The most stable TH-T complex is predicted to be quite strongly bound relative to its components, TH and T. At the B3LYP/TZ level employed primarily in this work, the radical hydrogen bond dissociation energy is 36.7 kJ/mol with and 41.6 kJ/mol without zero point vibrational (ZPV) correction. This is about twice the previous computational results in the literature , namely 18.4 and 21.3 kJ/mol, partly explainable by the superposition of London dispersion interaction on this hydrogen bond contact. Indeed, removing the D3(BJ,abc) correction at the B3LYP/TZ optimized structures reduces the attractive interaction by up to an order of magnitude. This suggests that London dispersion can promote the HAT process, by holding the two T radicals together when they share the hydrogen. More relevant for this work are relative energies, where some theory inaccuracies may partially cancel. On the other hand, already the size of the dispersion correction lowers the expectation on quantitative performance of the DFT calculations. Table thus contains relative energies of molecules, molecular complexes and transition states discussed in this work with (∆E 0 ) and without (∆E e ) zero-point vibrational energy at B3LYP/TZ level, also introducing their nomenclature. One can see that the OH group in TH has a strong preference to point towards the more o(pen) pair of flanking methyl groups (Figure ). It coordinates more or less equally well to the o(pen) and t(ight) faces of T, but the associated HAT transition state T-H-T which switches the roles of the nitroxyl radical and the hydroxylamine is significantly lower in the degenerate o-o case than in the conformation-changing o-t case. The absolute value of this barrier is seen to be sensitive to the quality of the theoretical treatment, underscoring the importance of the present experimental work in looking for evidence for barrier crossing.