Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C6H6···X (X = O2, N2, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser–Parr–Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predictions. Ab initio restricted open shell Hartree–Fock calculations for the cation vibrational frequencies show excellent correspondence with the observed infrared bands. The theoretical infrared intensities show some remarkable differences between the neutral and ionized species. For example, the CH in-plane bending modes and CC in-plane stretching vibrations are predicted to increase by several orders of magnitude upon ionization. Experimental infrared intensities have been estimated and certain bands do show a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified infrared emission bands from interstellar space is discussed briefly.

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