AmericanChemicalSociety.com

This past summer was productive and my research has shifted slightly to focusing on the keto-enol tautomerization of acetylacetone in various solvents. Our group has NMR data on the keto-enol ratio in the solvents cyclohexane, benzene, chloroform, tetrahydrofuran, pyridine, acetone, methanol, acetonitrile, nitromethane, dimethylsulfoxide, and water.  Analysis of the NMR data indicates there is a linear correlation between the percent enol and the dielectric of the solvent.  As the dielectric strength of the solvent increases the percent enol decreases; thus, the stronger dielectric solvents are better able to stabilize the dipole of the diketone. However, ab initio calculations with electron correlation, triple-zeta basis, diffuse functions, and polarization functions using a polarized continuum model fail to reproduce the experimental data. An explicit solvent study was carried out combining an effective fragment potential with a polarized continuum. Configuration space is sampled by a simulated annealing process to identify minima. Initially, 400 geometries were generated for acetylacetone with solvent molecules using a simulated annealing process sampling 560,000 structures. From the lowest energy configurations identified with simulated annealing, the 40 lowest energy structures were optimized at HF/6-31G(d) with a polarized continuum. The statistically significant geometries are optimized at M06-2X/6-311+G(d,p) with a polarized continuum to calculate the equilibrium concentrations of acetylacetone. This computational method was applied using common NMR solvents and the percent enol was compared to experimental NMR data. Inclusion of explicit solvent molecules is necessary to accurately calculate the acetylacetone tautomers in solution.