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Journal of Structural Chemistry

2011 year, number 2

Infrared spectra of diphenylphthalide and polydiphenylenephthalide

N. M. Shishlov, S. L. Khursan
Keywords: vibrational spectra, density functional theory, phthalides, polyarylenephthalides, phenolphthalein
Pages: 284-296


The splitting of the ν(С=О) absorption band (AB) of about 12 cm-1 is found in the IR spectra of diphenylphthalide (DPP) in the crystalline phase and CCl4 solution. In the crystalline phase, this splitting is likely to be caused by the inequivalence of DPP molecules in the crystallographic cell, while in the solution, by the dimerization of DPP molecules via dipole-dipole and/or hydrogen bonds. A theoretical low-frequency shift of the ν(С=О) AB for a complex of two DPP molecules (in comparison with a single molecule) is 14 cm-1 in the PBE/3ξ approximation, which is close to the experimentally observed splitting. In two quantum chemical approximations (B3LYP/6-311G(d,p) (I) and PBE/3ξ (II)) the optimal structure and vibrational spectrum of DPP are calculated. Approximation I better reproduces the intensities, whereas approximation II better reproduces the IR frequencies of the DPP spectrum. Almost all 48 ABs of the IR spectrum of DPP are assigned to theoretical normal vibrations (modes). Based on the potential energy distribution over natural coordinates and the visualization of vibrations, experimental ABs (and the corresponding modes) are assigned to the stretching and bending vibrations of certain bonds in the DPP molecule. In particular, ABs at 1107 cm-1 and 970 cm-1 are assigned to the ν(-OC-O-) and ν(-C-O-) stretching vibrations, respectively, of the DPP lactonic ring, which differs from the previously accepted assignment. The results of the interpretation of the DPP spectrum are used to assign a number of ABs in the IR spectrum of polydiphenylenephthalide (PDP), for which DPP is a model compound. According to the calculations in approximation II of the vibrational spectrum of a model valence-bonded dimeric molecule, the intense complex AB at 800-870 cm-1 in the IR spectrum of PDP is mainly due to the out-of-plane bending vibrations of C-Н bonds in the 1,4-substituted benzene rings of polymer biphenyl moieties and the bending vibrations of the lactonic ring.