HOMO-LUMO energy splitting in polycyclic aromatic hydrocarbons and their derivatives

The HOMO-LUMO gap of polycyclic aromatic hydrocarbons (PAHs) with and without functional group substitutions was computed using the B3LYP methods with the 6-311 + + G(d,p) basis set. The functional groups considered are ethyl (C2H5), phenyl (C6H5), hydroxyl (OH), vinyl (C2H3), carboxyl (COOH) and formyl (CHO). The purpose was to explore the impact of substitution on the electronic structure of PAHs and provide a rational explanation for the optical band gap values measured for nascent soot particles formed in flames. Results suggest a high sensitivity of the HOMO-LUMO gap on the type and number of functional groups in a PAH molecule. It was found that single -OH, -COOH and -CHO groups on PAHs can reduce the HOMO-LUMO gap by 0.2, 0.4 and 0.5 eV, respectively. Further functionalization of PAH by several CHO groups could reduce the HOMO-LUMO gap by as much as 0.7 eV. It is concluded that the impact of functional groups in PAHs cannot be neglected while interpreting the optical band gap measured for nascent soot particles. Published by Elsevier Inc. on behalf of The Combustion Institute.