Showing papers on "Transition state published in 1977"
TL;DR: In this article, a method of calculating the intrinsic reaction coordinate starting at a saddle point is proposed, which is used in combination with the analytical evaluation of the energy gradient for the calculation of the reaction coordinate on an ab initio potential energy surface.
Abstract: A practical method of calculating the intrinsic reaction coordinate starting at a saddle point is proposed. The method has been used in combination with the analytical evaluation of the energy gradient for the calculation of the reaction coordinate on an ab initio potential energy surface. The reaction coordinates are obtained for the HNC to HCN isomerization and the SN2 exchange reaction involving H−+CH4→CH4+H−.
748 citations
TL;DR: In this article, a statistically adiabatic model for chemical reactions involving a tight or loose transition state in the exit channel was used to obtain an integral equation for the individual reaction probabilities, i.e., for the magnitude of the S matrix elements.
Abstract: A statistically adiabatic model for chemical reactions involving a tight or loose transition state in the exit channel was used in Part I to obtain an integral equation for the individual reaction probabilities, i.e., for the magnitude of the S matrix elements. In the present paper this integral equation is explicitly solved for the general case of product orbital (l) and rotational (j) angular momenta constrained only by energy and angular momentum conservation. The reaction probabilities are shown to be related to a contour integral of a product of canonical partition functions. The theory includes an effect of the evolution of the bending vibrations of the transition state into free rotations of the product molecules. The distribution of final translational energy for the general (l,j) case is then obtained by averaging the reaction probabilities over various quantum states of the product molecules. The results are compared with the special cases in the literature for which (i) the transition state in the exit channel is loose (''phase space theory''), (ii) this case but with l>>j, and (iii) tight transition state theory with l>>j (Part I). The results are also compared with experimental data obtained from the molecular beam reaction F+(CH3)2C=CH2 -->F(CH3)2CCH2*-->CH3+FCH3C=CH2. The data and the theoretical results are now in better agreement. In the treatment described here and in Part I a loose transition state in the entrance channel was assumed. Expressions for the energy distribution are also given for the case when the entrance channel transition state is tight. Finally, a statistically adiabatic S matrix, which is useful for reactions proceeding through long-lived collision complexes having tight transition states, is described, and its possible application to angular distributions and angular momentum polarization experiments is discussed.
73 citations
TL;DR: The energy hypersurface for the attack of fluoride ion on methyl fluoride has been explored with ab initio LCAO-SCF calculations at a split-valence basis set level.
Abstract: The energy hypersurface for the attack of fluoride ion on methyl fluoride has been explored with ab initio LCAO-SCF calculations at a split-valence basis set level. Transition states for frontside and backside attack have been located. In addition to transition states, two possible F−-CH3F clusters have been identified. The transition state for the substitution of fluoride with retention of configuration is found to be 56 kcal/mol higher than the transition state for inversion of configuration. The transition state for hydride displacement with inversion is 62 kcal/mol above the transition state for fluoride substitution with inversion.
45 citations
TL;DR: In this paper, a 3,3-disubstituted α-enone reacts slowly with phosphonitrile (3 ) at the carbonyl carbon, CC double bond attack being inhibited by steric hindrance.
31 citations
01 Jan 1977
TL;DR: A free-radical reaction is a chemical process in which molecules having unpaired electrons are involved as discussed by the authors, and the radical species could be a starting compound or a product, but in organic chemistry, the most common cases are reactions that involve radicals as intermediates.
Abstract: A free-radical reaction is a chemical process in which molecules having unpaired electrons are involved. The radical species could be a starting compound or a product, but in organic chemistry, the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates or transition states in which all electrons remain paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur.
17 citations
TL;DR: In this paper, a general outlook on research on the Ei reactions of sulphilimines and the mechanism of the reaction are discussed in comparison with those of similar EI reactions which proceed via 5-membered cyclic transition states.
15 citations
TL;DR: In this paper, the reaction of the epoxides of the general formula with benzylamine at 40, 50 and 60°C has been examined and the determination of rate constants by variation of X has shown, that in the case of X = S or N no anchimeric acceleration occurred.
Abstract: Reaction of monosubstituted epoxypropanes with benzylamine
The reaction of the epoxides of the general formula
with benzylamine at 40, 50 and 60°C has been examined. The determination of rate constants by variation of X has shown, that in the case of X = S or N no anchimeric acceleration occurred. These results have been approved by product analysis and by synthesis of the possible reaction products. Transition states of the epoxide/benzyl-amine reaction are discussed by means of the experimentally determined activation energy and activation entropy.
12 citations
TL;DR: For some representative reactant pairs, chemical determinations have been made of the relative stereochemistry at the two new asymmetric centres which are set up when an allylic alcohol reacts with a cyclic orthoester.
Abstract: For some representative reactant pairs, chemical determinations have been made of the relative stereochemistry at the two new asymmetric centres which are set up when an allylic alcohol reacts with a cyclic orthoester. The results give directly the fractions of the total reaction which proceed through chair- and boat-type transition states; interpretations are provided in terms of steric interactions in these states. For some of the reactions there is evidence that the transition state (chair or boat) is not a symmetrical cyclohexane-like entity, the incipient new C–C bond being considerably longer than the C–O bond which is undergoing cleavage.
12 citations
TL;DR: In this article, the reaction of 2,2′-bipyridyldichloroplatinum(II) with thiourea is reported for a series of water-1,4-dioxan and water-tetrahydrofuran solvent mixtures, at 293.2 and 298.2 K respectively.
Abstract: Rates of reaction of 2,2′-bipyridyldichloroplatinum(II) with thiourea are reported for a series of water–1,4-dioxan and water–tetrahydrofuran solvent mixtures, at 293.2 and 298.2 K respectively. From an analysis of these kinetic results and data on the solubilities of the reactants, it is demonstrated that initial-state solvation changes play a pre-dominant role in determining the reactivity trends for this reaction in the solvent mixtures studied.
7 citations
TL;DR: In this article, it was shown that the very large variation in ΔS for the reaction between tetraethyltin and mercury(II) chloride is almost entirely due to initial state effects.
Abstract: Heats of solution are reported for tetraethyltin and mercury(II) chloride in methanol, methanol–t-butyl alcohol mixtures, and t-butyl alcohol. With previous data on free energies, these measurements lead to entropies of transfer as well as enthalpies of transfer from methanol to the other solvents of tetraethyltin, mercury(II) chloride, and the tetraethyltin–mercury(II) chloride transition state. It is shown that the very large variation in ΔS‡ for the reaction between tetraethyltin and mercury(II) chloride is almost entirely due to initial state effects. Heats of solution of t-butyl chloride in a number of pure alcohols have been determined and lead similarly to enthalpies and entropies of transfer of the solvolysis transition state from methanol to the other alcohols. Variations of the entropy of the [Et4Sn–HgCl2]‡ and [ButCl]‡ transition states with change of alcoholic solvent are quite small, whereas the entropy of dissociated species such as (Pr4N++ I–) or (Me4N++ Cl–) is considerably lower in the less polar alcohols than in methanol. It is suggested that these dissociated species are poor models for the particular transition states discussed, and that in general thermodynamic properties of polar, electrically neutral, transition states are not comparable quantitatively with those of dissociated species, as far as variation with solvent is concerned.
7 citations
01 Jan 1977
Abstract: Addition and elimination processes are the reverse of one another. In general, the two processes follow a similar mechanistic path in opposite directions, the final state of the system depending on the conditions. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that constitute a related addition-elimination pair:
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In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same for the reverse reaction as for the forward one under the same conditions. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as in the elimination reaction.
TL;DR: In this article, the stereochemistries of electron impact induced eliminations from 11 derivatives of the trans-4-t-butylcyclohexyl system exhibit a dichotomy, with the predominant cis elimination observed in every case is consistent with the substantial integrity of the cycloenyl ring prior to fragmentation, and with a cyclic transition state for hydrogen abstraction.
Abstract: The stereochemistries of nine electron-impact induced eliminations proceeding from derivatives of the cis-4-t-butyl system have been determined The predominant cis elimination observed in every case is consistent with the substantial integrity of the cyclohexyl ring prior to fragmentation, and with a cyclic transition state for hydrogen abstraction The stereochemistries of electron impact induced eliminations from 11 derivatives of the trans-4-t-butylcyclohexyl system exhibit a dichotomy The predominatn trans stereochemistry observed in six electron impact induced eliminations, and the nonstereospecific electron impact induced dehydration of trans-4′-t-butylcyclohexyl-ethanol are consistent with nonconcerted elimination from a chair-like cyclohexyl ring Conversely, the McLafferty rearrangement of trans-4′-t-butyl-cyclohexyl-2-propanone proceeds nonstereospecifically trans-4-t-Butylcyclohexyl acetaldehyde, 2-methyl-3-(trans-4′-t-butylcyclohexyl)-1-propane and trans-4-t-butylcyclohexyl-S-methyl xanthate exhibit predominant cis McLafferty rearrangement stereochemistry This result may be due to fragmentation through boat-like conformers in these compounds
TL;DR: In this article, the interaction between the molecules ethylene and cis-N2H2 has been studied using a gaussian basis in a series of ab initio SCF calculations, and the results indicated that the synchronous hydrogen transfer reaction is a one-step reaction having an activation energy of around 60 kcal/mol.
TL;DR: In this paper, the entropies of activation related to the reactions of aqueous malachite green with the nucleophiles water, hydroxide, hydrazine, ammonia, hypochlorite, and sulphite have been calculated from the temperature dependence of the rate constants appropriate to these reactions.
Abstract: The entropies of activation related to the reactions of aqueous malachite green with the nucleophiles water, hydroxide, hydrazine, ammonia, hypochlorite, and sulphite have been calculated from the temperature dependence of the rate constants appropriate to these reactions. The entropies of the transition states for these reactions have been characterized and the transition states shown to be 'reactant like'. A similar correlation, based on data for a series of nucleophiles attacking methyl iodide, suggests that the transition states for the SN2 displacement reactions are also 'reactant like' with respect to the attacking nucleophile. A relationship between two distinct parameters each of which is related to the position of transition states along the reaction coordinate is considered.
TL;DR: In this article, rate coefficients have been measured for both the base and acid-catalysed rearrangements of a series of methyl pseudo- 8-benzoyl-l-naphthoates and pseudo- 2-bensoylbenzoates in methanol.
Abstract: Rate coefficients have been measured for both the base- and acid-catalysed rearrangements of a series of methyl pseudo- 8-benzoyl-l-naphthoates and pseudo- 2-benzoylbenzoates in methanol at 60.0 °C to form the corresponding normal esters. The entropies and enthalpies of activation for the parent pseudo-esters have been evaluated, as have the solvent isotope and solvent (methanolic dioxan and dimethyl sulphoxide) effects for these esters. The reaction constants ρ for the methoxide anion-catalysed rearrangement are ca. 2.1 (naphthoates) and 1.1 (benzoates), respectively. The evidence is consistent with a stepwise mechanism in which the formation or decomposition of tetrahedral intermediates is rate-determining: for the pseudo-benzoates, the formation by addition of methoxide to the pseudo-ester carbonyl group; for the pseudo-naphthoates. the decomposition by dissociation of methoxide from the normal ester carbonyl group. This mechanistic switch appears to arise from greater strain in the five-membered ring affecting its initial state and primary addition product stabilities. The reaction constants ρ for the acid-catalysed rearrangement are ca.–0.1 (naphthoates) and –0.2 (benzoates), respectively. All the evidence for the latter reaction indicates an AAC2 path and likely transition states are discussed.
TL;DR: The steric effect of o-methyl groups is much more pronounced for N-alkylation with Mel, Etl, and PriI or aza-aromatic compounds.
Abstract: The steric effect of o-methyl groups is much more pronounced for N-alkylation with Mel, Etl, and PriI or aza-aromatic compounds than for S-alkylation of heterocyclic thiones. This is explained by comparison of van der Waals interactions in the transition states of chosen homomorphic systems; an angular transition state is shown to conform with the measured kinetic constants.
TL;DR: In this paper, the potential energy surface of planar square and rectangular arrangements of the four-electron system H4 is discussed in relation to the bimolecular isotope exchange reaction H2+D2-2HD.
Abstract: Floating spherical Gaussian orbital (FSGO) open-shell calculations have been made to determine the potential energy surface of planar square and rectangular arrangements of the four-electron system H4. This surface is discussed in relation to the bimolecular isotope exchange reaction H2+D2- 2HD. The changes in energy and geometry accompanying the coplanar approach of two hydrogen molecules interacting chemically have also been investigated. Calculations on the electronic energies of planar T-shaped and kite arrangements of H4 of various sizes show that it is unlikely that these configurations can serve as transition states for the exchange reaction. However, the energy curve for linear configurations of H4 (HHH … H), calculated as a function of the H3 … H distance with the symmetric linear H3 (H-H-H) unit fixed at the internuclear distance of 1.9080 a.u., is found to have a deep minimum (−1.9176 a.u.) at an r(H3 … H) distance of 1.5846 a.u. The overall results suggest that the following mechanism for the exchange reaction, H2+H2H2+H+HH3+HH+H2+ HH2+H2 could be advantageous as it requires a barrier height of 0.1604 a.u. which is significantly lower than that calculated from the saddle point energy (0.1950 a.u.). However, the problem of reconciling this with the experimental activation energy of 0.0685 a.u. still remains.
TL;DR: Transition state dissociation constants are currently considered, utilizing stopped flow equipment as discussed by the authors, and the underlying theory is briefly reviewed, relating the ideas to steady state kinetics of enzyme systems.
Abstract: Transition state dissociation constants are currently considered, utilizing stopped flow equipment. The underlying theory is briefly reviewed, relating the ideas to steady state kinetics of enzyme systems. The ideas are further analyzed under the consideration of chemical relaxation. Test conditions are described which would allow an investigation of the concepts of transition state dissociation constants by chemical relaxation techniques. A discussion concerning the way in which the concepts of transition state dissociation constants relate to other theories which assume short-lived, but real, dissociation constants is included. The theory is rigorously analyzed (in a second part), revealing the nature of the assumption of a transition state dissociation constant: While they may be written in a formal manner, they are not based on reality—on kinetic grounds direct interconversions between transition states are practically impossible. This applies also to transition state dissociation constants involving protons.
TL;DR: In this paper, the reaction of 2,2′-bipyridyldichloroplatinum(II) with thiourea is reported for a series of water-1,4-dioxan and water-tetrahydrofuran solvent mixtures, at 293.2 and 298.2 K respectively.
Abstract: Rates of reaction of 2,2′-bipyridyldichloroplatinum(II) with thiourea are reported for a series of water–1,4-dioxan and water–tetrahydrofuran solvent mixtures, at 293.2 and 298.2 K respectively. From an analysis of these kinetic results and data on the solubilities of the reactants, it is demonstrated that initial-state solvation changes play a pre-dominant role in determining the reactivity trends for this reaction in the solvent mixtures studied.
01 Jan 1977
TL;DR: In this article, the effect of substituents on the transition state energy of a benzene reaction is examined. But the authors focus on the effects of the substituent on the rate-determining step of the reaction.
Abstract: The complete study of a reaction mechanism entails a knowledge of the transition state, particularly that of the rate-determining step. The transition state is, however, by definition the highest energy, least stable configuration through which the reactants must pass on the route to the formation of the products of that step of the reaction. As a consequence transition states are short lived, of low concentration, and typically not amenable to direct study. A technique commonly used in organic chemistry is to examine the effect of substituents on transition state energy. This is particularly appropriate if charge is developed or lost as the reactants form the transition state. However, neighboring substituents may exert a steric effect on solvation at the reaction site, as well as an electronic effect on transition state stability, and the two effects may be difficult to separate. For this reason a number of such studies have been carried out with the reaction site present as a functional group attached to a benzene ring in a series of reactant molecules having various meta and para substituents. This provides varying degrees of electronic (polar and resonance) stabilization or destabilization of the transition state, with a constant steric factor around the reaction site, and is reflected in increased or decreased values of the measured rate (or equilibrium) constant. The experimental data are fitted to the Hammett equation, and conclusions can often be drawn concerning the nature of the transition state (1), insofar as the anionic or cationic character is concerned.
01 Jan 1977
TL;DR: Several specifically functionalized alpha-cyclodextrins are synthesized, designed to stabilize the trigonal bipyramidal transition state for an in-line displacement at the phosphorus of a phosphate monoester.
Abstract: Inspection of the active sites of the many enzymes whose structures are known at high resolution leads to the unsurprising conclusion that an enzyme may provide an environment that exquisitely stabilizes the transition state for an elementary catalytic step that is expected to be difficult In an effort both to mimic such an environment and to have the opportunity of investigating the thermodynamic and kinetic consequences of juxtaposing polar and non-polar loci in the same molecule, we have synthesized several specifically functionalized alpha-cyclodextrins One of these is designed to stabilize the trigonal bipyramidal transition state for an in-line displacement at the phosphorus of a phosphate monoester This cyclodextrin contains three symmetrically disposed ammonium groups on the 'top' (at C-6) of the hydrophobic cavity formed by the hexa-glucose torus and the remaining 15 hydroxy groups are methylated The thermodynamic consequences of adjacent hydrophobic and hydrogen-bonding and/or electrostatic binding sites are investigated using several charged and uncharged ligands The feasibility of building host species explicitly to stabilize reaction transition states is discussed
TL;DR: In this article, the electronic structures of ethylamine and related species have been calculated by means of the INDO method to examine the scope and limitations of the so-called fragmentation rules in organic mass spectrometry.
Abstract: The electronic structures of ethylamine and the related species have been calculated by means of the INDO method to examine the scope and limitations of the so-called fragmentation rules in organic mass spectrometry. Both the unpaired electron in the molecular ion and stabilization by the conjugation in the reaction transition state are confirmed to play important roles in fragmentation reactions. Localized MO representation of the reaction transition states and applicability of bond indices are also discussed.