Visualization of intermolecular interaction by electron density mapping
using synchrotron radiation
Hiroyuki Ohsumi, Fumiko Yoshida, Nobuhiro Mizuno, Kennichi Kato, Masaki Takata
RIKEN Harima Institute, SPring-8 Center, Japan

X-ray diffraction data allows us to determine not only an atomic arrangement
but also charge density distribution in materials. Low divergent and highly brilliant
synchrotron radiation of SPring-8 provides an accurate diffraction data, which has
considerably improved the precision of structural materials science. Our developed
MEM/Rietveld method[1], which is an analytical method of charge density distribution,
has been successfully applied to the structure analysis of organic conductors[2],ferroelectric materials[3], manganites[4] and so on. The obtained MEM charge density
enabled us to observe valence state, orbital occupation and bonding nature of atoms andmolecules. Recently, we have succeeded in developing elelectrostatic potential andelectric field imaging based on MEM charge density analysis[5, 6]. Our new method isalso conducive to visualizing the association between atomic arrangement and materialsproperties. In the presentation, I will give a talk about following two topics.
1. Direct observation of charge transfer in α-(BEDT-TTF)2I3
F. Yoshida, K. Kato(Structural Materials Science Lab.); R. Koto (Condensed Molecular Material Lab.)A molecular conductor α-(BEDT-TTF)2I3 has peculiar physical property, forexample, the metal-insulator transition at 135 K, photo-induced transition, andsuperconductivity under uniaxial pressure. However, the origin of the metal-insulatortransition has not yet been revealed. Recent research proposed the charge ordering may
be occurred in the insulator phase[7]. To ascertain this model, we conducted the charge
density level structure analysis on α-(BEDT-TTF)2I3 using MEM/Rietveld method.
We have obtained the reliable powder-diffraction data at BL02B2, Spring-8.
Results of the MEM/Rietveld analyses yielded the electrostatic potential and the electricfield images of α-(BEDT-TTF)2I. There were apparent differences in the electrostatic
potential at I3 molecular position between high temperature conducting phase and low
temperature insulating phase. Temperature variation of the electric field was also seenaround the boundary between conductor layer (ET molecular region) and non-conductorlayer (I3 molecular region). These suggested that the charge transfer from I3 molecules
to ET molecules occurs at the metal-insulator transition of 135 K. This finding revealed
the charge doping to the conductor layer is realized in this material.

2. Charge density distribution study on high-molecular-weight organic molecules
N. Mizuno(Structural Materials Science Lab.); Z. Hou(Organometallic Chemistry Lab.)

The application of MEM charge density mapping to protein molecule is ournext target. We are now striving to realize a research on charge transfer and related
function in protein molecules. Since the MEM charge density mapping requires
accurate diffraction data, the reliability of single crystal diffraction data should bevalidated with close attention. To analyze current condition, we have performed precise
structure analyses of two high-molecular-weight organic molecules, the rare earth metal
hydride cluster (MW:1500) and the supramolecular porphyrin box (MW:1700).
We have succeeded in visualizing the hydride ligands in the metal hydridecluster. The high quality data, in which standard deviation of integral intensity is lessthan several %, made it possible to apply the MEM charge density mapping tohigh-molecular-weight organic molecules. On the other hand, we have attained aremarkable improvement of the structural model of the porphyrin box by means of theMEM charge density mapping. It is noteworthy that the structural model of solvent area
has been successfully determined. These accomplishment motivate further studies on
relatively large molecules such as Rubredoxin (MW:5700) and Cytochrmoe c3(MW:11500).
[1] M. Takata, et al., Z. Kristallogr. 216, 71 (2001).
[2] S. Aoyagi, et al., Angew. Chem. Int. Ed. 43, 3670 (2004).
[3] Y. Kuroiwa, et al, Phys. Rev. Lett. 87, 217601 (2001).
[4] K. Kato, et al, J. Phys. Soc. Jpn. 74, 2137 (2005).
[5] H. Tanaka, et al., Phys. Rev. B74 (2006) 172105.
[6] K. Kato, et al, Phys. Rev. B 77, 081101 (2008).
[7] H. Seo, J. Phys. Soc. Jpn. 69, 805 (2000).