Nitrogen dioxide NO2 is a polar molecule with an odd number of valence electrons, which results in a free radical species. The electronic structure and bonding model of NO2 can be described using the molecular orbital MO theory, which provides insight into its molecular orbital diagram and spectroscopic properties.Electronic Structure and Bonding Model:The nitrogen atom has 5 valence electrons, and each oxygen atom has 6 valence electrons. In total, NO2 has 17 valence electrons. The central nitrogen atom is sp2 hybridized, forming three sp2 hybrid orbitals. Two of these hybrid orbitals form sigma bonds with the oxygen atoms, while the third hybrid orbital contains a lone pair of electrons.The remaining unpaired electron is located in a non-hybridized p-orbital on the nitrogen atom, which is perpendicular to the plane of the molecule. This unpaired electron is responsible for the free radical nature of NO2.Molecular Orbital Diagram:The molecular orbital diagram of NO2 can be constructed by combining the atomic orbitals of nitrogen and oxygen atoms. The sigma bonds between nitrogen and oxygen are formed by the overlap of sp2 hybrid orbitals from nitrogen and the p-orbitals from oxygen. The pi bonds are formed by the overlap of the remaining p-orbitals on nitrogen and oxygen atoms.In the molecular orbital diagram, the bonding molecular orbitals are lower in energy than the atomic orbitals they originated from, while the antibonding molecular orbitals are higher in energy. The 17 valence electrons fill the molecular orbitals in the order of increasing energy, resulting in one unpaired electron in the highest occupied molecular orbital HOMO , which is a * pi antibonding orbital.Spectroscopic Properties:The spectroscopic properties of NO2 are influenced by its electronic structure and molecular orbital diagram. The presence of an unpaired electron in the * orbital results in a strong absorption in the UV-Vis region, giving NO2 its characteristic reddish-brown color.NO2 also exhibits a characteristic electron paramagnetic resonance EPR spectrum due to the unpaired electron, which provides information about the electronic structure and the environment of the unpaired electron.Infrared IR spectroscopy can be used to study the vibrational modes of NO2. As a polar molecule, NO2 exhibits strong IR absorption bands corresponding to its asymmetric and symmetric stretching and bending vibrations.In summary, the electronic structure and bonding model of NO2 involve sp2 hybridization, sigma and pi bonding, and an unpaired electron in a * orbital. The molecular orbital diagram reflects this electronic structure, and the spectroscopic properties of NO2, such as UV-Vis, EPR, and IR spectra, provide further insight into its bonding and molecular structure.