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Abstract

Trimethylphenylammonium superoxide (1) and tetrabutylammonium
superoxide (2) were prepared by ion-exchange reaction in liquid
ammonia. Both compounds were structurally characterized by
single-crystal X-ray diffraction. The crystal structure of 2 contains
solvent ammonia molecules that are hydrogen bonded to the superoxide
ion and therefore may influence the bonding properties of the
superoxide ion. The crystal structure of 1 does not contain any solvent
molecules. Therefore, it represents the best known approximation to the
virtually isolated superoxide ion in the solid state to date. The O-O
bond length is 1.332(2) Angstrom in 1 and 1.312(2) Angstrom in 2.
Magnetization measurements show that the susceptibilities of both
compounds follow an ideal Curie law down to 2 K reflecting an absence
of intermolecular exchange effects between the superoxide ions. The
effective magnetic moments of both compounds are larger than the
spin-only value due to contributions of the orbital momentum in the
superoxide ion. The values of the magnetic moment comply well with the
g factors obtained from electron paramagnetic resonance spectra. The g
tensors themselves reflect the anisotropic environment of the
superoxide ions. The Pi(g) energy levels which are degenerate in the
free superoxide ion split up in crystal fields of lower than tetragonal
symmetry. The energy splitting is estimated from the diagonal elements
of the g tensor of 1.