ϕ denotes the isotopic fractionation parameter that signifies the tendency of a hydrogenic site acting as a proton shuttle to exchange with D + relative to H + and Z is the number of hydrogenic sites functioning as proton relays. If proton shuttling is in place, a plot of jn/j0 vs. n will show a slightly parabolic relationship and the data can be modelled with Z >1. In contrast, if there is no shuttling in place, there will be a linear jn/j0 vs. n curve and Z = 1. Proton inventory experiments on the Co Terpy-RF illustrated that under catalytic conditions, the data set can be fitted to an equation with ϕ = 0.61 and Z = 1.26 (Fig. ). In contrast, conducting the same experiment just positive of the Co(II/I) redox couple, where proton shuttling is not expected to be in play, yielded a fit of ϕ and Z = 1 (Fig. ).