GLOBAL CONFORMATIONAL-ANALYSIS AND THE ANOMERIC INTERACTIONS OF METHANEDIOL, METHANEDIAMINE AND AMINOMETHANOL

The global internal rotation potentials of methanediol, methanediamine , and aminomethanol have been obtained at the MP2/6-311 + g(2d,p) leve l by scanning through the dihedral angles of the two functional groups with the remaining nuclear coordinates being energy-minimized at the MP2/6-31G* level. The intramolecular hydrogen bonding between the two functional groups is represented by the general functional forms of t he electric dipole-dipole, dipole-quadrupole, and quadrupole-quadrupol e interactions. The through-direct-bond potentials between the functio nal group and its adjacent -CH2O or -CH2N molecular fragment are repre sented by the conventional three Fourier terms. It is found that the g lobal conformational potentials of these molecules can be adequately r epresented by the general functional forms of these two types of poten tials. The obtained electrostatic interaction strengths are in good ag reement with the predictions of the theoretical local dipole and quadr upole moments of the functional groups calculated by the Hirshfeld cha rge population analysis. The present energy-decomposition analysis sug gests that both the electrostatic interactions and the charge-delocali zation interaction of the lone-pair electrons of either oxygen or nitr ogen atom to its adjacent molecular fragment are equally important in determining the global conformational potentials, It also suggests tha t the origin of the anomeric effect of these compounds can be quantita tively explained in terms of these interactions. The relationship betw een the present energy-decomposition analysis and the other proposed i nteraction models for the anomeric and exo-anomeric effects is discuss ed.