Theoretical conformational analysis of thiacrown macrocycles

A gas phase conformational analysis was performed on four sulfur-containing macrocycles (9-thiacrown-3, 12-thiacrown-4, 15-thiacrown-5, and 18-thiacrown-6) using a combination of empirical and ab initio methods. Candidates for low-lying conformers were initially generated from high-temperature molecular dynamics simulations. A more computationally manageable subset of conformations was selected for further study based on their relative energies at successively higher levels of ab initio theory, The highest level of theory included second-order perturbation theory with the aug-cc-pVDZ basis set. The lowest conformation of 9-thiacrown-3 was found to have an exodentate C-2 structure with an electronic energy;that is 4 kcal/mol below the C-3 crystal structure. For 12-thiacrown-4, the lowest energy structure possesses D-4 structure in both the gas and crystal phase. In the case of 15-thiacrown-5 the lowest energy conformer possesses an oblong, partially exodentate, gas phase structure with C-2 symmetry. The crystal structure has C-1 symmetry and lies 3 kcal/mol higher in energy. 18-Thiacrown-6 has an exodentate C-2 symmetry, which is estimated with a large uncertainty to be 1 kcal/mol below the folded C-2 structure of the crystal. Zero-point vibrational effects shift relative energies by up to 0.3 kcal/mol across an energy span of 5-7 kcal/mol, but the effect on close-lying conformers is less.