Molecular mechanics calculations bicyclo nonane

The introduction of 1- and 5-substituents usually does not influence significantly the conformational behavior of bicyclo[3.3.1]nonanes. Molecular mechanics calculations (39) show that the presence of two methyl groups at the 1 - and 5-positions (compound 64) leads to some decrease (of approximately 0.03 A) of the C3 - C7 distance as compared with bicyclo[3.3.1]nonane itself. The substituents in these positions decrease the possibility of the wings flattening and could somewhat increase the interaction between substituent groups or lone electron pairs at endo-3 and endo-1 positions. 

The BB conformation is predicted by ab initio calculations to be located 10-11 kcal mol -1 above the CC one and optimizations of twist-twist forms with C2 symmetry led to a boat-boat conformer of C2v symmetry for all starting deformations chosen (5). This result is in contradiction with molecular mechanics calculations predicting the energy of the BB form of bicyclo[3.3.1]nonane to be 10.7kcal mol-1 and the TT form 8.2kcal mol-1 above that of the CC conformer (24,33). 

The potential surface for the conformational transition CC CB in bicyclo[3.3.1]nonane calculated using the MM1 molecular mechanics program has also been published (37). 

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