The structure, modelling and dynamics of hindered 5,6-diarylacenaphthenes

A series of 5,6-diarylacenaphthenes, 2-4, has been investigated using a var iety of theoretical and experimental methods. The purpose of the investigat ion was to gain a thorough understanding of the dynamics of atropisomer int erconversion in these molecules. Quantum chemical calculations were perform ed at different levels, including Hartree-Fock theory, density functional t heory (B3LYP), and a semi-empirical method (AMI). Basis sets used ranged fr om STO-3G to 6-31 + G*, The structures of energy minima and transition stat es for anti-syn interconversion were fully optimised. A geometrical compari son of the single-crystal X-ray structures of syn-2b, anti-2c and anti-2d w as made with the results from the calculations, and revealed excellent agre ement in most cases. Theoretical barriers to rotation were compared with th ose derived experimentally by NMR spectroscopy. Again excellent correlation between theoretical and experimental values was found. In the transition s tates a significant deviation from planarity for the acenaphthene moiety wa s calculated. The transition state structures indicate why bulky substituen ts, such as Bu-t, in the 3' and 3 " positions of the peri-aryl rings are no t able to prevent atropisomer interconversion. The structures provide a goo d explanation for the trends observed in experimental barriers to rotation.