 Jan 1986
TL;DR: In this paper, the activation energy and the transition state structure depend strongly on the transition element as well as ligand, and the migration group has been found to be the methyl group in the former and the hydride in the latter.
Abstract: The optimized structures will be reported for the transition states as well as the products and the reactants for two important elementary organometallic reactions, carbonyl insertion reaction and olefin insertion/² -elimination reaction . The model reactions studied are M(CH3)(H)(CO)(PH3) → M(COCH3(H)(PH3) (M = Pd, Pt) and M(H)2(CH2CX2)(PH3)⇄ M(CH2CHX2)(H)(PH3) (M=Ni, Pd, Pt for X=H and M=Pd for X=F). The migrating group nas been found to be the methyl group in the former and the hydride in the latter. In each reaction, the activation energy and the transition state structure depend strongly on the transition element as well as ligand. Factors that control such changes will be discussed.
TL;DR: In this paper, a simple model for the diffusion mediated reaction of an asymmetric molecule with ambient reactants was devised for the particular model in which the surface reactivity is assumed to vary as 1+β(cos)θ, where θ is the polar angle and 0≤β≤1, an analytic solution in the form of an infinite continued fraction was obtained for the steady state reaction rate.
Abstract: A simple model is devised for the diffusion mediated reaction of an asymmetric molecule with ambient reactants. For the particular model in which the surface reactivity is assumed to vary as 1+β cos θ, where θ is the polar angle and 0≤β≤1, an analytic solution in the form of an infinite continued fraction was obtained for the steady state reaction rate. This fills a gap in the theory of reaction rates of asymmetric molecules which generally have been investigated by numerical computation. In agreement with previous studies on other models with smoothly varying reactivity, it is found that the steady state reaction rate depends primarily on the surface averaged reactivity. The effect due to asymmetry is only of secondary importance for such models and even this tends to get washed out as the rotational diffusion coefficient increases.
TL;DR: The fundamental group of reaction mechanisms as mentioned in this paper is an algebraic framework for quantum chemical molecular design, for computer-based synthesis planning and for the prediction of new reaction mechanisms and synthetic routes.
Abstract: Reaction paths which lead from a common reactant to a common product through the same sequence of transition structures and intermediates are equivalent in a chemical sense and belong to the same reaction mechanism. In the language of topology, all reaction paths which are not separated by high energy domains of a potential-energy hypersurface, are homotopically equivalent, that is, these paths are continuously deformable into one another below some energy bound A. If A is the maximum energy available to the molecular systems, then each family of equivalent reaction paths represents a reaction mechanism. Reaction mechanisms on any potential-energy hypersurface form a group: the fundamental group of reaction mechanisms. This group is not determined by molecular symmetry groups, rather, they represent all relations among reaction mechanisms admitted by the given potential-energy hypersurface. The fundamental group of reaction mechanisms serves as an algebraic framework for quantum chemical molecular design, for computer-based synthesis planning and for the prediction of new reaction mechanisms and synthetic routes.
TL;DR: In this paper, the MNDO SCF-MO method was used for the analysis of proton exchange between β-fluorinated ethanols and ethoxide ions.
Abstract: Proton exchange between β-fluorinated ethanols and ethoxide ions has been studied using the MNDO SCF-MO method. Calculations were performed on reactions of ethoxide ion with ethanols substituted in the β-position with 0, 1, 2, and 3 fluorine atoms as well as on reactions where both the ethanol and the ethoxide ion were substituted with the same number (1, 2, 3) of fluorine atoms in the β-position. The energies obtained for the ion–molecule reactant complexes and the transition states from these reactions have been analyzed using the Marcus equation. Through the calculated force-constant matrices of reactants and transition states we also calculated the kinetic isotope effects for the proton-transfer reactions. The semiclassical isotopic rate constant ratios (kH/kD)s were found to be of rather normal magnitude and showed a variation with the energy of reaction. The calculated ratios of tunnel correction factors, QtH/QtD, proved to be unrealistically high. These factors were also calculated with the frequen...
TL;DR: In this paper, the reactions of atomic hydrogen with monosubstituted halomethanes (CH 3 X, X Cl, Br, I) have been studied using the MNDO semi-empirical SCF MO method.
TL;DR: In this paper, molecular orbital calculations on the transition states and barrier heights for the addition of atomic hydrogen to silaethylene are carried out, and the activation energy for addition to the silicon site is lower than that to the carbon site, while the exothermicity is smaller.
TL;DR: In this article, the first step of the BAC3 and ABAC3 reactions to the tetrahedral anionic intermediate T 0 is assumed to be in equilibrium or in a steady-state condition with the neutral intermediate T0[R1C(OH)2(OR2)].
Abstract: The mechanism for the exceptional acid-catalysed A-BAC3 hydrolysis is considered in detail on the basis of the estimated free energy levels of transition states and intermediates on the possible reaction paths for the hydrolyses of ethyl trichloroacetate and methyl acetate by the acid-catalysed AAC2 and ABAC3 and the neutral BAC3 mechanisms. It is concluded that the first step of the BAC3 and A-BAC3 reactions to the tetrahedral anionic intermediate T–[R1C(OH)(O–)(OR2)] is the same. T– is assumed to be in equilibrium or in a steady-state condition with the neutral intermediate T0[R1C(OH)2(OR2)]. It is proposed that, in the case of the A-BAC3 mechanism, a concerted general base–general acid-catalysed decomposition of T0 leads to the observed acid catalysis which thus follows the sequence of steps Ester → T–→ T0→ Products. Kinetic data for the acid hydrolyses of chloromethyl and 2,2-dichlorovinyl acetates together with other available data for exceptional, acid-catalysed hydrolyses were treated by a non-linear least-squares procedure.
TL;DR: In this paper, the preparation and silver-assisted acetolyses of the 1-methoxy (5 ), 1-methyl (6), 1-carbomethoxy (7 ), and 1-nitro (8 ) derivatives of trans-7,8-dichlorodibenzobicyclo [2.2] octa-2,5-dienes are described.
TL;DR: MINDO/3 calculations have been made on the potential energy surfaces for the attachment of radicals to benzene (1) and naphthalene (2) in the vapor state as discussed by the authors.
Abstract: MINDO/3 calculations have been made on the potential-energy surfaces for the attachment of OH. radicals to benzene (1) and naphthalene (2) in the vapor state. The activation energies of these reactions are calculated as 88 and 58 kJ/mole. while the enthalpies at 298K are calculated as −211 and −199 kJ/mol. The transition states in (1) and (2) lie closer to the reagents than the products on the reaction coordinate, while (1) has an earlier transition state than does (2). The transition states in these reactions have high dipole moments: 3.1 and 3.6 D, respectively, which are due to charge transfer from the hydrocarbons to the OH.. Quantum-chemical calculations and kinetic data on the reactions of aromatic hydrocarbons with OH. in aqueous solution indicate that the mechanism is probably not one involving electron transfer and a rate-limiting stage in the attachment. These processes are of high performance because the radicals are of high stability, while polar effects determine the selectivity.
Journal Article•
TL;DR: In this article, MNDO calculations were carried out to determine reactant complexes and transition states of the reactions of where X = F, Cl, CN and Y = CN, OH, F and Cl.
Abstract: MNDO calculations were carried out to determine reactant complexes and transition states of the reactions of where X = F, Cl, CN and Y = CN, OH, F, Cl. The leaving group ability was found to vary inversely with the activation barrier, which in turn was mainly ascribable to the deformation energies accompanied with bond stretching of C-X bond and inversion of group. The nucleophilicity was shown to be in the order $Cl^->F^->OH^->CN^-$ but the effect on the activation barrier was relatively small compared with that of the leaving group. The bond breaking and bond formation indices and energy decomposition analysis showed that the TS for the reaction of Cl occurs in the early stage of the reaction coordinate relative to that of F. It has been shown that the potential energy surface (PES) diagrams approach can only accommodate thermodynamic effects but fails to correlate intrinsic kinetic effects on the TS structure.
01 Jan 1986
TL;DR: The intrinsic paths of the disproportionation reactions were calculated by the 4-31G UHF SCF procedure, and MRD-CI calculations have been carried out at several points along the paths.
Abstract: The intrinsic paths of the disproportionation reactions OH + OH → H2O + O(3P) and OH + OH → H2O +O(1D) have been calculated by the 4-31G UHF SCF procedure, and MRD-CI calculations have been carried out at several points along the paths. Both reactions are found to proceed through the initial formation of hydrogen-bonded complex OH·OH followed by the attainment of coplanar transition states. The net activation barrier heights obtained for the triplet and singlet channels of the reaction are ca. 3 and 35 kcal/mol, respectively. On the basis of the transition state characteristics deduced from the ab initio computations, the bimolecular rate constants have been evaluated from the conventional transition state theory. The results obtained in the temperature range 300-2000 K are found to be in good agreement with the experimental data which apparently exhibit a strong non-Arrhenius behavior.
TL;DR: In this article, the kinetics of an irreversible reaction on a lattice is analyzed when the reaction rate at a site depends upon the number of its unreacted nearest neighbors in the lattice.
Abstract: The kinetics of an irreversible reaction on a lattice is analyzed when the reaction rate at a site depends upon the number of its unreacted nearest neighbors