Hoppe became famous through his synthesis of the stable noble gas compound XeF (xenon difluoride), reported in November 1962.  His work followed the previous synthesis of by xenon hexafluoroplatinate by Neil Bartlett, in an experiment run on March 23, 1962 and reported in June of that year.  Until then, everyone had assumed that compounds of such kind would not exist, the reason being, first, unsuccessful experiments attempting to synthesize such noble gas compounds and, second, the concept of the "closed octet of electrons", according to which noble gases would not participate in chemical reactions.
Through the properties of the interhalogen compounds it had become obvious that noble gas fluorides were the only accessible ones. Since 1949/50, a research group in Münster had carried out in-depth discussions on the possibility of the formation and the properties of xenon fluorides. This research group was convinced, already in 1951, that XeF and XeF should be thermodynamically stable against the decomposition into the elements.
For a long time it was planned to occasionally perform synthetic experiments targeted at the xenon fluorides. Technical and conceptional difficulties, however, interfered in Münster. On the one hand, xenon was not accessible in sufficient purity; on the other hand, the researchers believed that only pressure syntheses would be successful, for which steel bottles with compressed F were needed. Since 1961, those F-pressure cylinders had been promised by American friends but the transfer could not take place until 1963 because the valves of non-standard U.S. pressure cylinders were not allowed in Germany and vice versa.
Nevertheless, Hoppe’s research group was able to generate XeF in the form of transparent crystals in early 1962. To do so, they let electric sparks impact on xenon-fluorine mixtures. Neil Bartlett tried a similar experiment for the first time in the USA on August 2, 1962. After a few days, he gained xenon tetrafluoride, XeF.