Molecular and vibrational structure of 2,4-dinitrophenol: FT-IR, FT-Raman and quantum chemical calculations

The molecular vibrations of 2,4-dinitrophenol (DNP) were investigated in polycrystalline sample, at room temperature, by Fourier transform infrared (FT-IR) and FT-Raman spectroscopies. In parallel, semiempirical, ab initio and various density functional theory (DFT) methods are used to determine the geometrical, energetic and vibrational characteristics of DNP. Both the experimental and theoretical data suggest a rather strong intramolecular H-bond (HB) involving the hydroxyl and its neighbor nitro group. Semiempirical PM3 and ab initio methods fail to describe the HB while among different exchange and correlation functionals used in conjunction with DFT methods, only the B3LYP combination is able to quantitatively reproduce the effects of intramolecular HB, as reflected in FT-IR and FT-Raman spectra. The vibrational experimental bands were assigned to normal modes on the basis of DFT calculations at the B3LYP/6-31G(d) and B3LYP/6-311G(df,p) levels of theory and by comparison with vibrational spectra of phenol, nitrobenzene, 2-nitrophenol and p-nitrophenol. The second nitro group brings a large atomic charge to the phenol ring, especially on C4 atom and this is mainly reflected in frequency sequence and intensity pattern of some nu(CC) vibrations with nu(NO2) symmetric stretch contributions. The characteristic marker IR bands of DNP are discussed, and the intensities of several bands have been explained. It is also shown that in spite of its simplicity, AM1 semiempirical method is able to give surprisingly good agreement with experiment, especially for hydrogen bonding parameters in DNP. (C) 2004 Elsevier B.V. All rights reserved.