The same trend was also observed for D2, for which the signals appear as negative peaks in the difference spectrum (Figure ). The peak position of the small peak for non-metal adsorption sites is almost invariant for all Cu I M-MFU-4l frameworks at ~2970 cm -1 , whereas the position of the peak for D2 strongly bound to Cu I is located at 2360, 2390, and 2440 cm -1 for Cu I Zn-MFU-4l, Cu I Mn-MFU-4l, and Cu I Cd-MFU-4l, respectively. The ratios of the H2:D2 stretching frequency for the corresponding adsorption site are consistent with the change in the reduced mass between H2 and D2 within 3% error for both Cu I sites and non-metal adsorption sites. Analysis of variable-temperature IR data under gas dosing can also provide site-specific thermodynamic parameters of H2 adsorption through use of the van't Hoff equation. However, we encountered difficulties in integrating the peak area to perform a van't Hoff analysis for H2 adsorption in the Cu I M-MFU-4l frameworks owing to: (i) the broad asymmetric shape of the IR band for the chemically activated H2 and (ii) changes in signals associated with framework vibration due to the slight change in the coordination geometry of Cu I upon gas binding. In this work and our previous work, we used a D2-dosed spectrum as a baseline for the corresponding calculations were performed for a cluster model of a pentanuclear secondary building unit. In this model, all btdd 2-linkers were truncated to benzotriazolate (bta -), a single Cu I ion was substituted at one of the four peripheral metal sites, and chloride capping anions were placed at the other peripheral M II sites, resulting in an uncharged cluster of formula Cu1M4Cl3(bta)6 (M = Zn, Mn, Cd; Figure ). By normalizing the capping anion and Cu populations in each structure, we can understand how the identity of the central M II ion affects H2 binding at the peripheral Cu I site.