Anti conformations, force field methods

Meso- and D,L-2,3-dimethoxy-2,3-diphenylsuccinonitrile 39 were obtained in a 1 1 ratio by a preparative dimerization procedure. Their configurations were assigned by crystal structure analysis 74). According to nmr both diastereomers have a configuration with the phenyl groups in the anti position. From a conformational analysis by the force field method it was concluded that these conformations remain the preferred ones also for the isolated molecules meso-39 was calculated to be 1.2 kcal mol-1... 

Molecular modeling calculations (force-field methods, MMFF94s) at room temperature and NMR experiments have shown that the preferred orientation of the functional group [=N-N02] in 3 is in the trans-position the (Z)-isomer with lowest energy is more than 2.6 kcal mol above the optimal (F)-isomer [47]. Calculations as well as X-ray structure analysis have shown that the three C-N bonds involving atom C5 have some double bond character. The N-methyl group of 3 can flip easily from the anti- into the syn-position. The energies of its respective conformers, relative to the optional structure, are below 1.5 kcal mol . ... 

Note that there are two conformers, one with the two H8 atoms on the same side of the coordination plane (syn isomer) and the other with one H8 atom on each side of the coordination plane (anti isomer). Refine and save both using the MOMEC97 force field. Section 17.14 describes how to enforce planarity of a coordination compound. Two dimensional NMR methods can be used to determine which isomer dominates - as long as interconversion of the isomers is not rapid on the NMR time scale. The data used here are hypothetical and we have assumed that one isomer dominates to the exclusion of the other and that there is no interconversion, i. e., the observed NMR spectrum is that of an isomerically pure compound. 

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