Showing papers on "Transition state published in 1971"

Collision Dynamics and the Statistical Theories of Chemical Reactions. II. Comparison of Reaction Probabilities

Abstract: The reaction probability obtained as a function of total energy from transition‐state theory for classical collinear atom, diatomic‐molecule exchange reactions is compared with the results of trajectory calculations for the same potential surface. For a well‐defined barrier in the symmetric H, H2 reaction, there is excellent agreement up to rather high energies. Other reactions (e.g., H2+Br⇄HBr+H) yield considerably larger deviations; these can be shown to result from a transmission coefficient that is less than unity, from a nonequilibrium distribution in the transition region, or from both effects.

Substitution at saturated carbon. Part VIII. Solvent effects on the free energy of trimethylamine, the nitrobenzyl chlorides, and the trimethylamine–nitrobenzyl chloride transition states

Abstract: Standard free energies of transfer from ethyl benzoate to 17 other solvents of trimethylamine, p-nitrobenzyl chloride, and the trimethylamine–p-nitrobenzyl chloride transition state have been calculated (at 298 K). The transition state is stabilised by polar solvents and is destabilised by non-polar solvents; dipolar aprotic solvents are much more effective in stabilising the transition state than are the aliphatic alcohols. Similar calculations have been carried out for the substitution of o-, m- and p-nitrobenzyl chlorides by trimethylamine for 6 solvents (at 303 K). A comparison of solvent effects on the free energy of the trimethylamine–p-nitrobenzyl chloride transition state, the t-butyl chloride solvolysis transition state, and the Et4N+l– ion pair indicates that the solvolysis transition state markedly resembles an ion pair in behaviour, but that the transition state for substitution of p-nitrobenzyl chloride by trimethylamine is very much less polar.

Model calculations of isotope effects in proton transfer reactions

Abstract: An electrostatic charge-cloud model, including an empirical repulsive potential is used to calculate the properties of initial, final and transition states for the reaction X–H++ Yδ–→X–+ Yδ–H+, where X– and Yδ– are spherical charge distributions. The real and imaginary frequencies thus derived are used to calculate hydrogen isotope effects as a function of δ and hence of the energy change in the reaction. It is concluded that the observed variation of isotope effects cannot be accounted for in terms of the real vibrations of the transition state, but are primarily determined by the tunnel correction. The model also accounts for the dependence of isotope effects upon steric hindrance.

Substitution at saturated carbon. Part IX. Free energies of transfer from methanol to aqueous methanol of tetra-alkyltins and the transition states in the bimolecular substitution of tetra-alklytins by mercuric chloride

Abstract: Standard free energies of transfer (on the molar scale) from methanol to various methanol–water mixtures have been determined for tetramethyl-, tetraethyl-, tetra-n-propyl-, and tetra-n-butyl-tin. When combined with similar previous data for mercuric chloride, and with the known free energies of activation for reaction (1), these free energies R4Sn + HgCl2→ RHgCl + R3SnCl (1) of transfer yield values of ΔGt°(Tr), the standard free energy of transfer from methanol to aqueous methanol of the corresponding transition states in reaction (1). It is shown that the reductions in the free energy of activation of reaction (1) observed when solvent methanol is replaced by aqueous methanol are due to very large increases in the standard free energy of the initial states on transfer from methanol to aqueous methanol.Standard entropies of transfer (on the molar scale) have also been calculated for the reactants and transition state in reaction (1; R = Et). The influence of various methanol–water mixtures on the value of ΔS‡ is shown to be due to a combination of initial-state and transition-state effects.

Substitution at saturated carbon. Part XII. Calculation of the electrostatic contribution to free energies of transfer from methanol to aqueous methanol and to water of solutes and transition states. A new method for the examination of transition states

Abstract: Standard free energies of transfer, from methanol to aqueous methanol and to water, of solutes and transition states have been divided into a nonelectrostatic contribution, ΔGn°, and an electrostatic contribution, ΔGe° The value of ΔGe° for an uncharged transition state is considered to be related directly to the degree of charge separation (δ±) in the transition state It is shown that uncharged transition states in solvolyses thought to proceed by mechanism SN1 are characterised by high values of δ±(0·8), whereas uncharged transition states in presumed SN2 solvolyses are characterised by low values of δ±(ca 0·3); it is suggested that the values of ΔGe° and of δ± for uncharged transition states in nucleophilic solvolyses can be used as a criterion of reaction mechanismValues of δ± for transition states in the electrophilic substitution of tetra-alkyltins by mercury(II) chloride are quite large, averaging ca 0·55 for substitution of tetramethyltin and tetra-n-propyltin, and ca 0·65 for the substitution of tetraethyltin It is suggested that these values of δ± are compatible with mechanism SE2(open) for these substitutions, but not with mechanism SE2(cyclic)

A Semi-Empirical Model of the Energy Barrier of Proton Transfer Reactions†

Abstract: The energy barrier in proton transfer reactions is described by a Johnston-type equation (1) (n = order of bond to be broken). The barrier model is discussed in terms of free energies. The Vi values are free energies of ionic cleavage in aqueous solution of the XH and YH bonds; they are computed from eqns. (4c) and (4d). The values of p1 and p2 affect curvature (absence or presence of maximum) and symmetry of the barrier. It is postulated that pi is a typical constant of the reacting bond and can be transferred from one transition state to another. With the aid of eqn. (1) and its first derivative, values of pi and nm (bond order at maximum of barrier) can be based on quantities determined experimentally, Δ≠ and ΔG. For OH bonds, pi ≈ 1.0. For CH bonds pi is larger than 1.0 and depends on the structure of the carbanionic moiety (influence of resonance and inductive effects). As there cannot be a maximum if p1 = p2 = 1.0, the suggested model of the barrier leads to a better understanding why proton transfer must be ‘fast’ in some reactions and ‘slow’ in others. The computed values of nm may be utilized to gain some insight into the nature of the transition states; they supply a basis for the discussion of primary hydrogen isotope effects.

Catalysis by hydrogen halides in the gas phase. Part XXI. Butylamine and hydrogen bromide

Abstract: The hydrogen bromide-catalysed decomposition of t-butylamine into isobutene and ammonia has been studied in the temperature range 395–460°C. Individual runs follow the first-order rate law. The reaction is homogeneous, of the first order in each reagent, and molecular. Possible transition states for the catalysed reaction are examined. The variation of rate constant with temperature is described by the equation k2= 1012·21 exp (–29,329/RT) cm3 mol–1 s–1

Mechanisms of elimination reactions. Part III. Rate coefficients for elimination from substituted 2-phenylethyl arenesulphonates with potassium in t-butyl alcohol

Abstract: The rate coefficients for elimination from 24 substrates in the series 2-Z-phenylethyl p-Y-arenesulphonate with potassium t-butoxide in t-butyl alcohol at 40 °C have been correlated with the Hammett equation. The reaction constants ρz decrease linearly with the electronic properties of the group Y only for the most electron-withdrawing substituents, indicative of a shift to transition states with less carbanion character. The reaction constants ρY become less positive as Z becomes more electron-withdrawing, suggesting that increased Cβ–H bond breaking is accompanied by decreased Cα–X bond breaking, and a shift to a transition state with greater carbanion character. This result supports earlier secondary deuterium isotope studies in establishing a new interpretation of the reactivity ratio kOTs/kBr as a measure of C–X bond breaking.