The symmetric parts of these tensors are depicted by the ovaloid surfaces in figure . The asymmetric disposition of the hydrogen atom breaks the symmetry; the numbering of the atoms is such that the H atom is closer to O4 than to O2. The calculated values of the isotropic parts of the shielding tensors are given in table A. The quantity ⟨σ iso ⟩ is the average of the isotropic C shielding factors over all four positions. Since the shielding σ and chemical shift δ have opposite signs, the increase in mean isotropic shielding when Sq 2-is protonated corresponds roughly to the decrease in the isotropic chemical shift δ iso when the pH is decreased (see figure ).