AMIDINE N-C(N)-N SKELETON - ITS STRUCTURE IN ISOLATED AND HYDROGEN-BONDED GUANIDINES FROM AB-INITIO CALCULATIONS

Geometries of guanidine and eight of its N-imino derivatives (CH3, SiH 3, OH, CN, F, Cl, CHO, and NO2) were calculated using ab initio molecu lar orbital techniques. MP2, MP4, and CISD geometries indicate that th e guanidine molecule is pyramidal at amino groups and pi-conjugation t hrough the amidine skeleton is modest. MP2 structures of the eight N-i mino guanidines reveal that substituting the hydrogen atom of the imin o nitrogen by a functional group leads to a concerted variation of the CN bond distances, Topological electron density analyses indicate tha t the structural changes calculated upon N-imino substitution are larg ely due to changes of conjugation onto the amidine pi-skeleton. Interm olecular hydrogen bonding involving the imino and amino groups is also found to affect the amidine geometry. Special attention has been paid in the work of 2-cyanoguanidine. The centrosymmetric dimer and a more extended hydrogen-banded complex have been proposed as simulation of the crystal environment and the respective ab initio geometries (HF/4- 31G(d) and MP2/6-31G(d)) are discussed. The structural changes due to self-association have been rationalized supposing that the contributio n of polar canonical forms increases upon hydrogen-bonding formation.