Investigation of the spin-lattice relaxation of 13CO and 13CO2 adsorbed in the metal-organic frameworks Cu3(btc)2 and Cu3-xZnx(btc)2

The C-13 nuclear spin-lattice relaxation time of (CO)-C-13 and (CO2)-C-13 molecules adsorbed in the metalorganic frameworks (MOFs) Cu2.97Zn0.03(btc)(2) and Cu-3(btc)(2) is investigated over a wide range of temperatures at resonance frequencies of 75.468 and 188.62 MHz. In all cases a mono-exponential relaxation is observed, and the C-13 spin-lattice relaxation times (T1) reveal minima within the temperature range of the measurements and both frequencies. This allows us to carry out a more detailed analysis of the C-13 spin relaxation data and to consider the influence due to the spectral functions of the thermal motion. In a model-free discussion of the temperature dependence of the ratios T-1 (T)/T-1,T-min we observe a motional mechanism that can be described by a single correlation time. In relation to the discussion of the relaxation mechanisms this can be understood in terms of dominating translational motion with mean jump distance being larger than the minimum distances between neighboring adsorption sites in the MOFs. A more detailed discussion of the jump-like motion observed here might be carried out on the basis of self-diffusion coefficients. From the present spin relaxation measurements activation energies for the local motion of the adsorbed molecules in the MOFs can be estimated to be 3.3 kJ/mol and 2.2 kJ/mol, for CO and CO2 molecules, respectively. Finally, our findings are compared with our recent results derived from the C-13 line shape analysis. (C) 2013 AIP Publishing LLC.