Calculation of the charge transfer transition energy of the mesitylene–ICl complex by ab initio and TDDFT methods: Comparison with other methylbenzene–ICl complexes

Abstract

Of the four plausible isomeric structures of the mesitylene–ICl charge transfer (CT) complex, the most feasible one was determined by a detailed ab initio and DFT study at the HF, B3LYP and MPW1PW91 levels using the 6-31++G(d,p) basis set. Potential energy surface scans followed by frequency calculation and full optimization revealed that the I–Cl bond, with its I atom oriented towards the aromatic ring, stands vertically above an unsubstituted C-atom, being inclined at about 6° to the C 3-axis. Complexation increases the I–Cl bond length. Correction for basis set superposition error through a counterpoise calculation yields a binding energy close to the experimental value. The electronic CT transition energy (hν CT) with this ground-state structure as input was calculated in vacuo by the CIS method and in carbon tetrachloride medium by the TDDFT method under the polarizable continuum model. In a similar way the values of hν CT were calculated for complexes of ICl with p-xylene, durene and hexamethylbenzene. Throughout the series of methylbenzene complexes, the TDDFT-calculated values of hν CT were less than the experimental values and such underestimation may be attributed to the inherent difficulties of DFT to take into account long-range interactions. However, the trend of the variation of hν CT with the number and position of methyl groups in the series was reasonably similar to the trend followed by the experimental CT transition energies.