Matrix reactions of alkaline earth metal atoms with oxygen molecules: Infrared spectra of the metal superoxide and metal dioxide species

Abstract

Infrared spectra of the reaction products of oxygen molecules and alkaline earth metal atoms in argon and nitrogen matrices have been recorded. The ν₂ interionic modes of barium, strontium, and calcium superoxides, MO₂, have been located at 541, 474, and 498 cm⁻¹, respectively, in nitrogen matrices. The metal oxide, MO, was isolated for each metal, and several bands of the metal oxide dimer were measured. Frequencies observed for the antisymmetric modes of (BaO)₂ at 487 and 393 cm⁻¹ indicate a D_2h structure with an O−Ba−O angle of approximately 102°. Also, a band attributable to ν₃ of a bent O−M−O metal dioxide species has been identified near 752 cm⁻¹ for barium, 730 cm⁻¹ for strontium, and 743 cm⁻¹ for calcium, leading to a bond angle near 140° for all three species. A mechanism is proposed for the formation of the metal oxide from molecular oxygen, which includes MO₂M and (MO)₂ intermediates. It was also determined that nitrogen is much more effective as a matrix material in stabilizing ionic compounds than argon.