Y-Conjugated Compounds: The Equilibrium Geometries and Electronic Structures of Guanidine, Guanidinium Cation, Urea, and 1,1-Diaminoethylene

Ab initio calculations at the MP2/6-31G(d) level of theory predict that the equilibrium geometries of the Y-conjugated compounds guanidine (1), guanidinium cation (2), urea (5), and 1,1-diaminoethylene (6) are nonplanar. 1, 5, and 6 have energy minimum structures with strongly pyramidal amino groups. The equilibrium geometry of the guanidinium cation 2b has D₃ symmetry; the planar amino groups are rotated by ∼ 15° out of the D_3h form 2a. The planar structure 2a becomes lower in energy than 2b when corrections are made for zero-point vibrational energies. The observed planar geometries of guanidine and urea in the crystal are probably caused by hydrogen bonding. The resonance stabilization of the Y-conjugated structures is not very high, because the rotation of one amino group leaves a subunit which is isoelectronic to the allyl anion. Yet, resonance stabilization in the Y-conjugated forms is important, as it is revealed by the calculated rotational barriers for the NH₂ groups and the substantial lengthening of the C-NH₂ bonds upon rotation. The energy difference between 1,1-diaminoethylene (6) and 1,2-diaminoethylene (7) is mainly due to conjugative stabilization in 6. The two isomers have nearly the same energy when one amino group in 6 is rotated. The calculated proton affinity of guanidine is only 237.7 kcal/mol. It is concluded that the very high basicity of 1 in solution is not caused by the resonance stabilization of 2, but rather by strong hydrogen bonding of the guanidinium cation.