PNNL

Abstract

Bond critical point, bcp, and local energy density properties for the electron density, ED, distributions, calculated with first principle quantum mechanical methods for divalent transition metal Mn-, Co- and Fe-containing silicates and oxides are compared with experimental model ED properties for tephroite, Mn2SiO4, fayalite, Fe2SiO4 and Co2SiO4 olivine, each determined with high energy synchrotron single crystal X-ray diffraction data. Trends observed between the experimental bond lengths, R(M-O), (M = Mn, Fe, Co), and the calculated bcp properties are comparable with those observed for non-transition M-O bonded interactions. The bcp, local total energy density, H(rc), and bond length trends determined for the Mn-O, Co-O and Fe-O interactions are virtually identical. A comparison is also made with model experimental bcp properties determined for several Mn-O, Fe-O and Co-O bonded interactions for organometallic complexes and several oxides. Despite the complexities of the structures of the organometallic complexes, the agreement between the calculated and the model experimental bcp properties is good in several cases. The G(rc)/p(rc) vs. R(M-O) trends established for non-transition metal M-O bonded interactions hold for the given transition metal M O bonded interactions with the G(rc)/p(rc) ratio increasing in value as H(rc) becomes progressively more negative in value and the shared character of the interaction increases. As observed for the non-transition metal M-O bonded interactions, the Laplacian, ?2p(rc), increases in value as p(rc) increases and as H(rc) decreases. The Mn-O, Fe-O, and Co-O bonded interactions are indicated to be of intermediate character with a substantial component of closed-shell character compared with Fe-S and Ni-S bonded interactions which show greater shared character based on the |V(rc)|/G(rc) bond character indicator. The atomic charges conferred on the transition metal atoms for the three olivines decrease with increasing atomic number from Mn to Fe to Co.