Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

The sterically hindered nitroxyl radical TEMPO is co-expanded with its hydroxylamine TEMPO-H in a supersonic jet and probed by FTIR spectroscopy. One major and one minor conformation of the 1:1 complex are identified by their OH stretching signatures, the major one exhibiting a weaker hydrogen bond. The acidic hydrogen atom in these structures can switch between the two TEMPO units in a more or less symmetric double minimum potential with a high barrier. Both conformations are experimentally shown to have a self-exchange quantum tunnelling period longer than 15 ps or 1500 OH vibrational periods even when excited by 41 kJ/mol along the OH stretching coordinate. The homodimer and more tentatively the monohydrate of TEMPO-H are also identified in the spectrum.