Potential surface of rotation-translation coupled systems: Me(NH{sub 3}){sub 6}(PF{sub 6}){sub 2}, Me=Ni,Co

• Main Authors: Schiebel, P., Prandl, W., Buettner, H. G., Kearley, G. J., Prager, M.
• Format: Conference Proceeding
• Language: English
• Subjects: AMMONIUM COMPOUNDS; COBALT COMPOUNDS; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; COUPLING; FCC LATTICES; GROUND STATES; INELASTIC SCATTERING; MOLECULES; NEUTRON DIFFRACTION; NEUTRON SPECTRA; NICKEL COMPOUNDS; PHASE SHIFT; PHASE TRANSFORMATIONS; POTENTIALS; PROTON DENSITY; ROTATION; SURFACES; SYMMETRY; TUNNEL EFFECT; VIBRATIONAL STATES
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.59476

One of the most recent developments in the field of rotational dynamics is the discovery of rotation-translation-coupling (RTC) for molecules in an environment of incompatible symmetry. The combination of incompatible molecular and environmental symmetry usually leads to almost free rotation. Ni-and Co-hexamminehexafluorophosphate thus show large ground state tunnel splittings (0.540 meV (Ni) and 0.542 meV (Co)), as observed by inelastic neutron scattering (INS). The tunnelling spectrum of Ni(NH{sub 3}){sub 6}(PF{sub 6}){sub 2} is split into three peaks. Under pressure their relative intensities change, but there is almost no shift of the peak positions. In contrast, the tunnelling spectra of Co(NH{sub 3}){sub 6}(PF{sub 6}){sub 2} shows only one peak, which does not change under pressure. The hexafluorophosphate compounds are unique among the hexammines, because they do not undergo a phase transition on cooling. From a crystallographic view, both compounds are isostructural (Fm3m). Nevertheless, the proton density distributions, observed by neutron diffraction, although almost circular, show an interesting difference, namely their maxima are rotated by 45: in the cobalt compound they are on the axes, whereas in the nickel compound they are on the diagonale at room temperature and on the axes at 5K. This is related with a phase shift between the two terms characterizing the potential surface. Obviously, the potential is very sensitive to weak changes of the environment.