Orbital Calculations of the Anomeric Effect

The description and understanding of the nature of stereoelectronic effects is an appropriate held for the application of oiganic quantum chemistry. Molecular orbital (MO) methods can describe the electron distribution in molecules, and the changes in internal rotation. In principle, they give the total potential energy of individual conformers completely, without the necessity to correct for various effects. Quantum chemical calculations offer a deeper insight into the orbital interactions in the molecule, and reveal the factors responsible for the stabilization of any conformation. 

—The Conformational Energy Map of Dimethoxymethane, With Energy Contour in kJ.mol.. Two Conformations ofDimethoxymethane Corresponding to a and e forms of 2-Methoxyoxane Are Also Shown. 

Owing to the complexity of the internal motion of carbohydrate molecules, the elucidation of their conformational properties by MO calculations requires a lessening of dimensionality to manageable proportions. Several small acyclic molecules have therefore been used as models for ab initio or semiempirical MO studies on the structural segments of carbohydrates. On the whole, calculations reproduce all of the main structural trends and conformational preferences observed experimentally in the crystal structures of carbohydrates and in solution. 

Calculated Relative Energies (kJ.mor ) of Stable Confonners of R OCHjOR with Respect to the (sc, sc) Rotauier Compared with Experimental Values for Dimethoxymethane 

Furthermore, there are four minima on the conformational surface representing the conformations of the type (ap, sc), and one minimum corresponding to the ap, ap) conformation. The map calculated for methanediol is similar, but, in contrast to that for dimethoxymethane, the ap, ap) con-former of methanediol is found to be a local maximum. A potential surface has also been reported for methoxymethanol, a structural intermediate between dimethoxymethane and methanediol, with lessened symmetry due to the presence of two different rotors. For example, conformations ap, sc) and sc, ap) can be distinguished in this case, and both are only doubly degenerate. 

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