ESR of Mn2 and Mn5 molecules in rare-gas matricesa)

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Abstract
Mn2 and Mn5 have been isolated in argon, krypton, and xenon matrices and their X-band ESR spectra observed at 4 and up to 70 K, depending upon the matrix. As predicted by Nesbet, the lowest state of Mn2 is 1Σ, and the two atoms are exchange coupled (antiferromagnetically). The temperature behavior of the ESR bands in the higher spin states (S=1,2,3) was approximately in accord with a Lande interval rule, and a value of J=−9±3 cm−1 was obtained from the S=2 intensity variations. Each fine structure line appears with a superimposed 11-line hyperfine pattern with splitting one-half that of isolated 55Mn atoms (30 G). The anisotropic exchange interactions fit the Judd–Owen relationship with De=−0.043(2) and Dc=−0.001(4) cm−1. Assuming De arises solely from magnetic dipole interaction, the interatomic distance in Mn2 is calculated to be 3.4 Å. Mn5 appears in more concentrated matrices as a highly oriented axial molecule with its axis perpendicular to the flat sapphire substrate surface. The observed fine structure indicates that the molecule contains 25 unpaired electrons (S=25/2) and has magnetic parameters g⊥=1.997(1), g∥=1.979(2), D=−0.013(1) cm−1. All 55Mn hyperfine structure lies within the linewidths and is therefore unresolved. It was concluded that the molecule most probably contains five Mn atoms and is a plane pentagon. With this model an attempt was made to rationalize the observed g shifts.