Showing papers on "Transition state published in 1973"

Reactions of F atoms and aromatic and heterocyclic molecules: Energy distribution in the reaction complex

Abstract: This paper reports studies of the reactions of fluorine atoms and several aromatic and heterocyclic molecules. As in earlier work we have used the method of crossed molecular beams to create long‐lived reaction intermediates and to deduce from the angular distribution of the product molecules information about the energy distribution in the reaction complex. The data reported in this paper complement those collected earlier. They show that when a light group is emitted from a reaction complex randomization of the internal energy of the complex is not more rapid than reaction. When a heavy atom is emitted from the reaction complex the limited data available indicate that almost all vibrational degrees of freedom of the complex participate in the reaction. This latter observation provides an interesting connection between vibration‐translation energy exchange and the mechanism of chemical reaction.

Potential energy surfaces for the reaction CH2+ H2→ CH4

Abstract: Approximate ab-initio potential surfaces have been obtained for the reaction CH2+ H2→CH4, with CH2 both in its lowest singlet and lowest triplet states. The surfaces have been studied mainly for configurations of symmetry C2v, but the occurrence of orbital crossing indicates that the transition states have lower symmetry than this. The calculations show that the insertion of triplet methylene into H2 to give the lowest triplet-state of methane (calculated to have a square planar structure) is energetically unfavourable by approximately 300 kJ/mol, and there is probably a small barrier to the reverse reaction. The insertion of 1CH2 into H2 is found to be energetically favourable with little or no activation barrier when the system is allowed to distort from C2v structures.

Relative reactivities and activation parameters for the Vilsmeier–Haack formylation of five-membered heteroaromatic compounds containing the Group VI elements

Abstract: Rate constants and activation parameters are determined for the formylation by the HCONMe2–COCl2 complex in CHCl3, of furan, thiophen, selenophen, and tellurophen. ΔS‡ Values are identical within experimental error, and the reaction is enthalpy controlled. The activation entropies are discussed in terms of the position of the transition states along the reaction co-ordinate, whereas ΔH‡ values are correlated with ‘empirical resonance energies’ and ΔEloc. The results indicate that the transition states for all four compounds lie in a similar position along the reaction co-ordinate and that the differences in ground state energy play a fundamental role in determining the relative reactivities of the α-positions.

Substitution at saturated carbon. Part XV. The effect of t-butyl alcohol–methanol mixtures on the free energy of reactants and transition states in the substitution of tetraethyltin by mercury(II) salts

Abstract: Standard free energies of transfer from methanol to t-butyl alcohol–methanol mixtures have been determined for tetraethyltin, mercury(II) chloride, mercury(II) iodide, mercury(II) acetate, and for the transition states in reaction (i; X = Cl, I, and OAc). It is shown that the increases in ΔG‡ on change of solvent from methanol to t-butyl Et4Sn + HgX2→ EtHgX + Et3SnX (i) alcohol–methanol mixtures observed for reaction (i; X = Cl, I, and OAc) are due largely to increases in the free energy of the transition states. An analysis of reaction (i; X = Cl) in terms of the Kirkwood function (Iµ– 1)/(2Iµ+ 1) suggests that the [Et4Sn–HgCl2]‡ transition state possesses the very high dipole moment of ca. 14 D.

Medium effects on heavy-atom kinetic isotope effects. II. Cell model with external--external and certain external--internal coordinate interactions

Abstract: The method of Stern, Van Hook, and Wolfsberg for the calculation of vapor pressure isotope effects was applied to model calculations of medium effects on /sup 13/C kinetic isotope fractionation in the decompositions (via several reaction coordinates) of a hypothetical nonlinear triatomic; molecular parameters were given values characteristic of organic compounds, while the necessary external force constants were adjusted to yield associated frequencies typicaI of moderate H bonding (30-- 90 cm/sup -1/). Influences of interactions between external coordinates were studied, as were those between external coordinates and an irternal coordinate which was not a component of the reaction coordinate. TAM was selected as the model because of its small size, all such interaction effects decreasing rapidly with increasing molecular mass and complexity. In no case studied would the medium effects calculated be measurable given present techniques; indeed, for molecules of ordinary mass and complexity, such effects would not even be a detectble increment or decrement to isotope rate effects determined with reasonable precision. Because the effects are additive, subject to certain restrictions, it is possible that medium effects of experimental signicance could be produced more » if: the isotopic molecules were relatively light; reaction temperatres were low (say, below 25 deg C); solvent-- solute interactions were drastically different in magnitude in reaction and transition states; one or more of the component interactions includes coupling to an internal coordinate (preferably isotopic) which is part of the reaction coordinate; and the force constants associated with such interactions are substantially larger than those employed here. If the last two conditions are met, the assumptions of the cell model will have been strained and some more natural technique for handling such strong interactions should be attempted. (auth) « less

Effects of variations in reaction coordinate eigenvalue and eigenvector on the rate and hydrogen isotope effects of the reaction H+Cl2 → HCl+Cl

Abstract: Effects on the rate constant k, and on the temperature independent and temperature dependent factors of kh/kD, of variations in the eigenvalue (λ1) and eigenvector (L1) associated with the reaction coordinate have been studied by computation for the reaction H+Cl2 → HCl+Cl A slightly bent transition state was assumed and its diagonal force field maintained invariant at partial bond levels The properties of the transition state were varied by systematic manipulation of the three off‐diagonal force constants Values of the several temperature dependent quantities calculated were controlled largely by the interaction force constant between the H···Cl stretch and the H···Cl···Cl bend Curving the potential barrier affects drastically the range of reaction coordinate ``compositions'' which correspond to acceptable transition states, but neither k nor kH/kD is sufficiently sensitive to barrier curvature to allow an estimate of a ``best value'' for λ1 by comparison of calculated and experimental values Eigenv

Structural direction of the heterodiene synthesis in the reaction of oxazoles with acetylenic dienophiles

Abstract: 1. Using the Huckel approximation for calculating the molecular orbitals, we calculated the energy levels and theπ-electron densities of some substituted oxazoles, and also the stabilization energies of the transition states when these oxazoles were reacted with acetylenic dienophiles. 2. The calculated data are in agreement with the experimental results.

Potential energy surfaces for the reaction ch2+h2-ch4

Abstract: Approximate ab-initio potential surfaces have been obtained for the reaction CH2+ H2→CH4, with CH2 both in its lowest singlet and lowest triplet states. The surfaces have been studied mainly for configurations of symmetry C2v, but the occurrence of orbital crossing indicates that the transition states have lower symmetry than this. The calculations show that the insertion of triplet methylene into H2 to give the lowest triplet-state of methane (calculated to have a square planar structure) is energetically unfavourable by approximately 300 kJ/mol, and there is probably a small barrier to the reverse reaction. The insertion of 1CH2 into H2 is found to be energetically favourable with little or no activation barrier when the system is allowed to distort from C2v structures.