TRANS-DECALIN; A CONFORMATIONALLY-LOCKED SYSTEM
Although simple cyclohexanes, in general, have two conceivable chair conformations, this is not true for trans-decalin. The conformation shown below is the only low energy possibility. The reason becomes clear........
....when you try to convert one of the two rings to the "alternative" chair structure. The two carbons belonging to the second ring must go axial, and it is then impossible to complete the second ring without employing completely unrealistic bond lengths and/or angles (see below).
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