X-ray crystallographic analysis and DFT calculations of three ‘propylene linker’ dimers linked by one polystep reaction

In this manuscript, we report the synthesis, NMR and single-crystal structures of three propylene linking dimers related with the hydrolytic degradation of one 5,6-dehydronorcantharimide dimer. Special attention was paid to the conformation of propylene linkers in order to understand their changes in the reaction. Statistical analysis of CSD database revealed that a-a, g-a and g-g conformations may have similar stability in most cases and various complicated unpredictable non-covalent interactions may play important role in the formation of final rotamers . In order to reproduce all stable conformations and the energy barriers separating them, full range two-dimensional fully relaxed potential-energy surfaces (PES) scans of six ‘propylene linker’ dimers were calculated starting from the most stable crystal structures. The PES were scanned along both bridge C C single bond torsional angles (denoted as θ 1 and θ 2 ), while all other internal coordinates were optimized at the DFT/B3LYP/3-21G* level in gas phase. Then all energy minima were re-optimized again at the DFT/B3LYP/6-311 + G(d,p) level both in gas and ethanol solutions in order to evaluate the really stable rotamers. At last, 1D or 2D relaxed PES scans were performed between local stable rotamers to get reliable energy barriers. This method represents a less time-consuming and more reliable approach to the determination of conformational stability of propanediyl bridging chains. The combination of experimental, statistical and theoretical results shows that the observed conformation is jointly determined by the energy levels of the minima, energy barriers separating them, non-covalent interactions and somewhat randomness.