Knowledge Bank

Pure rotational spectra of the HOOO and DOOO radicals have been observed in a supersonic jet by using a Fourier transform microwave (FTMW) and FTMW-millimeter wave double resonance spectroscopy with a pulsed discharge nozzle. From a total of 12 observed rotational transitions with fine and hyperfine splittings, precise molecular constants have been determined. The experimentally determined molecular structure is $trans$ planar, which was not expected before the present experimental detection, since the large number of $ab$ $initio$ calculations so far reported have predicted its structure to be $cis$ planar. The determined bond length between the HO and O$2$ moieties, HO-OO, is fairly long, 1.688 \AA, and the molecular structure of HOOO is similar to that of an isoelectronic radical, FOO, which also has a fairly long FO bond.