Dimethoxymethane torsional potentials

Fig. 4. Modelling of the torsion potential of the exo anomeric effect using the results of the ab initio calculation of dimethoxymethane. The difference between the non bonded terms and the ab initio calculation is fitted with the general function of torsion potentials to yield the equations for the oc-and the P-anomers discussed in Sect. 2.5.3 for the HSEA method...

For such an asymmetric structure as dimethoxymethane with one torsion angle fixed at 60 °, additional sine terms are necessary, in order to account for the lack of symmetry about 0 = 180° (see Ref. 94). The individual components V,(0), V2(0), and V3(0) of the total potential function V(0) can be identified with specific physical effects of similar periodicity. For example, the onefold term, V,(0) = 0.5 V ( 1 — cos 0), moves from a maximum v ue to a minimum value as 0 changes by 180 °. The same variation with torsion shows dipolar or steric interactions. The twofold term, V2(0), changes from a maximum to a minimum as 0 changes by 90°. This periodicity frequently corresponds to the change of delocalization interactions. Finally, the threefold term, V3(0), moves from a maximum value to a minimum value as it changes by 60°. This is generally attributed to the intrinsic torsion potential. 

Figure 1. Theoretical potential-energy curves for rotation about the anomeric bond from ah initio Hartree-Fock 431-G calculations on dimethoxymethane. is the glycosidic O-5-C-l-O-l-CH i torsion angle. is the C-5-0-5-C-1-0-1 torsion angle and is 60° for a-D-pyranosides, 180° for fl-v-pyranosides (see...

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