Interactions between Thiourea and Imines. Prelude to Catalysis

The interaction between thiourea and a series of imines was examined via high-level ab initio calculations. For each imine, there is a set of stable complexes that represent minima on the potential energy surface. One type is characterized by a pair of symmetric NH···N hydrogen bonds (HBs), with both NH groups of thiourea approaching the imine N from above and below its molecular plane. Another geometry category combines a linear NH···N with a CH···S HB. A third, which is less stable, has the S approaching the imine's CH2 group, and a stacking arrangement is present in the fourth. Interaction energies vary from ∼2 kcal/mol to a maximum of 13.5 kcal/mol. The formation of the complex tends to elongate the C-N bond within the imine by as much as 0.004 Å, but there are certain dimers that cause a small contraction of this bond.