## 3 Results and Discussion
### Theoretical Analysis of the Water Dissociation Reaction Rate  
Calculations summarized in Table 2 show that, when both water and methanol are present in the system, methanol associates to the metal center and water binds to the organic linker. Water alone can associate with both the metal linker and organic linker almost equally, but the MeOH has a stronger binding (association) to the metal center than water. In fact, MeOH does not interact with the oxygen atom of the linker, in contrast to water. Consequently, the OH group of MeOH binds to the metal center upon cooperative adsorption of MeOH and D\({}_{2}\)O, while the D atom from water binds to the oxygen atom of the linker since the metal site is occupied, and the OD part of D\({}_{2}\)O binds to the CH\({}_{3}\) group of MeOH, as illustrated in Figure 3a. The dissociation reaction then proceeds as follows: First, there is a lengthening and thus weakening of the C-O bond in MeOH, which is in agreement with the observed redshift of \(\
u\)(C-O) from 1033 cm\({}^{-1}\) to 1003 cm\({}^{-1}\) and facilitates subsequent OH exchange. Indeed, at the transition state, the OD group from the neighboring D\({}_{2}\)O molecule approaches the methyl group, while the other D atom bonds to the oxygen of the ligand. As OH (from methanol) bonds to the Zn center very strongly (see Table 2) and D (from D\({}_{2}\)O) binds strongly to O (see Table 2), the OD group is transferred to the methyl group to complete the hydroxyl exchange on the methyl group. While such an exchange is not common, the strong energetic gain drives this reaction. As MeOD is formed, a single D remains at the oxygen of the linker, characterized by the well-studied 970 cm\({}^{-1}\) IR fingerprint [3]. Note that in this cooperative reaction the energy barrier for dissociation is also reduced by a clustering effect [14]. The products of the dissociation reaction (OH on the Zn\({}^{2+}\) site or D on oxygen site) bind much more strongly than the initial reactants (H\({}_{2}\)O/D\({}_{2}\)O and MeOH) that are only bound through weaker van der Waals interactions, as shown in Table 2.  
Figure 2: **(Top)** Absorbance IR absorption spectra of Zn-MOF-74 loaded with a mixture of 8 Torr methanol and 8 Torr D\({}_{2}\)O as a function of temperature and time. **(Bottom)** Detailed reaction evolution at 180 \({}^{\circ}\)C from 0 to 8 h (dashed arrows indicate the time evolution). The mechanism by which MeOH can thus speed up the reaction is as follows: When water dissociates without any other guest molecules present, it is the water molecule bound to the metal center that dissociates, as shown in Figure 3c [3; 14; 30; 31]. In the presence of MeOH, this pathway is blocked, as MeOH takes the place on the metal site, which forces the water bound to the linker, as shown in Figure 3a. The weakening of C-O bond in MeOH, evidenced by the frequency shift, is even more pronounce when an OD group (from D\({}_{2}\)O) is in close proximity, which makes MeOH an effective precursor for D\({}_{2}\)O dissociation, leading to D bonding to the oxygen linker, with reformation of a methanol (MeOD) molecule. To further support the experimental observation of a faster reaction, the vibrational modes of the D\({}_{2}\)O/MeOH system were also calculated in conjunction with Zn-MOF-74 and characterized using J-ICE [41]. After applying a systematic scaling of 7% to all computed frequencies, a peak at 1030 cm\({}^{-1}\) was found for the C-O stretching mode of gas-phase MeOH; that mode was also calculated to red-shift to 1005 cm\({}^{-1}\) upon adsorption in the MOF at the Zn metal centers and interaction with D\({}_{2}\)O. This calculated 25 cm\({}^{-1}\) red shift is in excellent agreement with the measured 30 cm\({}^{-1}\) redshift, i.e., from 1033 cm\({}^{-1}\) to 1003 cm\({}^{-1}\), mentioned above. As pointed out, this red-shift is indicative of the C-O bond weakening, which is a requirement for hydroxyl