“No Barrier Theory” postulates that when only one thing happens there is a quadratic relation between energy and reaction progress, with no kinetic barrier. The kinetic barrier seen with essentially all real chemical processes results from the need for more than one thing to happen simultaneously. This approach permits calculation of free energies of activation for covalent hydration of carbonyl compounds, over the range of reactivity for which data are available, from formaldehyde to carboxamides. Acid- and base-catalyzed and uncatalyzed reactions can be treated with no adjustable parameters, with root-mean-square errors of 1.41, 1.30, and 1.50 kcal/mol in free energy of activation. The method requires equilibrium constants and distortion energies. The latter can be calculated by molecular orbital theory using relatively low levels of theory. The calculations can be inverted to calculate equilibrium constants from experimental rate constants.

Rate constants for the reaction of water or azide ion with various substituted benzylic carbocations can be calculated from the equilibrium constants for cation formation and distortion energies by means of the No Barrier Theory. Rate constants for the water reactions span 10 orders of magnitude. Most but not all of the azide reactions are diffusion controlled. The rate constants, and in particular those less than diffusion controlled, were successfully calculated. The set of equilibrium constants available from the literature was supplemented using alkyl chloride hydrolysis equilibrium constants and relative cation formation equilibrium constants derived from DFT/continuum calculations. The model used for these reactions allows for the entropic cost of bringing solutes together, the desolvational cost of losing hydrogen bonding solvation by water and the cost of moving the aryl ring through the solvent as the central atom changes from sp2 to sp3 hybridization and from planar to pyramidal geometry. Copyright © 2004 John Wiley & Sons, Ltd.

No barrier theory has been applied to the E2 reactions of five alkyl bromides with ethanolic ethoxide. The model used for these reactions is that the reaction proceeds from the encounter complex of base and alkyl halide to the product encounter complex of halide ion and alkene (and alcohol), and requires five simple processes, which combine to give the concerted elimination: transfer of a proton from carbon to base; a change in geometry at the carbon which loses a proton from sp 3 to sp 2 ; breaking the C-leaving group bond; a change in geometry at the carbon which loses the leaving group from sp 3 to sp 2 ; and a change in the length of the carbon-carbon bond. The free energy of activation can be calculated with an rms error of 2.58 kcal mol -1 (1 cal = 4.184 J).Résumé : On a appliqué la théorie sans barrière à l'étude des réactions E2 de cinq bromures d'alkyles avec de l'éthanolate éthanolique. Le modèle utilisé pour ces réactions est celui d'une réaction qui se produit à partir d'un complexe de rencontre de la base et de l'halogénure d'alkyle et qui conduit au complexe de rencontre du produit avec l'ion halogénure, l'alcène (et l'alcool) et qui nécessite cinq processus simples qui se combinent pour conduit à une éli-mination concertée: le transfert d'un proton du carbone à la base; une modification de la géométrie au niveau du carbone qui perd un hydrogène et qui passe de sp 3 à sp 2 ; un changement de géométrie au niveau du carbone qui perd le groupe partant et qui passe de sp 3 à sp 2 ; la rupture de la liaison du carbone-groupe partant et un changement dans la longueur de la liaison carbone-carbone. On a calculé l'énergie libre d'activation avec une erreur de 2,58 kcal/mol (1 cal = 4,184 J).

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