In total, we found 59 TS structures across the 11 sites, which are reported in Figures S5-S15 of the SI. We identified two categories of TS configurations across sites, which we call C I and C II , and one example of each is reported in Figure . ure 8b, except for the 2M site where the whole two-member ring moves to create more space for the reactants. The C II structures are found on sites where the Nb --O wagging motion has a vibrational frequency below or around 100 cm -1 (the frequencies for all sites are reported in Table of the SI). This low energy motion allows the reactants to "push away" the oxo group and increase the available volume in the pocket. The only exception to this trend is site 9, where the Nb -O wagging is in the direction of a nearby silanol that makes the area sterically hindered and cannot accommodate a TS structure. In site 1 (ν N b=O = 100.4 cm -1 ), the vibration includes the motion of the entire twomember ring.