FULL AB-INITIO CONFORMATIONAL SPECTRUM OF ALPHA,ALPHA'-DIAMINOACETONE

The ab initio conformational spectrum of alpha,alpha'-diaminoacetone, (NH2CH2)?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 pl anar heavy-atom structure and two acceptor-bifurcated NH2:::O:::H2N hy drogen bonds is the global minimum at all levels of theory: AM1, HF/ST O-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 hyd rogen bonding. The relative energies of higher conformational states s how a distinct basis set dependence. Significantly, HF/6-31G calculati ons do not reproduce the full HF/6-31+G conformational spectrum, sinc e 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.