The structure of cinnamic acid and cinnamoyl azides, a unique localized p system: The electronic spectra and DFT-treatment

The electronic absorption spectra of cinnamic acid and some cinnamoyl azides have been recorded in absolute methanol and investigated to explore the structure of the titled compounds. Cinnamic acid and its derivatives have a double bond, -C=C-, between the aromatic ring and the carboxyl group which disturbs the p electron system of the molecule and inhibits electron delocalization as compared with styrene or benzoic acid. The azide group is neither a strong electron donor nor a strong electron acceptor but it increases conjugation in the molecule. The observed spectra confirm that each of the cinnamic acid and cinnamoyl azide molecules is one of a kind of unique disturbed p-system and not of different independent p systems, each on a fragment of the molecule as predicted by the quantum theory of atom in molecule calculations. The spectra of cinnamic acid and its derivatives are not the additive spectra of the different fragments of the molecule. The spectra are characterized by few number, low intensity, and high-energy electronic transitions (absorption bands) in the UV-vis region. Molecular orbital calculations confirmed the spectral observations. The optimized geometry of the ground state of the studied compounds is calculated using the DFT/B3LYP/6-31G** level of theory and an explicit molecular orbital analysis is carried out. Excited states are calculated using the TD/DFT procedure as implemented by the Gamess 2009 package of programs. The correspondence between calculated and the observed transition energies is adequate. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012