DFT study of electronic properties, structure and spectra of aryl diazonium cations

We report here density functional theory calculations providing the electronic structure, IR absorption and Raman bands for a series of aryl diazonium cations, X-C6H4N2+, with different substituents (X = Cl, Br, F, NO2, NH2, CH3, CF3, CN, OH, OCH3, COOH, COOC2H5, etc.) in the ortho-, meta- and para-positions. The DFT study allows the complete assignment of IR absorption bands, with good agreement with the experimental data, for the few cases where such data are available. For some IR bands we discuss the correlations between the vibrational frequencies and intensities and the nature and force of the mesomeric effect of the substituent. We also analyze the influence of the substituent, to better understand any changes in the activity of the cation as a reagent in the Meerwein reaction. The study of the N-N vibration frequency of the radical and the analysis of the orbital structure showed that the SOMO has a bonding character along the C-N link, hindering nitrogen elimination. (c) 2009 Elsevier B.V. All rights reserved.