The desolvated (3,24)-connected metal-organic framework (MOF) material, MFM-160a, [Cu-3(L)(H2O)(3)] [H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid)], exhibits excellent high-pressure uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C-2 hydrocarbons from CH4 at 1 bar pressure. Henry's law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid-state H-2 NMR spectroscopic studies on partially deuterated MFM-160-d(12) confirm an ultra-low barrier (similar to 2 kJ mol(-1)) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 x 10(6) Hz cf. 10(11)-10(13) Hz). Upon introduction of CO2 or C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO2 and C2H2 around the triazine core are entirely consistent with the H-2 NMR spectroscopic observations.