First O−H−N Hydrogen Bond with a Centered Proton Obtained by Thermally Induced Proton Migration

Within a range of 0.1 Å, the H atom in the O-H-N hydrogen bond of the adduct 4-methylpyridine⋅pentachlorophenol could be shifted by a simple adjustment of temperature. At approximately 90 K the H atom is exactly centered between the O and the N atoms, as could be shown by stepwise monitoring by using variable-temperature single-crystal neutron diffraction.