text: Strong hydrogen bonds are revealed by downfield shifts in the H NMR spectrum. For example, the acidic proton in the enol tautomer of acetylacetone appears at  15.5, which is about 10 ppm downfield of a conventional alcohol.
  In the IR spectrum, hydrogen bonding shifts the  stretching frequency to lower energy (i.e. the vibration frequency decreases). This shift reflects a weakening of the  bond. Certain hydrogen bonds - improper hydrogen bonds - show a blue shift of the  stretching frequency and a decrease in the bond length. H-bonds can also be measured by IR vibrational mode shifts of the acceptor. The amide I mode of backbone carbonyls in α-helices shifts to lower frequencies when they form H-bonds with side-chain hydroxyl groups. The dynamics of hydrogen bond structures in water can be probed by this OH stretching vibration. In the hydrogen bonding network in protic organic ionic plastic crystals (POIPCs), which are a type of phase change material exhibiting solid-solid phase transitions prior to melting, variable-temperature infrared spectroscopy can reveal the temperature dependence of hydrogen bonds and the dynamics of both the anions and the cations. The sudden weakening of hydrogen bonds during the solid-solid phase transition seems to be coupled with the onset of orientational or rotational disorder of the ions.