Mössbauer studies related to the solid-state dynamics of C₆₀

C₆₀ (Buckminsterfullerene) is isolated from organic solvents above 260 K as an f.c.c. solid with complete orientational disorder. The rotational dynamics are diffuse and isotropic, with a short (12 ps) correlation time at 300 K. Below 260 K, the solid forms in a simple cubic lattice with Pa3 symmetry. The librational motion occurs by activated jumps with a correlation time of about 60 ns at 200 K. To gain further insight into the dynamics of C₆₀, the compound C₆₀Fe(CO)₄ (I) (generously made available to us by A. Stephens and M. Green of Oxford University) has been studied by ⁵⁷Fe Mössbauer spectroscopy. As expected, the resonance at 90 K consists of a doublet with an isomer shift (relative to metallic iron) of 0.034(1) mm s^-1 and a quadrupole splitting of 1.607(5) mm s^-1. There is no significant temperature-dependent intensity asymmetry of the doublet. The temperature dependence of the effect magnitude is well fit by a linear regression over the range 85 < T < 210 K. To elucidate the relationship of these results to the effect of the C₆₀ moiety in I, further lattice dynamical Mössbauer studies have been carried out on (maleic anhydride) Fe(CO)₄ (II), one of the few isolatable tetracarbonyl complexes of iron. For (II), the quadrupole splitting is 1.371(5) mm s^-1 at 90 K, and the temperature dependence of the isomer shift is smaller than it is for (I), while the temperature dependence of the recoil-free fraction is significantly larger in (I) than in (II).