Later, it was found that this single empirical equation could not describe an entire reaction family . In solvents which support ionization, some hydrogen abstraction reactions, for example that of 2,2'-methylene-bis(4methyl-6-tert-butylphenol) (BIS) with DPPH•, proceeded by the SPLET mechanism . This led to a reaction which is zero-order in DPPH•. Because this is not true for the reactions of all phenols with DPPH•, it was suggested that properties of the reacting phenol play a role. One such property is the intramolecular H-bond in BIS, which may slow the reverse proton-transfer reaction, thus leading to the unusual effect observed. Furthermore, Nielsen and Ingold found that the β H 2 scale does not account for solvents' anion solvating properties, and thus reactions involving proton transfer do not quite follow the Snelgrove-Ingold correlation . The Taft β scale gives better correlations for these proton transfer reactions.