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

A series of cationic magnesium radicals (Mg-N(+), N=1-6) was studied by electron spin resonance (ESR) spectroscopy. The ESR data indicate that 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 with ab initio calculations. The minimum energy structures for Mg-2(+) and Mg-3(+) and the transition-state structure for Mg-3(+) were optimized using complete active space self-consistent field (CASSCF) wave functions. The minimum energy structure for Mg-3(+) was determined to be linear and the barrier for the pseudorotation between equivalent minima was calculated. Good agreement between theory and experiment is obtained if averaging between the center and end position of Mg: is assumed to occur in the ESR experiment in spite of the 6 kcal/mol calculated energy barrier.