H0₂; H₂0⁺; H₃0⁺; HOOH; OH; OH⁺

Several radicals, ions, and neutral molecules whose chemistry is related to that of water have been found in the interstellar medium. These include water itself, hydrogen peroxide (HOOH), H₂O⁺, HO₂, OH, OH⁺, and H₃O⁺. For many of these species, the pure rotational transitions are in the terahertz frequency range, so that their detections have followed the recent availability of high-altitude or space-based telescopes such as APEX and Herschel. Details of such recent observations may be found in the links from the table of interstellar and circumstellar molecules in the entry Molecules in Space. In contrast, the hydroxyl radical(OH) was the very first molecular species detected by radio astronomers, observing at wavelengths around 18 cm the lambda-doublet transitions which arise from interaction between the molecular rotation and the motion of the unpaired electron. These lines have traditionally been used to estimate the water...