Biuret and its sulfur analogs: Structures and energies

The calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO–MO method at the Hartree–Fock level. The molecular structure of B was fully optimized using standard 3‐21G and 6‐31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3‐21G* and 6‐31G* levels. Only trans‐isomers were found to be minimum structures on the HF/3‐21G* potential‐energy surfaces, whereas cis‐forms are the first‐order transition structures. The relative energies of the cis‐conformers at MP 2/6‐31G**//HF/6‐31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol−1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis‐conformers are predicted to be stabilized in polar solvents.