Figure 5.

Arrhenius plots for the oxygen four-site exchange process, assuming first, rotations about the four P–O C₃ axes (with identical exchange rates k, red open squares), and second, a faster C₃ rotation about the P–O1 axis (k₂, orange open diamonds), and slower C₃ rotations about the P–O2, P–O3^d, and P–O3^a axes (k₁, black open circles) of the phosphate ion. The red, black, and orange solid lines indicate the Arrhenius equation least-squares fits to the rates k, k₁, and k₂, giving E_a = 0.43 ± 0.05, 0.50 ± 0.07, and 0.21 ± 0.06 eV, respectively, in the temperature range 79–214 °C (for k₂, 98–214 °C). Error bars indicate errors of 5% except 20% for data points at lower temperatures (79 and 98 °C). The blue solid line corresponds to the ¹H two-site exchange data obtained in our earlier study²⁰ (rate k_ex, blue open triangles) with E_a = 0.70 ± 0.07 eV (167–214 °C) which is shown here for comparison.