X-RAY-DIFFRACTION STUDY OF THE CORRELATION BETWEEN ELECTROSTATIC POTENTIAL AND K-ABSORPTION EDGE ENERGY IN A BIS(MU-OXO) MN(III)-MN(IV) DIMER

The mixed-valence compound u-oxo)tetrakis(2,2'-bipyridine)dimanganese( III,IV) is a small molecule model compound for the oxygen-evolving cen ter, an enzyme which converts water into dioxygen in photosystem II in the photosynthesis of green plants and algae. Crystallographic data: M(r) 1077.47 g/mol, monoclinic, P2(1)/c, Z = 4, a = 13.584(2) Angstrom , b = 14.058(4) Angstrom, c = 23.622(5) Angstrom, beta = 105.25(2)degr ees, V = 4352(4) Angstrom(3) at T = 9 K. A quantitative estimate for t he valence contrast, i.e., the difference in electrostatic potential o f the two Mn atoms in this compound, has been established by two diffe rent experimental methods which both involve single-crystal X-ray diff raction. A resonant synchrotron X-ray diffraction experiment examining the Bragg scattering for energies of the incoming beam close to the M n absorption edge at 6539 keV (similar to 1.9 Angstrom wavelength) sho ws a 3.7 eV difference in ionization energies of the first K-shell ele ctron for Mn(IV) and Mn(III), respectively. The Mn(TV) edge has the hi gher energy. Low-temperature (9 K) X-ray diffraction data allowed a mu ltipolar refinement of the electron density distribution, The most rem arkable feature of the deformation density maps is a large accumulatio n of density at the mu-oxygen atoms inside the planar 4-membered Mn(II I)-O-Mn(IV)-O ring. From the electron density, the electrostatic poten tial at each of the manganese nuclei has been calculated, first using a direct space summation and, second, a Fourier summation combined wit h an atomic cluster calculation. When corrected for the energy term, w hich originates from relaxation of the remaining electrons upon remova l of one electron, the two calculations give estimates of valence shif ts of the ionization energy of 4.0 and 2.9 eV, respectively. These val ues closely bracket the value of 3.7 eV obtained in the resonant synch rotron X-ray diffraction experiment, and they are in good accordance w ith the result, 3,2 eV, of an all-electron ab-initio model calculation . It is to the authors' knowledge the first attempt to compare values of shifts in core-ionization energies as found from the two different types of diffraction experiments.