CONFORMATIONAL STUDY OF GLYOXAL BIS(AMIDINOHYDRAZONE) BY AB-INITIO METHODS

We report the first ab initio molecular orbital study on the ground st ate of the endiamine tautomer of glyoxal bis(amidino-hydrazone) (or gl yoxal bis(guanylhydrazone), GBG) free base. The calculations were perf ormed at the following levels of theory: Hartree-Fock, second-order Mo ller-Plesset perturbation theory and density functional theory (B-LYP and B3-LYP) as implemented in the Gaussian 94 software. The standard b asis set 6-31G(d) was found to be sufficient. The default fine grid of Gaussian 94 was used in the density functional calculations. Molecula r properties, such as optimized structures, total energies and the ele ctrostatic 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-s tate structure. The second conformer with respect to rotation around t he 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 ener gy increased monotonically from the trans conformation to the cis conf ormation, 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 th e bond charge increments were found to be easily transferable between structurally related systems. (C) 1997 Elsevier Science B.V.