We find small differences in δiso( 13 C) = 78.0-79.0 ppm upon ethylene coordination in Cu(I)-MFU-4l and 80.5 ppm for ethylene coordination in Cu(I)-ZrTpmC*(η 2 -C2H4) (see Table ). For hydride substituents in Cu(I)2.2Zn2.8(Cl/H)1.8(btdd)3(η 2 -C2H4), the C shifts are slightly higher with 78.0 ppm (OCHO -: 78.5 ppm) indicating stronger binding for the electron donating hydride compared to the bulkier chloride ligand, and with the bidentate formate ligand falling in the intermediary range. This emphasizes the sensitivity of C NMR spectra for detecting small changes in the metal-organic framework that can impact the nature of adsorbate binding, for example showing slightly stronger π-backbonding. To elucidate the response of the guest to confinement at the Cu(I) site, we measured C NMR spectra as a function of temperature and magnitude of magnetic field (Figure ). At both 11.7 and 16.4 T, good fits were obtained with the CSA and 13 C- C dipole-dipole coupling parameters, as derived from DFT. At lower temperatures (seeFigure S1), the spinning side bands observed at low ppm increase in relative intensity, indicating a slight increase in δ11, while the center of the spectrum shifts to larger chemical shifts (i.e., δiso decreases). The best fit simulating (c in Figure ) the experimental data in Figure illustrates that from room temperature down to 255 K (see Figure ) the line shape of ethylene is best fit with a model that assumes the ethylene molecule rapidly rotates when bound to the metal center and effectively averages σ11 and σ22. This is consistent with a small rotation barrier height of 2.0 kJ/mol obtained from DFT simulations (see Fig. ). Thus, via C MAS NMR combined with DFT calculations we find that ethylene is adsorbed at the Cu(I) center while spinning around the C3 z-axis in Cu(I)-MFU-4l, effectively averaging out anisotropy in the xy plane.