Theory and spectroscopy of an incarcerated quantum rotor: The infrared spectroscopy, inelastic neutron scattering and nuclear magnetic resonance of H 2@C 60 at cryogenic temperature

Abstract

The supramolecular complex, H 2@C 60, represents a model of a quantum rotor in a nearly spherical box. In providing a real example of a quantum particle entrapped in a small space, the system cuts to the heart of many important and fundamental quantum mechanical issues. This review compares the predictions of theory of the quantum behaviour of H 2 incarcerated in C 60 with the results of infrared spectroscopy, inelastic neutron scattering and nuclear magnetic resonance. For H 2@C 60, each of these methods supports the quantization of translational motion of H 2 and the coupling of the translational motion with rotational motion and provides insights to the factors leading to breaking of the degeneracies of states expected for a purely spherical potential. Infrared spectroscopy and inelastic neutron scattering experiments at cryogenic temperatures provide direct evidence of a profound quantum mechanical feature of H 2 predicted by Heisenberg based on the Pauli principle: the existence of two nuclear spin isomers, a nuclear spin singlet (para-H 2) and a nuclear triplet (ortho-H 2). Nuclear magnetic resonance is capable of probing the local lattice environment of H 2@C 60 through analysis of the H 2 motional effects on the ortho-H 2 spin dynamics (para-H 2, the nuclear singlet state, is NMR silent). In this review we will show how the information obtained by three different forms of spectroscopy join together with quantum theory to create a complementary and consistent picture which strikingly shows the intrinsically quantum nature of H 2@C 60.