charge density analysis and dipole moment enhancement in Mna (2-methyl-4-nitroaniline)

Andrew Whitten,^a Mark Spackman,^a Peter Turner,^b Wim Klooster,^c Ross Piltz,^c and Masaru Tachibana^d

^aChemistry, School of Biological, Biomedical and Molecular Sciences, University of New England, Armidale, NSW 2351, Australia; ^bSchool of Chemistry, University of Sydney, NSW 2006, Australia; ^cBragg Institute, Building 58, ANSTO, PMB 1, Menai, NSW 2234, Australia; ^dDepartment of Physics, Yokohama City University, 22-2, Seto, Kanazawa-ku, Yokohama, 236-0027, Japan (awhitten@une.edu.au)

MNA belongs to a class of substituted benzene derivatives, which possess exceptional non-linear optical properties. Such compounds exhibit large microscopic second order non-linear susceptibilities, and lack a centre of symmetry in the solid state. Another interesting property of these compounds is the apparent enhancement of the dipole moment due to hydrogen bonding and general crystal-field effects. For MNA, ab initio calculations of the free molecule give values of the dipole moment of about 8.2 D,^[1] while in-crystal estimates obtained from a previous charge density analysis are around 25(8) D,^[2] suggesting a three-fold enhancement of this molecular property. This experimental result has been widely cited, but whether the magnitude of the dipole moment enhancement exhibited there is reasonable is of some debate because of difficulties encountered in the analysis of X-ray diffraction data for non-centrosymmetric space groups; the large experimental error associated with the value also raises questions about the ability of the multipole refinement procedure to reliably retrieve such values.

To independently verify this important observation, and hopefully to improve on the results obtained in the previous study, we are making use of experimental data that was unavailable a decade ago, including neutron diffraction data collected on the four-circle diffractometer (2TANA) at HIFAR, as well as charge density quality
X-ray diffraction data collected on the Bruker SMART 1000 CCD system at the University of Sydney, all at 100 K. We are also performing Hartree-Fock CRYSTAL98 calculations to aid in the analysis of the experimental data, and for comparison with experimental results. We will discuss the difficulty in obtaining sensible estimates of one-electron properties for non-centrosymmetric compounds from multipole refinements, and present in detail the results obtained from this study.

References

1 Daniel, C. and Dupuis, M. (1990) Chem. Phys. Lett. 171, 209.

2 Howard, S.T., Hursthouse, M.B., Lehmann, C.W., Mallinson, P.R., and Frampton, C.S. (1992) J. Chem. Phys 97, 5616.