For the B3LYP/TZ method, the predicted wavenumber sequence of the two experimentally detected TH-T isomers is more trustworthy than a sub-kJ/mol energy ranking. Therefore, the more intense signal is attributed to the THo-Tt isomer, where the hydroxylamine OH group points along the o(pen) direction and coordinates the radical from its t(ight) side. As Figure (upper part) shows, hydrogen transfer from this conformation initially leads to an energetically less favourable THt-To isomer. Assuming that relaxation to a lower isomer is hindered by a broad barrier involving large amplitude motion of the radical scaffolds, it is very unlikely that this isomer exhibits a tunnelling-split OH stretching vibration, although the OH stretching excitation effectively lowers the residual HAT barrier quite substantially. The process is likely to be in the ns or slower regime and therefore remains undetectable at the moderate resolution of the IR spectra. Dynamically more interesting is the THo-To isomer, which we attribute to the weaker, more downshifted signal. Here, HAT leads to a degenerate To-THo isomer and does not involve much heavy atom motion. Furthermore, the barrier is now predicted to be substantially lower than for the dominant THo-Tt species (bottom part of Figure ). We note that the electronic HAT barrier is still around 60 kJ mol -1 , but it is lowered by zero point motion to slightly more than 45 kJ mol -1 in line with vibrational stabilisation in heavy-light-heavy systems. Previous barrier values using a different density functional and smaller basis set for the purpose of HAT modelling in solution (Tab. 1) were considerably higher , underscoring the importance of the present experimental investigation in the gas phase. In view of the expected barrier height and narrowness, it is thus conceivable for the THo-To isomer that tunnelling is fast enough after OH stretch excitation to show up as a spectra doublet. The frequency splitting ∆ν corresponds to the inverse of the tunnelling period. Based on the observed resolution and the absence of such a resolved doublet, the period must be longer than 15 ps, equivalent to 1500 OH oscillations. By lowering the barrier through suitable chemical substitution, it is conceivable that a tunnelling splitting may appear in the corresponding spectra of such derivatives.