ELECTRON-SPIN-RESONANCE INVESTIGATION OF SMALL MAGNESIUM CLUSTER CATION RADICALS, MG-N(- COMPARISON WITH AB-INITIO CALCULATIONS(), IN NEON AND ARGON MATRICES AT 4 K )

A series of cationic magnesium radicals (Mg-N(+), N=1-6) was studied b y electron spin resonance (ESR) spectroscopy. The ESR data indicate th at all Mg nuclei are equivalent on the ESR time scale in each of these clusters. The nuclear hyperfine interactions obtained for Mg-25(N)+(N =1-3) in neon and argon matrices from ESR measurements were compared w ith ab initio calculations. The minimum energy structures for Mg-2(+) and Mg-3(+) and the transition-state structure for Mg-3(+) were optimi zed using complete active space self-consistent field (CASSCF) wave fu nctions. The minimum energy structure for Mg-3(+) was determined to be linear and the barrier for the pseudorotation between equivalent mini ma was calculated. Good agreement between theory and experiment is obt ained if averaging between the center and end position of Mg: is assum ed to occur in the ESR experiment in spite of the 6 kcal/mol calculate d energy barrier.