Showing papers on "Transition state published in 1976"

On simple saddle points of a potential surface, the conservation of nuclear symmetry along paths of steepest descent, and the symmetry of transition states

Abstract: We show that if the transition state of a chemical reaction is a simple saddle point of the potential surface linked directly with reactants and products by paths of steepest descent, then the symmetry of the transition state is limited to the joint symmetry of reactant and product, unless reactant and product are physically indistinguishable and themselves symmetry related, in which case the transition state may have those additional symmetries that interchange reactant and product. We outline some of the general consequences of this symmetry restriction on transition states.

Deuterium and tritium exchange in enzyme kinetics.

Abstract: The theory of the isotopic exchange of deuterium and tritium between an enzyme-substrate complex and the solvent is derived for 16 different types of experiment involving measurements of initial velocities and of the isotopic content of the reactants and products as a function of the extent of reaction. It is shown how the data from these experiments can be analyzed to obtain the rate constants for the individual steps and thereby the Gibbs free energies of the intermediates and transition states in the reaction. The effects of isotopic substitution on each intermidiate and transition state are also found and this allows conclusions to be drawn as to the extent to which a proton is in flight in a particular transition state. Neither substrate handling (that is, on-off steps), nor the isotopic exchange with the solvent, is assumed to be rapid.

Ab initio molecular orbital study of simple 1,3-dipolar reactions

Abstract: Ab initio molecular orbital calculations with minimal and extended basis sets have been carried out for the 1,3-dipolar addition of fulminic acid to acetylene, ethylene, ethynamine and propynenitrile Optimized geometries are reported for the transition states HCNO+C2H2, HCNO+C2H4, HCNO+ C2HNH2, for the adducts isoxazole and 2-isoxazoline, and for nitrosocyclopropene as a possible intermediate The calculations indicate that (a) these 1,3-dipolar reactions are synchronous processes, (b) the geometry of the transition state is insensitive to substitution and (c) of the isomeric substituted adducts, 5-aminoisoxazole and isoxazole-4-carbonitrile should be formed preferentially

Mechanism of cyclopropane-propene isomerization

Abstract: A SINDO method was used to investigate the sequence of rearrangements in the cyclopropane-propene isomerization. Geometries of reactant and product were optimized for a complete set of 21 independent internal coordinates. Then the bond angle of the migrating H atom was chosen as reaction coordinate and all other geometrical parameters were energy minimized along this reaction coordinate. In the neighborhood of the transition state a Newton type iteration process is initiated to locate the transition state. The latter is characterized by a CCC bond angle close to 90 ° and an almost finished migration of the H atom. The activation energy was found about 25% lower than the experimental value of 65 kcal/mole. A sequence of rearrangements was established, suggesting the initiation of the reaction by a combination of two normal modes: a torsional vibration of the methylene groups together with an asymmetric stretch of the carbon framework. Our calculations classify the reaction as concerted.

13C kinetic isotope effects and reaction coordinate motions in transition states for SN2 displacement reactions

Abstract: Reaction coordinate motions and 13C kinetic isotope effects at 25 °C have been calculated for the SN2 reactions of methyl iodide with iodide, cyanide, and chloride ions and for the SN2 reaction of ...

Pyrolysis of t-butyl N-arylcarbamates in the gas phase, and correlation of the elimination rates with those obtained in solution

Abstract: Rates of pyrolysis of t-butyl N-arylcarbamates in the gas phase at 548.8 K correlate linearly with their rates of pyrolysis in diphenyl ether at 450.7 K and with σ0 values (ρ+ 0.55). The ρ factor for the gas-phase reaction is almost identical (after correcting for the temperature difference) to that obtained in solution. Both the substituent effects and the ρ factor are therefore largely independent of solvation. Solvation does however increase the reaction rate as expected, and by factors ranging from 4(dodecane) to 25 (decanol). Comparison of the gas-phase data with those for carbonates show that NH is more effective at transmitting conjugative effects than is O, which confirms similar observations deduced from the pKa values of substituted acetic acids (measured in solution); these results confirm that the Hammett ρ factor depends not only upon the charge developing at a reaction site but also upon the ability of the system to transmit electronic effects. The magnitudes of the ρ factors for pyrolysis of t-butyl benzoates and carbamates confirms that the transition states for the latter are more polar. The kinetic isotope effect for the reaction, is 1.78 at 569.2 (gas phase) and 2.56 at 468.7 K (diphenyl ether); this is less than for the pyrolysis of secondary acetates and provides further evidence that the breaking of the C–H bond is less kinetically important, the more polar the transition state.

Kinetic isotope effects in the reduction of 2- and 3-alkyl-substituted-indanone-(tricarbonyl)chromium complexes by NaBH4 and NaBD4. Evidence for displacement of the transition state depending on the substrate

Abstract: The kinetic, primary, H–D isotope effects, measured when 2- and 3- alkyl-substituted-indanone-(tricarbonyl)chromium complexes are reduced with NaBH4 and NaBD4, exhibit significant variations of kH/kD values depending on the degree of steric hindrance around the ketone groups; this suggests that displacements of the transition states are involved in this type of reaction.

Transition States and Reaction Mechanisms in Organic Chemistry

Abstract: The conventional description of the chemical reaction as a single equation $$Reactant\left( s \right) \to Product\left( s \right) $$ actually encompasses a vast number of microscopic phenomena. These phenomena occur throughout the entire reaction process but it is convenient to distinguish successive stages in which a given phenomenon tends to predominate as in Figure 1. The first stage is vibrational excitation of the reacting molecule — here trans 1,2 dideuterocyclopropane — by multiple collisions with other molecules M in the reaction vessel. The second stage is exchange of acquired vibrational energy between the various modes of the molecule ; overall rotation is certainly involved in this exchange. The third stage is the actual deformation of the molecule along the reaction coordinate — rather than along “useless” vibrational coordinates. Classically speaking, in this third stage the molecule explores, via kinematic trajectories, the potential energy surface.

A study of the transition state for the unimolecular decomposition of a triatomic molecule

Abstract: A quantitative examination of the transition state model of unimolecular reactions has been made using an explicit three-dimensional potential. The results obtained using RRKM theory have been applied to the three principal experimental areas of unimolecular decay for the decomposition of water. The results of shock-tube studies of the decomposition of water imply that one of the rotational degrees of freedom is active in energy randomization. The consequences of assuming tight and loose transition states for long-lived complexes in molecular beam studies have been examined. The distribution of energy amongst the products of reaction in a mass spectrometric decay has been calculated and the importance of the initial rotational energy on this is emphasized.

Mercury photosensitization of 1-hexene and cis-2-octene vapor. Intramolecular hydrogen abstraction of vibrationally excited π,π* triplet state

Abstract: Upon mercury photosensitization, the vibrationally excited triplet of the simple alkenes 1-hexene and cis-2-octene undergoes an olefinic type II reaction consisting of an intramolecular 1,5-hydrogen abstraction via a six-membered transition state and subsequent reaction of the resulting biradical, in sharp contrast to the well established allylic C—C and C—H bond cleavage of alkenes without a γ-hydrogen. The cis–trans photoisomerization of cis-2-octene and other intramolecular hydrogen abstractions of minor importance occurring through unfavorable transition states were also observed. The mercury photosensitization of various cyclic and acyclic alkenes so far reported are comparatively discussed.

The E2C mechanism in elimination reactions. 8. Interaction of conjugating substituents with E2C- and E2H-like transition states

Abstract: Rates and olefinic products of dehydrotosylation of secondary tosylates under conditions suitable for E2C, E2H, and solvolysis (El) reactions, respectively, have been measured. The kinetic products are compared with those from equilibration. Quite different proportions of olefins are obtained according to the reaction conditions and this has obvious value for synthetic work. The tosylates studied contain groups, e.g., phenyl, acyl, viny1, capable of conjugating with the developing double bond in the transition state leading to olefins. The product distribution from E2Clike reactions is not entirely consistent with the concept of a very product- (olefin-) like E2C transition state.

Ab initio molecular orbital study of simple 1,3-dipolar reactions

Abstract: Ab initio molecular orbital calculations with minimal and extended basis sets have been carried out for the 1,3-dipolar addition of fulminic acid to acetylene, ethylene, ethynamine and propynenitrile. Optimized geometries are reported for the transition states HCNO+C2H2, HCNO+C2H4, HCNO+ C2HNH2, for the adducts isoxazole and 2-isoxazoline, and for nitrosocyclopropene as a possible intermediate. The calculations indicate that (a) these 1,3-dipolar reactions are synchronous processes, (b) the geometry of the transition state is insensitive to substitution and (c) of the isomeric substituted adducts, 5-aminoisoxazole and isoxazole-4-carbonitrile should be formed preferentially.