Vibrational studies of 2,3,5,6-tetrachloro-1,4-benzoquinone and its reactivity: An ab-initio computational method

Abstract

The IR spectrum of 2,3,5,6-Tetrachloro-1,4-benzoquinone (p-chloranil) has been recorded in the solid phase in the range 4000 – 400 cm-1. ab initio calculations have been performed using Gaussian 03 program to compute optimized geometry, harmonic vibrational frequencies along with intensities in IR and Raman spectra and atomic charges at RHF/6–31+G*, B3LYP/6-31+G*, B3LYP/6-31++G** and B3LYP/6-311++G** levels for p-chloranil and parent molecule p-benzoquinone. The optimized molecular structures are found to be planar and possess D2h point group symmetry for both the molecules. The C=C bond lengths are larger whereas the C=O bond lengths are shorter in p-chloranil as compared to p-benzoquinone which is also reflected in the corresponding vibrational frequencies. The atomic arrangements in p-benzoquinone and p-chloranil possess two concentric hexagonal arrays and both the arrays of p-benzoquinone contain mixed charges. In the case of the p-chloranil molecule, the outer array is negatively charged and the inner array is positively charged which leads to increased susceptibility at the sites to which chlorine atoms are attached in p-chloranil towards the nucleophile. Consequently, p-chloranil is a good candidate for synthesis of many derivatives of 1,4-benzoquinone. The optimized geometrical structures along with the atomic charges on the atomic sites of p-benzoquinone and p-chloranil reveal the reactivity of p-chloranil molecules towards nucleophilic substitution reactions as compared to p-benzoquinone.