Formation of CO(2)(-) Radical Anions from CO(2) 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, CO(2)(-), by reaction Of CO(2) 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 CO(2) with oxide anions at low-coordinated sites and formation of surface carbonates, CO(3)(2-); (2) interaction of CO(2) with ``free{'' electrons trapped at low-coordinated sites of the MgO surface with fort-nation of adsorbed CO(2)(-); and (3) interaction of CO(2) with (H(+))(e(-)) centers and formation of CO(2)(-) species bound near an adsorbed proton (OH group). CO(2) 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 CO(2)(-). Several adsorption sites have been considered, but based on the comparison of measured and computed hyperfine ((13)C and (17)O) and superhyperfine ((1)H) coupling constants we conclude that CO(2)(-) bound at (H(+))(e(-)) centers formed near cationic corners is the most abundant species observed in the experiment.}