Molecular structures and infrared spectra of p-chlorophenol and p-bromophenol.: Theoretical and experimental studies

The molecular structures of p-chlorophenol and p-bromophenol have been calculated with the MP2, DFT(hybrid), and HF methods using the extended 6-311++G (df, pd) basis set. The geometrical parameters of p-ClPh and pBrPh in the gas phase have not been reported as yet. The results show that substitution of phenol with sigma -electron-withdrawing groups (Br and Cl) leads to small shortening of the C-C and C-O bonds and small changes in the CCC angles. The structural changes of the phenol ring are governed mainly by the electronegativity of the para-substituent and, to a lesser extend, by resonance factors. The FT-IR spectra of p-ClPh, p-BrPh, and their OD counterparts were measured in CCl4 and cyclohexane solutions in the frequency range of 3700-400 cm(-1) and the integrated infrared intensities were determined. The theoretical harmonic frequencies and infrared intensities were calculated for all the molecules using the DFT and HF methods. The best overall agreement between the calculated and experimental spectra has been obtained at the B3LYP/6-311++G(df,pd) level. A clear-cut vibrational assignment is made on the basis of the calculated potential energy distribution (PED). The effect of the p-Cl and p-Br substituents upon the characteristic phenolic frequencies and infrared intensities is discussed; in particular, it is shown that both the OH and OD torsional frequencies are related to the nature of the substituent.