Structures and energies of seleno derivatives of biuret. Ab initio comparative studies of diselenobiuret, selenobiuret, and selenothiobiuret

Ab initio studies (LCAO-MO method) on conformers of three seleno derivatives of the biuret molecules diselenobiuret [I], selenobiuret [II], and selenothiobiuret [III] were carried out at the Hartree-Fock (HF) and MP2 levels. The molecular geometries of these species were fully optimized at the HF level and characterized by analysis of the harmonic vibrational frequencies using a split-valence triple-zeta basis set augmented by a set of d polarization functions on heavy atoms and p polarization functions on hydrogen atoms [TZP(d, p)]. The total energies of the HF-optimized structures were calculated at the MP2 (frozen core) level using a larger TZP (2df, 2pd) basis set. The potential energy searches revealed a total of 11 minimum-energy conformers (assigned as trans-trans, trans-cis, cis-trans, and cis-cis) and seven transition-state species for the title molecules. The two predicted conformers for diselenobiuret (Ia = trans-trans and Ic = cis-cis) are characterized by C-2 and the third by C-s symmetry. For selenothiobiuret two forms (IIIa = trans-trans and IIId = cis-cis) possess C-1 and two (IIIb = trans-cis and IIIc = cis-trans) possess C-s symmetries, respectively. For selenobiuret, four forms IIa = trans-trans (C-1), IIb = trans-cis (C-s), IIc = cis-trans (C-1), and IId = cis-cis (C-1), were obtained as a result of gradient optimization. Comparison of the relative energies for the considered species indicated that the cis-trans forms are the most stable conformations for all three systems at both the HF and MP2 levels of theory.