A stable argon compound

The noble gases have a particularly stable electronic configuration, compri sing fully filled s and p valence orbitals. This makes these elements relat ively non-reactive, and they exist at room temperature as monatomic gases. Pauling predicted(1) in 1933 that the heavier noble gases, whose valence el ectrons are screened by core electrons and thus less strongly bound, could form stable molecules. This prediction was verified in 1962 by the preparat ion of xenon hexafluoroplatinate, XePtF6, the first compound to contain a n oble-gas atom(2,3). Since then, a range of different compounds containing r adon, xenon and krypton have been theoretically anticipated and prepared(4- 8). Although the lighter noble gases neon, helium and argon are also expect ed to be reactive under suitable conditions(9,10), they remain the last thr ee long-lived elements of the periodic table for which no stable compound i s known. Here we report that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which w e have identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy. Extensive ab initio calculations indicate that HArF is intrinsically stable, owing to significa nt ionic and covalent contributions to its bonding, thus confirming computa tional predictions(11-13) that argon should form a stable hydride species w ith properties similar to those of the analogous xenon and krypton compound s reported before(14-18).