Using 'molecular surgery', remarkable complexes have recently been synthesised in which a small molecule such as H2 or H2O is entrapped inside a fullerene cage, typically C60. [1] In providing a practical example of an entrapped quantum particle, the system goes to the heart of quantum mechanics in displaying the wave-like nature of molecules. There is quantisation of both rotational and translational energy and the systems are proving to be invaluable as models for a quantum translator-rotator as well as for their intrinsic interest. [2,3] Inelastic neutron scattering (INS) has the exceptional ability to mediate spin-restricted transitions. As a result, INS provides a unique experimental probe for investigating the dynamics of symmetrical quantum rotors. H2 and H2O molecules are both characterised byortho- and para- nuclear spin-isomers, namely quantum states that have correlated rotational and nuclear spin eigenfunctions arising from the antisymmetry requirement for the wavefunction under exchange of the pair of indistinguishable protons. INS experiments conducted on the IN4, IN5 and IN1-Lagrange spectrometers at the Institut Laue-Langevin will be described. The emphasis will be on low temperature investigations of the small molecule endofullerenes H2@C60, H2@C70 and H2O@C60. In H2O@C60, the electric dipole moment plays a role and the system provides the unique opportunity to study the H2O molecule in isolation without hydrogen bonding. On IN1-Lagrange, high resolution spectra have been obtained in the range 12 – 300 meV, while IN4 and IN5 permit detailed interrogation of the low-lying states &lt;30 meV. In H2@C60 we observe splittings arising from the coupling of translational and rotational angular momentum while also revealing the first experimental evidence for selection rules in INS. [3] For H2@C60 and H2O@C60 we observe symmetry breaking in the ground ortho- state arising from inter-cage interactions and the crystal field. More substantial effects of lowering the cage symmetry or molecular symmetry are evident in H2@C70 and HF@C60 respectively. The INS spectra will be interpreted to provide quantitative insight into the cage potentials and the role of coupling between translational and rotational angular momentum. [1] K. Kurotobi, Y. Murata, Science 333 (2011) 613: K. Komatsu et al, Science 307 (2005) 238 [2] S. Mamone et al, Coord.Chem.Rev. 255 (2011) 938 [3] A.J. Horsewill et al; Phys.Rev.Letters 113 (2014) 123001: Phys.Chem.Chem.Phys. 16 (2014) 21330

Quantum translator-rotator dynamics of small molecule endofullerenes: Neutron scattering investigations of the nuclear spin-isomers of H2 and H2O

251st National Meeting (2016)

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