ELECTRON-DENSITY DISTRIBUTION IN LIB3O5 AT 293 K

The electron density distribution in lithium triborate LiB3O5 has been studied at room temperature by X-ray diffraction using Ag K alpha rad iation up to 1.02 Angstrom(-1) [1439 unique reflections with I > 3 sig ma(I)]. Conventional refinements with a free-atom model yield R(F) = 0 .0223, wR(F) = 0.0299, S = 1.632. Atom charge refinements show that th e lithium should be considered a monovalent ion. Multipolar refinement s were undertaken up to fourth order, imposing local non-crystallograp hic symmetry constraints in order to avoid phase problems leading to m eaningless multipole populations due to the non-centrosymmetry of the structure (space group: Pna2(1)). The residual indices decreased to: R (F) = 0.0147, wR(F) = 0.0193, S = 1.106. The net charges are in good a greement with what can be expected in berate chemistry. Deformation de nsity maps are analysed in terms of sigma and pi bonding. The experime ntal electron distribution in the p(z) orbitals of triangular B atoms and surrounding O atoms has been analysed by introducing idealized hyb ridized states. In parallel, the electron density has been determined from ab initio Hartree-Fock calculations on fragments of the structure . Agreement with the X-ray determination is very good and confirms the nature of bonding in the crystal. The amount of transfer of pi electr ons from the oxygen to the triangular B atoms is estimated to be 0.22 electrons by theory.