Theoretical conformational analysis for chain systems with two conjugated double bonds in the gas phase and in solution

Conformational preferences for butadiene-1,3, acrolein, glyoxal, methyl-1-propenyl ketone, acrylic and pyruvic acids with conjugated double-bond substructures have been studied at the IEF-PCM/B97D/aug-cc-pvqz and IEF-PCM/CCSD(T)/CBS theoretical levels in dichloromethane, acetonitrile and water. Overall agreement were achieved by the two quantum mechanical methods with deviations in the relative conformational free energies up to a few tenths of a kcal/mol. Considering explicit-solvent Monte Carlo simulations, the favorable conformations agree with those from IEF-PCM results with an exception for pyruvic acid. Barriers to rotation about the central CC bond were calculated at 1.2–8.5 kcal/mol. The conformational equilibria are under thermodynamic control with predominating s-trans form in general. Carlo simulations predict non-negligibly different relative solvation free energies when a reaction field model or the Ewald correction has been applied to account for the long-range electrostatic interactions. Acids favorably adopt the dimeric form with doubly hydrogen-bonded structures in dilute solutions. The calculated degree of association is highly sensitive to the accepted atomic charges, indicating the importance of the polarization of the constituents within the dimer.