Figure 4:

Magnetic field dependence (B_o = 18.1 to 35.2 T) of the observed ¹⁷O isotropic frequency shown using the (a) observed isotropic frequency from the 2D MQMAS spectra, and (b) using the equation${\delta }_{iso}={\delta }_{3Q}+\frac{3}{850}\frac{P_Q^2}{{\nu }_0^2}x{10}^{-6},$where$P_Q=C_Q\sqrt{1+\frac{{\eta }^2}{3}}$and v₀ is the Larmor frequency of ¹⁷O and is calculated from the values in Table 1. A projection of the isotropic dimension at each field is shown to the left of the isotropic frequencies for each field in (a). The dashed lines (Black, Red, Blue and Purple) are linear fitted lines to the isotropic frequencies of each site.