THE MOLECULAR DIPOLE-MOMENT OF THE NONLINEAR-OPTICAL 3-METHYL 4-NITROPYRIDINE N-OXIDE MOLECULE - X-RAY-DIFFRACTION AND SEMIEMPIRICAL STUDIES

The 3-methyl 4-nitropyridine N-oxide molecule (POM) is characterised b y its weak dipole moment. The low polarity of this molecule results ma inly from the electronic competition of the two electron-accepting gro ups, N-oxide and nitro. An accurate electron density distribution and the electrostatic potential around the molecule have been calculated f rom a high-resolution X-ray diffraction study. The data were collected at 123 K using graphite-monochromated Mo-K alpha radiation to sin(the ta)/lambda = 1.24 Angstrom(-1). Crystal data: C6H6O3N2, orthorhombic, space group P2(1)2(1)2(1), a = 20.890, b = 6.094, c = 5.123 Angstrom, mu = 0.2 mm(-1), lambda = 0.71 Angstrom. The integrated intensities of 8000 reflections were measured and reduced to 2296 independent reflec tions with I greater than or equal to 3 sigma(I). The experimental dip ole moment is in a reasonable agreement with the semi-empirical calcul ations of the isolated molecule. The calculated net charges, the dipol e moment and the electrostatic potential around the molecule deduced f rom this study reveal the nature of the intramolecular charge transfer due to the electron-accepting nitro and N-oxide groups.