Conformations and Conformational Processes of Hexahydrobenzazocines by NMR and DFT Studies

Conformational processes that occur in hexahydrobenzazocines have been studied with the ¹H and ¹³C dynamic nuclear magnetic resonance (DNMR) spectroscopy. The coalescence effects are assigned to two different conformational processes: the ring-inversion of the ground-state conformations and the interconversion between two different conformers. The barriers for these processes are in the range of 42–52 and 42–43 kJ mol^–1, respectively. Molecular modeling on the density functional theory (DFT) level and the gauge invariant atomic orbitals (GIAO)-DFT calculations of isotropic shieldings and coupling constants for the set of low-energy conformations were compared with the experimental NMR data. The ground-state of all compounds in solution is the boat–chair (BC) conformation. The BC form adopts two different conformations because the nitrogen atom can be in the boat or chair parts of the BC structure. These two conformers are engaged in the interconversion process