Specifically, we have shown that TEMPO-H forms a hydrogenbonded homodimer engaging nitrogen as the acceptor site, in contrast to the solid state, where oxygen is the predominant acceptor due to cooperative effects. We have also shown that there are two conformations of the complex of TEMPO-H with TEMPO, both engaging oxygen as the acceptor. The slightly less stable one based on experimental intensities leads to an equivalent structure after hydrogen atom transfer between the two oxygen atoms with rather little distortion of the organic scaffold. This should favour tunnelling, in particular when the complex is excited along the hydrogen transfer coordinate by OH stretching motion. Still, the complex can empirically undergo far more than 1000 vibrations before the hydrogen is transferred. The present results suggest that further lowering of the self-exchange barrier for hydrogen between two nitroxyl radicals may lead to the direct spectral manifestation of hydrogen atom tunnelling by a splitting of the OH stretching fundamental transition. This might be achieved by replacing the alkyl substitution present in TEMPO with aryl substitution. Phenoxy radicals may also lead to lower barriers 9 due to the aromatic conjugation. Self-exchange complexes based on phenoxy radicals are sterically less hindered and have already been characterised in the solid state , which is more challenging to model accurately by theory. Our gas phase approach promises to lift the contact between theory and experiment in the field of degenerate hydrogen atom transfer complexes to a higher level of accuracy.