Electron Density Distributions of Intermolecular N...O Contacts in Crystals of Biphenyl-3,5-diyl bis(t-butyl nitroxide)

Department of Applied Physics and Chemistry, The University of Electro-Communications
○Masanori Yasui Shinichiro Mashiyama Kazuhiro Ozawa Hiroaki Hosoya Hirokazu Nishimaki Gentaro Kurokawa Takayuki Ishida Takashi Nogami

The title compound gave two crystalline phases (a- and b-phase). Though this biradical molecule has a triplet ground state, the a-phase crystals exhibit unusual diamagnetic properties. The very large intermolecular antiferromagnetic interactions between N-O radical moieties are ascribed to the diamagnetic properties.(1) In order to study these strong intermolecular interactions, the experimental electron density distributions were examined using the atoms-in-molecule method. The multipole refinements were successfully completed using the program XD to give R value of 0.034 for observed 10249 reflections up to 2q=100°. Both N-O radical moieties have short intermolecular contacts with those of another molecules related by the center of symmetry to form infinite chains in the crystals. The N...O distances are 2.3051(8) and 2.3418(8) Å. The topological analysis of the electron densities gave bond critical points and bond paths between the N...O contacts. The total energy densities at the BCPs (Hcp) are -6 and -4 kJ/mol. These values mean that the intermolecular N...O interactions are "covalent". This observation is consistent with the strong intermolecular antiferromagnetic interactions.
The crystals of another polymorph (b-phase) have no intermolecular N...O interactions and exhibits paramagnetic properties.(2) Charge density studies of b-phase crystals were also carried out. Theoretical studies of electron densities for a-phase structure are now in progress. The comparison of the experimental and theoretical electron densities between two phases will be presented.
(1) Kurokawa, G., et al. (2004) Chem. Phys. Lett. 392, 74. (2) Nishimaki, H., et al. (2006) Chem. Mater. To be published.