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.