Matrix ESR spectra of polyatomic alkali metal clusters

ESR spectra have been obtained for polyatomic sodium and potassium clusters in argon and nitrogen matrices. The spectra, tentatively assigned to alkali septemer molecules, show a large isotropic hf (a1) from two equivalent alkali nuclei plus a much smaller splitting (a2) from an additional (at least) five equivalent nuclei. For an argon matrix, the measured hf constants and g values are a1=118.16(4) G, a2=6.66(12), g0=1.9994 (1) and a1=30.55(6) G, a2=1.79(2) G, g0=2.0017(1) for Na7 and K7, respectively. The ESR spin populations, ρi=ai/a(atom), are compared with the orbital symmetries predicted by the simple Huckel approximation for a range of M7, M9, M11, and M13 structures. Aside from the relatively unstable ‘‘T-shaped’’ arrangement, only a few three-dimensional structures give a satisfactory agreement with the ESR spectra. For M7, a likely geometry is the pentagonal bipyramid. The ESR spin populations for Na7 are almost identical in magnitude to those of K7. If a2<0, the total isotropic spin population is Σiρi≂0.64, which implies an ∼ 36% p character for the unpaired electron.