Inelastic neutron-scattering measurements of the torsional levels of C${\mathrm{H}}_3$N${\mathrm{O}}_2$ and C${\mathrm{D}}_3$N${\mathrm{O}}_2$ are presented as functions of pressure and temperature. In contrast to all previously observed pressure dependence of hindered rotors, the ground-state tunnel splitting increases and the energies of the bound torsional levels decrease with pressure. A potential which reproduces these anomalous effects is found and the source of the anomaly explained.

• Received 28 September 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.193