Model building caution

It should be remembered that a crystal structure represents an average not only over all the molecules in the crystal, but also over the time course of data collection. Dynamic and static disorder can lead to weak electron density in some regions, complicating the building and refinement of a structural model [9,125]. Caution should be exercised, and such factors considered, when a... 

The isomeric hydrocarbons A and B (see margin) both exhibit preferred conformations in which the methyl groups are equatorial, yet B is more stable than A by 2.3 kcal mol . What is the origin of this difference (Caution Are A and B ting flip isomers Hint Build models and look at the nature of the conformations of the methyl substituted cyclohexane ring.)... 

How many signals would you expect in the H NMR spectrum of bromocyclohexane (Caution Even considering fast ring flip, do the hydrogens located cis to the bromine ever become equivalent to those located trans Build a model )... 

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