The quantity ⟨ ∆σ iso ⟩ in table A is the mean difference in isotropic shielding factor between adjacent 13 C sites, neglecting the sign. This quantity corresponds roughly to the parameter ∆δ used in the two-site calculations (table ). Table . Quantum chemistry estimates for the isotropic shielding parameters for the 13 C sites in Sq 2-and SqH -, assuming a water solvation model. The sites are numbered so that the hydrogen is attached to O1 and is closer to O4 than to O2. The row labelled ⟨σ iso ⟩ gives the average isotopic shielding of all four 13 C positions. The row labelled ⟨ ∆σ iso ⟩ gives the average unsigned difference in isotropic shielding factors between adjacent 13 C positions.