Rotational Motion and Nuclear Spin Interconversion of HO Encapsulated in C Appearing in the Low-Temperature Heat Capacity.

The heat capacity of HO encapsulated in fullerene C is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in HO@C undergoes quantum rotation at low temperature, and the ortho-HO and para-HO isomers are identified by labeling the rotational energy levels with the nuclear spin states. A rounded heat capacity maximum is observed at ∼2 K after rapid cooling due to splitting of the rotational J = 1 ground state of ortho-HO. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-HO to para-HO. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with E ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ ≈ 1.5 h and τ ≈ 11 h.