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A theoretical study of tautomerism in the gas phase and aqueous solution a combined use of state of the art ab initio quantum mechanics and free energy perturbation methods

A theoretical study of tautomerism in the gas phase and aqueous solution a combined use of state of the art ab initio quantum mechanics and free energy perturbation methods

Journal of the American Chemical Society 109(21): 6283-6289

The combination of ab initio quantum mechanical calculations together with the free energy perturbation method implemented in molecular dynamics simulations gives a good description of tautomeric equilibria for 2-oxopyridine, 2-oxopyrimidine, and cytosine tautomers in water solution. Such an approach appears to be general and applicable to most systems where equilibrium constants and free energy differences of solvation between structural isomers are needed. The strength of this method is further demonstrated by showing that one can "close the thermodynamic cycle" by simulating transitions from one species to another going in different directions. In the case of gas-phase tautomeric energy differences, one still has uncertainties of the order of 1-2 kcal/mol at the highest level of ab initio theory used by us (6-31G* geometry optimization with energies at the MP2/6-31G* level) applied to the 2-oxopyridine tautomerism. Uncertainties of .apprx. 1 kcal/mol are found in the solvation contribution to free energy differences. Nonetheless, this work demonstrates the power of combining these two methods to study chemical equilibrium in the gas phase and under various solvent conditions.

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Accession: 004640783

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DOI: 10.1021/ja00255a010

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