Ring-flip molecular model

Using molecular models as well as structural drawings, explain why trans-decalin is rigid and cannot ring-flip, whereas ds-decalin can easily ring-flip. 

With the aid of molecular modelling methods we have developed and compared models for the antagonist binding site of the receptor, based on steric, electrostatic and hydrophobic properties of various adenosine receptor antagonists [44,45]. In one of the favoured models theophylline binds to the adenosine receptor in a flipped orientation, i.e. the ring atoms Nl, N3, N7 and N9 in adenosine coincide with C2, C6, N9 and N7, respectively, in theophylline (Fig. 2). This implicates that the domain where the ribose moiety of adenosine binds must be adjacent to N7 in xanthines. 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that then undergoes a ring flip to the diequatorial conformation of rn r-l,2-dibromo-cyclohexane. However, when the unsaturated bicyclic compound I is the alkene, instead of cyclohexene, the addition product is exclusively in a stable diaxial conformation. Account for this. (You may find it helpful to build handheld molecular models.)... 

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