Amidine N−C(N)−N Skeleton: Its Structure in Isolated and Hydrogen-Bonded Guanidines fromab InitioCalculations

Geometries of guanidine and eight of its N -imino derivatives (CH 3, SiH 3, OH, CN, F, Cl, CHO, and NO 2 ) were calculated using ab initio molecular orbital techniques. MP2, MP4, and CISD geometries indicate that the guanidine molecule is pyramidal at amino groups and π-conjugation through the amidine skeleton is modest. MP2 structures of the eight N -imino guanidines reveal that substituting the hydrogen atom of the imino nitrogen by a functional group leads to a concerted variation of the CN bond distances. Topological electron density analyses indicate that the structural changes calculated upon N -imino substitution are largely due to changes of conjugation onto the amidine π-skeleton. Intermolecular 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-bonded 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 contribution of polar canonical forms increases upon hydrogen-bonding formation.