Ab initio calculated gas-phase basicities of polynuclear aromatic hydrocarbons

The gas-phase basicities (GBs) of polynuclear aromatic hydrocarbons (PAHs) have been calculated using the semiempirical Austin model 1 method, ab initio quantum mechanical methods at the HF/3-21G, HF/6-31G(d), MP2/6-31G(d), MP2/6-31+G(d,p) levels, and the B3LYP/6-311G(d,p) density functional method. GBs calculated at these levels of theory are compared to experimentally known GBs measured by equilibrium mass spectrometric methods. Theoretically calculated entropies for PAHs and their protonated carbocations are used to reevaluate original experimental equilibrium data producing a revised set of experimental GBs and proton affinities for PAHs and related hydrocarbons. Experimental GBs for 40 hydrocarbons are found to correlate well with AMI, Hartree-Fock, DFT and MP2 calculated GBs, with regression analyses showing standard errors of 2.12, 1.53, 1.36, and 1.55 kcal mol(-1) for each of the four methods, respectively. The results permit an evaluation of the reliability of experimental data and suggest a need for new experimental work for some molecules. Predictions are made for GBs for 12 new PAHs whose GBs have not yet been measured. (C) 2000 Elsevier Science B.V.