Ring current shielding and molecular motions in Piperazine-2,5-diones

Abstract

In 3-(phenylmethyl)piperazine-2,5-dione (cyclo(Phe-Gly)) the C6 methylene hydrogens are shielded by the aromatic ring which lies above them. As the temperature is raised, this shielding is reduced but the aromatic ring retains its gross position with respect to the methylene. The reduction in shielding is believed to be caused by aromatic ring torsional vibrations, enhanced at higher temperatures, which cause the aromatic ring to spend less time face-on to the methylene. In the naphthalene analogue these vibrations are damped and the shielding is less dependent upon temperature.