Chromium metal, natural and isotopically enriched in53Cr (I=3/2), was vaporized and trapped in solid neon, argon, and krypton at 4 K. An extensive ESR spectrum of 16 lines between 200 and 7000 G in neon and argon matrices was found to be due to an axial molecule withS=3, Verbar;DVerbar;=0.110 cmminus;1,gperp;=1.9970,gpar;=1.9996.53Cr spectra showed essentially isotropic hyperfine interaction of the six unpaired spins with apparently only one nucleus and an accompanying electric field gradient at that nucleus. It is concluded that the molecule is a trigonally distorted tetrahedral Cr4or possibly a squarehyphen;based pyramidal Cr5. The unique apical atom is bonded relatively weakly to the lsquo;lsquo;basersquo;rsquo; and is the locus of almost all of thescharacter among the unpaired spins. In argon and krypton matrices, another species withS=3 is observed nearg=2.0 but with only a small zerohyphen;field splitting (Verbar;DVerbar;=0.0049 cmminus;1,gpar;=2.005,gperp;=2.0015 in argon). The possibilities of it being a weakly perturbed Cr atom in an asymmetrical matrix site or a Crnmolecule, wheren(ge;3) is the number of equivalent atoms, are discussed.
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