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.

Keywords:

• ab initio
• electronic structure
• quantum chemistry
• CT complex

Acknowledgements

Financial assistance from UGC, New Delhi, extended through the DSA project in chemistry, is gratefully acknowledged. The authors thank a referee for valuable suggestions.