Shape Selection in the Association of Diaminoguanidinium Cation with Counterions

Article abstract of DOI:10.1021/jo00294a008

X-ray crystal structure determination shows that the conformation of the diaminoguanidinium (DAG) cation ((NH2NH)2C(=NH2)⁺ is different in its salts with nitrate ion and with the conjugate base of 3-nitro-1,2,4-triazole-5-one (NTO).In both cases, the carbon atom and all five nitrogen atoms are coplanar.In the nitrate salt an S configuration is found where one of the terminal NH2 groups is cis to the central C=NH2 bond and the other is trans.In the NTO salt a W configuration is found where both of the terminal NH2 groups are cis to the central C=NH2 bond.The terminal NH2 are positioned in such a fashion that the hydrogens appear above and below the plane of the rest of the molecule and the lone pairs are directed toward the hydrogens of the C=NH2 moiety.AM1 calculations were performed for a variety of DAG conformations to give a qualitative indication of the relative energies.The S and W forms were not found to be true minima by this method because of a spurious tendency toward pyramidalization of the guanidine nitrogens.Ab initio molecular orbital calculations, however, show that both the S and W forms are potential energy minima in the absence of counterions, and that the S form is the more stable by about 0.5 kcal/mol.Other forms for the DAG cation were calculated, but were found to be higher in energy.Topological analysis of the S and W forms shows unusual N-N bond paths, rather than N-H bond paths expected for typical hydrogen bonds.Additionally, integrated populations are used to assess Y delocalization of charge.Energies of the complexes formed by the NTO conjugate base with the W and S forms of DAG were calculated using a electrostatic model.The association energiesshowed that the W form is complexed more strongly than the S form.The difference in this energy is larger and oppositely signed from the W/S energy difference as free cations.Thus, the calculations suggest that the nitrate salt contains DAG in its lower energy form, while favorable electrostatic interactions lead to selective binding of the less stable W form of DAG with NTO.