The molecular dipole moment of the non-linear optical 3-methyl 4-nitropyridine N-oxide molecule: X-ray diffraction and semi-empirical studies

Abstract

The 3-methyl 4-nitropyridine N-oxide molecule (POM) is characterised by its weak dipole moment. The low polarity of this molecule results mainly from the electronic competition of the two electron-accepting groups, N-oxide and nitro. An accurate electron density distribution and the electrostatic potential around the molecule have been calculated from a high-resolution X-ray diffraction study. The data were collected at 123 K using graphite-monochromated Mo-Kα radiation to sin(θ)/λ= 1.24 Å^–1. Crystal data: C₆H₆O₃N₂, orthorhombic, space group P2₁2₁2₁, a= 20.890, b= 6.094, c= 5.123 Å, µ= 0.2 mm^–1, λ= 0.71 Å. The integrated intensities of 8000 reflections were measured and reduced to 2296 independent reflections with I 3σ(I). The experimental dipole moment is in a reasonable agreement with the semi-empirical calculations of the isolated molecule. The calculated net charges, the dipole moment and the electrostatic potential around the molecule deduced from this study reveal the nature of the intramolecular charge transfer due to the electron-accepting nitro and N-oxide groups.