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Design to store gas molecules, such as CO2 , H2 , and CH4 , under low pressure is one of the most important challenges in chemistry and materials science. Herein, we describe the storage of CO2 in the cavities of a porous coordination polymer (PCP) using molecular rotor dynamics. Owing to the narrow pore windows of PCP, CO2 was not adsorbed at 195 K. As the temperature increased, the rotors exhibited rotational modes; such rotations dynamically expanded the size of the windows, leading to CO2 adsorption. The rotational frequencies of the rotors (k≈10-6 s) and correlation times of adsorbed CO2 (τ≈10-8 s) were elucidated via solid-state NMR studies, which suggest that the slow rotation of the rotors sterically restricts CO2 diffusion in the pores. This restriction results in an unusually slow CO2 mobility close to solid state (τ≥10-8 s). Once adsorbed at room temperature, CO2 is robustly stored in the PCP under vacuum at 195-233 K because of the steric hindrance of the rotors. We also demonstrate that this mechanism can be applied to the storage of CH4 .

In multiple-quantum magic-angle-spinning (MQMAS) NMR experiments, it has been argued that one should use as strong rf irradiation as possible in most situations so as to attain the better sensitivity. In this work, we theoretically derive formulas for the efficiency of MQMAS experiments for spin nuclei under strong rf irradiation, and experimentally examine the validity of the derived formulas. For this purpose, a microcoil was accommodated to magic-angle-spinning (MAS) experiments by developing a microcoil-based, coin-sized probe, which is attachable to an existing MAS module. The rf intensity of up to ∼1 MHz was realized, and the efficiencies of multiple-quantum (MQ)-excitation and conversion were studied for the 23Na (I = 3/2) nucleus in a polycrystalline sample of sodium sulfite, from which the theoretical formulas were confirmed. Moreover, it was found that the effect of rf inhomogeneity is significant for MQ-excitation when rf irradiation is strong, while it is not appreciable for MQ to single-quantum conversion. From the insight into the MQ-excitation and conversion processes, the optimal set of rf intensity and pulse width is proposed for a given quadrupolar coupling constant. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 115–123, 2008