Recently, it has been reported that the montmorillonite-catalyzed oligomerization of activated nucleotides exhibits remarkable enantioselectivity. In the current paper we investigate the structures and intrinsic energies of homochiral and heterochiral cyclic dinucleotides by means of accurate quantum chemical calculations in gas-phase and in bulk water. The steric effect of the clay is represented with geometrical constraints. Our computations reveal that the heterochiral dimer geometries are systematically less stable than their homochiral counterparts due to steric clashes inside the sugar-phosphate ring geometry. Thus we suggest that the homochiral selectivity observed in the cyclic dinucleotide formation in confined spaces may arise from the energetic destabilization of the heterochiral ring geometries as compared to their homochiral analogues. In the present paper we provide the first model of the 3D structure of d,l cyclic dinucleotides, which until now has eluded experimental observation.