Proton affinities of polybenzenoid aromatic hydrocarbons and those with five-membered rings

Proton affinities of PAHs including one five-membered ring are calculated by using the AM1 Hamiltonian for the determination of ΔH_f^o of the neutral and protonated molecules. The calculated PAs are compared to experimental PAs measured by chemical ionization mass spectrometry, using a new method based on competition between charge transfer and proton transfer occurring during the ionization process. A procedure is proposed to validate AM1-calculated PAs from experimental PAs after rescaling the calculated and measured PA values. The site of protonation is first determined on the criterion of the lowest loss of aromaticity, then on the criterion of the largest HOMO coefficient. For indene, the corrected result is compared to an ab initio calculation at the MP2/6-31G^*//HF/6-31G^* level and to a DFT calculation at the B3LYP/6-31G^* and the B3LYP/6-311 + G^** levels. Five new PAs are thus established and one published experimental PA is revised. © 1997 by John Wiley & Sons, Inc.