Observation of n→π* interaction in terminally protected proline

The presence of the n→π* interaction as a stabilizing factor in the absence of other non-covalent interactions has fairly been recognized in collagen, poly-L-proline type II (PP II) helix and even in crystal structures of small molecules. Herein, we report C=O ... c=o n→π* interaction between the trans conformers of a terminally protected proline molecule. For the first time, we have also found additional n→π* interaction between the lone pair of the N-atom in the pyrrolidine ring to the π orbital of the C=O group in the proline derivative studied here. (author)