Formation of CO2- Radical Anions from CO2 Adsorption on an Electron-Rich MgO Surface: A Combined ab Initio and Pulse EPR Study

We report the results of DFT cluster model calculations on the formation of carboxylate radical anions, CO2-, by reaction Of CO2 with an electron rich MgO surface. The theoretical results are used in conjunction with new pulse electron paramagnetic resonance (EPR) experiments to interpret recently reported experimental data obtained with continuous wave EPR (CW-EPR) (Chiesa, M.; Giamello, E. Chem. Eur. J. 2007, 13, 1261). Three cases have been considered: (1) interaction Of CO2 with oxide anions at low-coordinated sites and formation of surface carbonates, CO32-; (2) interaction of CO2 with "free" electrons trapped at low-coordinated sites of the MgO surface with fort-nation of adsorbed CO2-; and (3) interaction of CO2 with (H+)(e(-)) centers and formation of CO2- species bound near an adsorbed proton (OH group). CO2 prefers to form surface carbonates and only once most or all the low-coordinated O sites of the surface have reacted it will interact with trapped electrons to form CO2-. Several adsorption sites have been considered, but based on the comparison of measured and computed hyperfine (C-13 and O-17) and superhyperfine (H-1) coupling constants we conclude that CO2- bound at (H+)(e(-)) centers formed near cationic corners is the most abundant species observed in the experiment.