Full ab Initio Conformational Spectrum of α,α‘-Diaminoacetone

The ab initio conformational spectrum of α,α‘-diaminoacetone, (NH 2 CH 2 ) 2 CO, is obtained by systematic conformational space search at the HF/6-31+G* level. The 15 conformers are discussed in terms of near-neighbor interactions such as hydrogen bonding. Conformer 1 with a planar heavy-atom structure and two acceptor-bifurcated NH 2:::O:::H 2 N hydrogen bonds is the global minimum at all levels of theory: AM1, HF/STO-3G, HF/6-31G, HF/6-31G*, HF/6-31+G*, HF6-311++G**//HF/6-31+G*, HF/6-311++G(3df, 3pd)//HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, and MP2/6-311++G**//HF/6-31+G*. The double bifurcation appears to be a novelty in hydrogen bonding. The relative energies of higher conformational states show a distinct basis set dependence. Significantly, HF/6-31G calculations do not reproduce the full HF/6-31+G* conformational spectrum, since the pyramidality of amino nitrogens is clearly underestimated at the former level. The AM1 method predicts far too few minima and does not seem to properly describe the NH···O bond.