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 calc ulated 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 substitutio n of phenol with sigma -electron-withdrawing groups (Br and Cl) leads to sm all 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 elec tronegativity 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 we re measured in CCl4 and cyclohexane solutions in the frequency range of 370 0-400 cm(-1) and the integrated infrared intensities were determined. The t heoretical harmonic frequencies and infrared intensities were calculated fo r all the molecules using the DFT and HF methods. The best overall agreemen t 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 frequen cies 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.