RESUMO
The formation of environmentally persistent free radicals (EPFRs) is usually related to transition-metal oxides in particulate matter (PM). However, recent studies suggest that alkaline-earth-metal oxides (AEMOs) in PM also influence EPFRs formation, but the exact mechanism remains unclear. Here, density functional theory calculations were performed to investigate the formation mechanism of EPFRs by C6H5OH on AEMO (MgO, CaO, and BaO) surfaces and compare it with that on transition-metal oxide (ZnO and CuO) surfaces. Results indicate that EPFRs can be rapidly formed on AEMOs by dissociative adsorption of C6H5OH, accompanied by electrons transfer. As the alkalinity of AEMOs increases, both adsorption energy and the number of electron transfers gradually increase. Also, the stability of the formed EPFRs is mainly attributed to the electrostatic and van der Waals interactions between the phenoxy radical and surfaces. Notably, the formation mechanism of EPFRs on AEMOs is similar to that on ZnO but differs from that on CuO, as suggested through geometric structure and charge distribution analyses. This study not only elucidates the formation mechanisms of EPFRs on AEMOs but also provides theoretical insights into addressing EPFRs pollution.
RESUMO
Tetranuclear Cp4M4(CO)4 clusters have been synthesized for iron and vanadium but not for the intermediate first-row transition metals manganese and chromium. All of the low-energy structures of these "missing" Cp4M4(CO)4 (M = Mn, Cr) species are shown by density functional theory to consist of a central M4 tetrahedron with each of the four faces capped by a µ3-CO group. The individual low-energy structures differ in their spin states and in their formal metal-metal bond orders along the six edges of their central M4 tetrahedra. The two low-energy Cp4Mn4(µ3-CO)4 structures are a triplet structure with all Mn-Mn single bonds and a singlet structure with one Mn≡Mn triple bond and five Mn-Mn single bonds along the six tetrahedral edges. Related low-energy Cp4Cr4(µ3-CO)4 structures include a quintet structure with all Cr-Cr single bonds and a singlet structure with two Cr≡Cr triple bonds and four Cr-Cr single bonds. However, the potential energy surface of the Cp4Cr4(CO)4 system is complicated by three other structures of comparable energies including two triplet structures and one quintet structure with various combinations of single, double, and triple chromium-chromium bonds in the central Cr4 tetrahedron.