Effects of nuclear spin on quantum dynamics and symmetry: A case study for molecular boron rotors
Boron rotors 11B2@11B9-, 11B3@11B10+, 11B4@11B11-, 11B6@11B13+, La-[11B2@11B18]-La consist of inner molecular “wheels” embedded in outer “bearings”. Weak interactions allow almost free rotations of the “wheels” in the pseudo-rotating “bearings”, from one global minimum (GM) structure to many other equivalent GMs. These large amplitude motions correspond to delocalized spatial eigenfunctions with equal populations of all GMs. Total eigenfunctions consist of products of the spatial eigenfunctions times nuclear spin functions. As a consequence, nuclear spins cause decisive quantum effects: (i) The local symmetry of the GMs is changed to global molecular symmetry. (ii) The molecular rotors can never be localized in a single GM. (iii) The rotors’ nuclear spins prohibit laser transitions between different nuclear spin isomers. (iv) As a consequence, lasers cannot induce rotations of the boron rotors. (v) Isotopic labelling causes quantum non-equidistribution. Quantum effects (i)-(v) cannot be described by means of classical molecular dynamics simulations.