EPR spectroscopic study of the reaction of Al and Ga atoms with CO2 and CS2 in a rotating cryostat

Abstract

Al and Ga atoms have been allowed to react with CO₂, ¹³CO₂, CS₂ and ¹³CS₂ in a rotating cryostat at 77 K. The paramagnetic products resulting from these reactions have been investigated by electron paramagnetic resonance (EPR) spectroscopy. The two main products from the reaction of Al and CO₂ are a mononuclear, monoligand complex, Al[CO₂], with a small Al and a large ¹³C coupling constant and a species, C, with unresolved Al and ¹³C coupling constants. There is also evidence for the formation of three minor mononuclear complexes, A′, A″ and B, with 〈a〉(Al) values of ca. 34, 105 and –71 MHz, respectively. The highly anisotropic nature of the a tensor observed for B (|a_∥(Al)|) < (|a⊥(Al)|) indicates that a substantial amount (ca. 78%) of the unpaired spin density resides in an Al 3p orbital. Ga atoms and CO₂ give two mononuclear complexes with unresolved ¹³C coupling constants and ⁶⁹Ga coupling constants of –282 and 12 MHz, respectively. Both Al and Ga react with CS₂ to give species with small metal and unresolved ¹³C coupling constants. Structures are proposed for these complexes, and in the case of Ga and CO₂, the spin densities obtained from the experimentally derived EPR parameters are compared with those obtained by a local spin density functional method.