Conformational study of glyoxal bis(amidinohydrazone) by ab initio methods

We report the first ab initio molecular orbital study on the ground state of the endiamine tautomer of glyoxal bis(amidinohydrazone) (or glyoxal bis(guanylhydrazone), GBG) free base. The calculations were performed at the following levels of theory: Hartree–Fock, second-order Møller–Plesset perturbation theory and density functional theory (B-LYP and B3-LYP) as implemented in the Gaussian 94 software. The standard basis set 6-31G(d) was found to be sufficient. The default fine grid of Gaussian 94 was used in the density functional calculations. Molecular properties, such as optimized structures, total energies and the electrostatic potential derived (CHELPG) atomic charges, were studied as functions of C–C and N–N conformations. The lowest energy conformation was found to be all-trans, in agreement with the experimental solid-state structure. The second conformer with respect to rotation around the central C–C bond was found to be the cis conformer with an MP2//HF energy of 4.67 kcal mol⁻¹. For rotation around the N–N bond the energy increased monotonically from the trans conformation to the cis conformation, the cis energy being very high, 22.01 kcal mol⁻¹ (MP2//HF). The atomic charges were shown to be conformation dependent, and the bond charge increments and especially the conformational changes of the bond charge increments were found to be easily transferable between structurally related systems.