Equilibrium constants Experimental" correlation energy

Molecular mechanics calculations (MM2(85)) were employed to rationalize the relationship between stmcture and the equilibrium constants of the thiol-disulfide interconversion (Scheme 6) <1990JA6296>. An excellent correlation (r=0.93) between experimental AG values and calculated differences in strain energy A3 if was obtained, AG = 0.41 kj moF and /. SE = 0.5 kj moF thereby supporting the facile formation and stability of 1,2-dithianes. 

It is straightforward to calculate energies of hydration reactions as a function of the carbonyl compound and, once calibrated on the basis of available experimental data, use this as a criterion for selecting systems which might exist primarily as carbonyl compounds, primarily as carbonyl hydrates or anywhere in between. The disadvantage to such an approach (other than it requiring calculations on both the carbonyl compounds and their respective hydrates) is that it provides very little insight into the factors which influence the equilibrium. Another approach is to focus only on the carbonyl compounds (or only on the hydrates) and look for characteristics which correlate with the experimental equilibrium constants. This is the approach illustrated here. 

Table III. Minus the total Si crystal valence electron energy per atom with relaxation energy and pseudopotential corrections included, along with the equilibrium lattice constant, bulk modulus, and cohesive energy calculated with four different exchange-correlation functionals (defined in the caption of Table I) are compared with experimental values. The experimental total energy is the sum of Acoh plus the four-fold ionization energy.

A threedimensional diagram, expressing the amount adsorbed as a function of sorbate equilibrium concentration and total pressure, is drawn using both direct experimental data and computed data for C /N- mixtures on K-clinoptilolite at room temperature. The real adsorption isotherms follow the Freundlich law for Nj and Henry s law for C>2 Freundlich and henry constants are computed and correlated to sorbate equilibrium concentration. Selectivity coefficients,, equilibrium constants, K, and the changes in the standard Gibbs free energy, aG°, are determined. Selectivity towards N. is satisfactory and K-clinoptilolite could be used for industrial air enrichment. 

An exclusively computational method for obtaining enolization equilibrium constants in water has been described, based on gas-phase free energy changes, solvation energies and a correction for the latter via a parameterization scheme. In some cases where computed and experimental values disagree, the authors identify concerns with the experimental values. For 37 reactions, the correlation shows a root-mean-square error of 1.3 kcal mol . The report includes an examination of the relative stability of some E- and Z-enols. 

In so far as Westheimer s treatment correlates isotope effects with changes in force constants and the structure of the transition state, it cannot be tested without some experimental measure of these properties, and usually it has been assumed that, within a family of related reactions, the structure of the transition state varies smoothly with the rate constants and equilibrium constants of the reactions, with reactant-like transition states associated with reactive substrates and exothermic reactions. This assumption, which derives from observations of rate-equilibrium and reactivity-selectivity correlations [2, 3, 42], as well as calculations of semiempirical potential energy surfaces [43], is generally known (not quite accurately) as Hammond s Postulate [44]. It should be noted that while the postulate probably applies more generally to proton transfers than to other reactions, recent considerations of its scope and limitations [45, 46], based on extensive experimental experience, strongly suggest that departures... 

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