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) fromtwoequivalent alkali nuclei plus a much smaller splitting (a2) from an additional (at least)fiveequivalent nuclei. For an argon matrix, the measured hf constants andgvalues area1=118.16(4) G,a2=6.66(12),g0=1.9994 (1) anda1=30.55(6) G,a2=1.79(2) G,g0=2.0017(1) for Na7and K7, respectively. The ESR spin populations, rgr;i=ai/a(atom), are compared with the orbital symmetries predicted by the simple Huckel approximation for a range ofM7,M9,M11, andM13structures. Aside from the relatively unstable lsquo;lsquo;Thyphen;shapedrsquo;rsquo; arrangement, only afewthreehyphen;dimensional structures give a satisfactory agreement with the ESR spectra. ForM7, a likely geometry is the pentagonal bipyramid. The ESR spin populations for Na7are almost identical in magnitude to those ofK7. Ifa20, the totalisotropicspin population is Sgr;irgr;ibartil;0.64, which implies an sim; 36percnt;pcharacter for the unpaired electron.
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