Showing papers on "Transition state published in 1980"

Potential energy characteristics and energy partitioning in chemical reactions: Abinitio MO study of four‐centered elimination reaction CH3CH2F→CH2=CH2+HF

Abstract: The mechanism of energy disposal along the reaction pathway was discussed in relation to the characteristic features of potential energy surface. A simplified theoretical model was constructed in terms of the intrinsic reaction coordinate (IRC) and normal coordinates perpendicular to it. Ab initio MO calculations with the split‐valence basis set were carried out for the four‐centered elimination reaction CH3CH2F→HF+CH2CH2. The energy gradient method was used to optimize the stationary points on the potential surface and to trace IRC as well as to calculate the force constants. The origin of vibrational enhancement of HF in the reaction product was interpreted as the result of interchange of the main components of IRC in a local region where the IRC curvature is large.

Classical transition state theory: A lower bound to the reaction probability

Abstract: We derive a rigorous lower bound to the microcanonical reaction probability in classical collinear atom–diatom collisions. This lower bound complements the upper bound provided by transition state theory, and the information needed to calculate the bound is acquired automatically in the search for the periodic orbit dividing surfaces that are possible transition states for the reaction. Numerical calculations for F+H2 and H+Cl2 over a wide energy range show that the lower bound provides the best available estimate of the reaction probability, short of a full dynamical calculation.

Cycloaddition mechanism and the solvent dependence of rate

Abstract: The dependence of the rate constant on solvent polarity is an important mechanistic criterion which should always be used in conjunction with other diagnostics tool. The slow step of 2 + 2 cycloadditions of tetracyanoethylene (TCNE) with enol ethers [1], thioenol ethers [2] and trans-fixed 1,3-dienes [3] is the formation of a zwitterionic intermediate. The log k2 values are linear functions of the Dimroth-Reichardt parameter ET. The rate accelerations of TCNE cycloadditions in going from cyclohexane to acetonitrile amount to 29,000 for anethole, 10,800 for 1-ethoxyisobutene, 2,600 for butyl vinyl ether, 17,000 for ethyl 1-propenyl sulfide and 54,000 for verbenene. The violation of stereo-specificity becomes greater with increasing solvent polarity. In contrast, the additions of TCNE to anthracene [4] and related Diels-Alder reactions show only a minute solvent dependence. The same is true for 1,3-dipolar cycloadditions of diazoalkanes [5], phenyl azide [6], C-phenyl-n-methylnitrone and azomethine imines to various dipolarophiles. These concerted cycloadditions are characterized by early transition states, i.e., there is hardly any change of solvation energy during the activation process. A dichotomy of reaction paths was found for diazocarbonyl compounds and enamines. Whereas dialkylaminocyclohexenes produce cycloadducts, the corresponding enamines containing the cyclopentene ring undergo azo coupling furnishes enamino-hydrazones via a zwitterionic intermediate. The contrasting dependence of rate on solvent polarity supports different mechanism [7]. Dimethylketene combines with N-isobutenylprrolidine to give a 3-pyrrolidinocyclobutanone as 1:1 adduct and a δ-methylene-δ-lactone as 2:1 adduct. The dependence of the product ratio on the concentration of dimethylketene allows to disent angle a concerted pathway and a reaction via a zwitterionic intermediate which can be intercepted by a second molecule of dimethylketene (see formula Scheme) [8]. The different solvent dependencies of the two reaction branches confirm the mechanistic divergence.

Theoretical studies of the O+H2 reaction

Abstract: ab initio theoretical calculations of the potential energy surfaces and rates of reaction of the O+H2 reactions are reported. (AIP)

A theoretical study of paths for decomposition and rearrangement of dihydroxycarbene

Abstract: The transition states for fragmentation of dihydroxycarbene [C(OH)2] to H2 and CO2 and for the rearrangement of this carbene to formic acid were located by ab initio calculations. The relative energies of the transition states were determined at several levels of theory and the basis set dependence of the energies is discussed. At the best level of theory; using a basis set of double-zeta quality augmented by polarization functions and with the inclusion of extensive CI, we found that the transition state for fragmentation was considerably higher in energy than that for rearrangement. This finding is at variance with the predictions of the Woodward--Hoffmann rules because fragmentation represents an “allowed” reaction, whereas rearrangement is “forbidden.” In conformity with the Woodward–Hoffman rules, the transition state for rearrangement was found to be close in energy to H· + ·CO2H. The even higher energy of the transition state for concerted fragmentation to H2 and CO2 is attributed to the need for the latter fragment to remain substantially bent in order to permit H2 formation while maintaining a modicum of OH bonding. Difficulties in locating the transition state for concerted fragmentation are discussed and a new method for finding transition states is proposed.

Intra- and inter-molecular catalysis in the aminolysis of benzylpenicillin

Abstract: The rate law for the aminolysis of benzylpenicillin in water is reported. The Bronsted β-values for the uncatalysed, the amine-catalysed, and hydroxide-ion catalysed reactions are 1.0, 1.09, and 0.96 respectively. This indicates that in the transition states for all three pathways the amine nucleophile contains a unit positive charge which is consistent with the formation of a tetrahedral intermediate. Intramolecular general base catalysis occurs with the reaction of ethylenediamine and, despite the importance of general base catalysis in the aminolysis reaction, the effective concentration of the catalysing base is only ca. 1M, which is attributed to the ‘loose’ transition state involved in intermolecular catalysis. Intramolecular general acid catalysis occurs with the reaction of ethylenediamine monocation. This suggests that nucleophilic attack takes place from the least hindered α-side.

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines†

Abstract: The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C-X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M- XI' ions and reference ions arising from 3-hydroxypyrido[l,2alindole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X=H), which gives rise to a mixture of [M-HI' ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thennochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution readions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C-X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient. It has been shown recently2 that the partitioning of the excess potential energy of the transition states (or the activation energy of the reverse reactions F:) between the kinetic energy T, released during the reaction, and the internal energy F* of the products during the loss of ortho substituents from the molecular ions of substituted benz.alacetones depends strongly and systematically on the thermochemistry of the reaction or the position3 of the transition state on the reaction coordinate. Thus it appears that an investigation of the variation of the kinetic energy release and the energy partitioning quotient q = T/&f with structural changes of the reactant ions gives some insight into fundamental properties of elementary reactions of organic ions, which will be useful in testing mechanistic models of organic chemistry and linking them more closely to physical theories of chemical reactions. The loss of substituents from molecular ions of benzalacetones with formation of 2-rnethylbenzpyrylium ions4 can be regarded as an intramolecular aromatic substitution reaction,5 a well known reaction in organic chemistry. As a further example of this type of reaction the results of a detailed investigation of the loss of ortho substituents from the molecular ions of substituted 2-benzoyl pyridines la-lg (Scheme 1) are reported. The decomposition of the molecular ions of la-lg occurs by the three competing fragmentation pathways A-C shown in Scheme 1. Compound la exhibits large peaks due to [M-HI' and [M-CO]" ions in its 70eV mass spectrum, which are absent or of much lower relative abundance in the 70 eV mass spectra of

Molecular dynamics simulation of a chemical reaction in solution

Abstract: We have carried out a study, using the molecular dynamics simulation technique, of a simple model describing a chemical reaction in solution. The model consists of a three body reaction complex, in which a light particle transfers between two heavier species, surrounded by a ‘bath’ of 106 spherical liquid particles. Within the reaction complex we employ a three body potential with an effective barrier to transfer which depends strongly upon the separation of the heavy particles. We study the behaviour of a correlation function related to the kinetic rate constant for the transfer reaction and examine the validity of the transition state approximation to this quantity. It is found that transition state theory improves as the mass of the transferred particle increases. Arrhenius plots yield an effective activation energy, although the transfer is not best described as a barrier hopping process. We also compute the predicted incoherent neutron scattering spectrum due to light particle motion and compare with...

Secondary deuterium isotope effects on epoxide methanolysis reactions

Abstract: Secondary kinetic deuterium isotope effects have recently come into widespread use as probes of reaction mechanism and transition state structure. Isotopic substitution at the reacting carbon gives rise to the so-called ..cap alpha.. effect due to differences in the energetics of out-of-plane bending of C-H and C-D bonds; these effects are usually interpreted in terms of changes in hybridization at the reacting carbon as the reaction processes. Alternately isotopic substitution at carbons adjacent to the reacting carbon leads to a ..beta.. effect presumably arising from differences in the ability of C-H and C-D bands to interact with orbitals on the reacting carbon by means of hyperconjugation. In connection with mechanistic studies of the epoxide-diol pathway secondary deuterium isotope effects were used to compare epoxidation of olefins by cytochrome P-450 with olefin epoxidation by m-chloroperbenzoic acid. Observations of secondary deuterium isotope effects on the acid-and base-catalyzed methanolysis of p-nitrostyrene oxide, a chemical model system for the epoxide hydrase system were reported. This is the first observation of an ..cap alpha.. effect at the ..beta.. carbon or a secondary effect at a neighboring carbon not attributable to hyperconjugation phenomena. Such effects might be expected but are not observed in the solvolysis ofmore » ..beta..-arglethyl tosylates and in the thermal cic-trans isomerization of cyclopropane derivatives. Another interesting result is the parallelism that the major reactions in acid and base both involve late or product-like transition states, while both minor reactions involve early or reactant-like transition states. These studies have shown that secondary deuterium isotope effects provide a sensitive and predictable probe for mechanistic studies of epoxide reactions. 3 tables. (DP)« less

Influence of solvation on the initial and transition states for anation reactions of transition-metal complexes in mixed aqueous solvents

Abstract: A free-energy cycle has been applied to the rate-determining step in mixed aqueous solvents for the anation of cations having all aquo-ligands or only one aquo-ligand together with a variety of other ligands. Combining rate data with activities of water and calculated values for the equilibrium constants for outer-sphere complexes of the incoming ligand with the cation in the mixture, the relative effects of changes of solvation in the initial and transition states can be assessed. At mole fractions of co-solvent less than those where sharp structural changes occur in the mixture, changes of solvation in either the initial or in the transition state may dominate, depending on the identity of the incoming ligand and of the complex cation; but, in general, at mole fractions of co-solvent higher than those where the sharp changes in solvent structure occur, changes in solvation in the initial state have a dominant effect on the rates.

Cycloreversion of Cyclobutanes

Abstract: Semiempirical MO calculations with the method SINDO1 were performed to study the potential energy surface of cyclobutane and several substituted cyclobutanes with substituents F, OCH3 and CN. The reaction pathway with the lowest activation energy leading to two ethylenic fragments is nonconcerted. One carbon bond is broken after symmetric opening of two adjacent bond angles and twisting of the carbon framework. The first transition state is asymmetric and diradicaloid. The reaction proceeds to a diradicaloid, non-zwitterionic intermediate. The second transition state is characterized by bond breaking of the inner carbon-carbon bond. For the unsubstituted case, the barrier for free rotation of the outer methylenic groups was also calculated. In comparison, the unsubstituted reaction is characterized by transition states of almost equal energy, whereas in the substituted reactions the barriers for the second bond breaking are much higher than for the first bond breaking step.

An Electrophilic Addition Reaction in Solution: C2H4 + Cl2. Ab initio studies

Abstract: The C2H4 + Cl2 addition reaction has been studied from ab initio SCF calculations, assuming a perpendicular approach of Cl2 along the C2v axis of C2H4 and an intermediate ionic state C2H4 Cl+ + Cl-. It is found that, in gas phase, the energy of C2H4Cl+ + Cl- is much to high for a chemically possible intermediate state; the interaction with solvent molecules may allow the reaction according to this path. The potential energy curve is drawn for this first step of the reaction: within the accuracy of our calculations, no transition state appears in gas phase; the solvent effect is discussed, showing that one or several transition states may occur in solution.

Stereochemistry of diels-alder reactions at high pressure. ii. influence of high pressure on asymmetric induction in condensation of (-)-di-(r)-menthyl fumarate with butadiene and isoprene

Abstract: The high-pressure Diels-Alder condensation of (−)-di-(R)-menthyl fumarate with butadiene and isoprene, followed by reduction of the adducts with lithium aluminium hydride, produced (1S,2S)-(+)-4-cyclohexene-1,2-dimethanol and (1S,2S)-(+)-4-methyl-4-cyclohexene-1,2-dimethanol in 6.2–12.8% optical yield, depending on the pressure applied. Results of asymmetric synthesis were discussed in light of the concept of parallel transition states. An interpretation of the thermal and Lewis-acid-catalyzed asymmetric Diels-Alder condensations using (−)-di-(R)-menthyl fumarate was proposed.

Influence of solvation on the initial and transition states for anation reactions of transition metal complexes in mixed aqueous solvents

Abstract: A free-energy cycle has been applied to the rate-determining step in mixed aqueous solvents for the anation of cations having all aquo-ligands or only one aquo-ligand together with a variety of other ligands. Combining rate data with activities of water and calculated values for the equilibrium constants for outer-sphere complexes of the incoming ligand with the cation in the mixture, the relative effects of changes of solvation in the initial and transition states can be assessed. At mole fractions of co-solvent less than those where sharp structural changes occur in the mixture, changes of solvation in either the initial or in the transition state may dominate, depending on the identity of the incoming ligand and of the complex cation; but, in general, at mole fractions of co-solvent higher than those where the sharp changes in solvent structure occur, changes in solvation in the initial state have a dominant effect on the rates.

Quantum-chemical calculations of transition states and activation energies of molecular 1,2-elimination (1,2-addition) of HNO 2

Abstract: 1. Quantum-chemical calculations of the reactants and the transition states in the reaction of 1,2-addition (1,2-elimination) of HNO2 to a double bond have been carried out. The orbital interactions of the reactants were analyzed, and it was shown that in the course of addition electron density is transferred from HNO2 to the olefin, i.e., HNO2 acts as a nucleophile. 2. The regioselectivity of the addition reaction of HNO2 is determined by the character of the lower vacant MO of the olefin. The addition to nitroethylene should proceed with a lower energy of activation than in the reaction with ethylene, and in opposition to the Markownikoff rule, while the addition to alkyl-substituted ethylene is according to the Markownikoff rule. 3. The introduction of an additional nitro group to the molecule of nitroethane and also replacement of the methyl group in nitroethane by an amino group lead to a considerable decrease in the activation energy of the reaction of the 1,2-elimination of HNO2. 4. The dipole moment of the transition state of the 1,2-elimination of HNO2 is much smaller than in the initial nitroethane, which is explained by mutual compensation of the component due to the polarization of the HNO2-ethylene system and the dipole moment of the HNO2 molecule itself.

Electron impact induced intramolecular hydrogen transfer reactions in polymethylene glycol di‐p‐toluenesulfonates

Abstract: The mechanisms of formation of the ionized acid and the protonated acid fragments in the electron impact induced fragmentation of the title compounds were investigated. The well known mechanisms of hydrogen transfer through mono- and bicyclic transition states that occur in the molecular ions of carboxylic esters are not the main pathways giving rise to these fragments. The major component of the m/z 172 peak corresponding formally to ionized p-toluenesulfonic acid in fact has a different structure; its formation involves a complex mechanism including a double hydrogen transfer reaction.