Rotational Motion and Nuclear Spin Interconversion of H₂O Encapsulated in C₆₀ Appearing in the Low-Temperature Heat Capacity

The heat capacity of H2O encapsulated in fullerene C60 is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in H2O@C60 undergoes quantum rotation at low temperature, and the ortho-H2O and para-H2O 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 JKaKc = 101 ground state of ortho-H2O. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-H2O to para-H2O. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with Ea ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ1 ≈ 1.5 h and τ2 ≈ 11 h

Solid-State Spin Equilibrium of Ni(cyclam)₂ Complex: Magnetostructural Correlations in Two Polymorphs

Two crystal polymorphs of Ni­(cyclam)­I2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) were synthesized, and their magnetic properties were investigated. Temperature-dependent X-ray structural analysis and magnetic measurements revealed gradual spin transition in molecular-crystal polymorph trans-[Ni­(cyclam)­I2] (1a), whereas the zigzag-chain polymorph catena-[Ni­(cyclam)­(μ-I)]­I (1b) did not show an obvious spin transition. The entropy difference between high- and low-spin states of 1a estimated by assuming the spin-equilibrium model is much smaller than those in typical iron­(II)-based spin-crossover (SCO) complexes, suggesting that the normal mode softening is less remarkable in 1a. In this system, it is clearly evidenced that the interaction mode responsible to the spin equilibrium in octahedral nickel­(II) complexes is highly anistropic, i.e., z-elongation and x,y-shortening of the coordination octahedron