Intramolecular hydrogen bonding and anomeric interactions in HnXCH2YHm (XHn, YHm=OH, NH2, SH, PH2): A global conformational analysis

The global internal rotation potentials of the title compounds were obtaine d at the MP2/6-311+g(2d,p) level by scanning through the dihedral angles of the XHn and YHm functional groups with the remaining nuclear coordinates b eing energy minimized at the MP2/6-31G** level. The intramolecular hydrogen bonding between the XHn and YHm functional groups is represented by the ge neral functional forms of the electric dipole-dipole, dipole-quadrupole, an d quadrupole-quadrupole interactions. The through-direct-bond potentials be tween the functional group and its adjacent -CH2X or -CH2Y molecular fragme nt were represented by the conventional three Fourier terms. The general fu nctional forms of these two types of potentials could adequately represent the global conformational potentials of these molecules. The present energy decomposition analysis suggests that both the electrostatic interactions a nd the charge delocalization interaction of the lone-pair electrons of the X or Y atom to its adjacent molecular fragment are equally important in det ermining the global conformational potentials, and the origin of the anomer ic effect of these compounds could be quantitatively explained in terms of these two types of interactions. Quantitative comparisons of the anomeric i nteraction strengths and the related orbital interaction energies among the title compounds were emphasized. Their general implications on the related molecular systems reported in the literature were also discussed.