ABSTRACT:The potential energy surface of curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was explored with the DFT correlation functional B3LYP method using 6-311G* basis. The single-point calculations were performed at levels up to B3LYP/6-311ϩϩG**//B3LYP/6-311G*. All isomers were located and relative energies determined. According to the calculation the planar enol form is more stable than the nonplanar diketo form. The results of the optimized molecular structure are presented and compared with the experimental X-ray diffraction. In addition, harmonic vibrational frequencies of the molecule were evaluated theoretically using B3LYP density functional methods. The computed vibrational frequencies were used to determine the types of molecular motions associated with each of the experimental bands observed. Our vibrational data show that in both the solid state and in all studied solutions curcumin exists in the enol form.

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ABSTRACT:The infrared (IR) spectra of sulfanilamide 1 and of its azanion (counter ion Na ϩ ) 2 have been measured in dimethyl sulfoxide (DMSO) solutions. The density functional theory (DFT) B3LYP 6-31ϩϩG(d,p) computations performed adequately describe the experimental spectroscopic data, including the corresponding frequency isotopic shifts. In agreement between theory and experiment, the conversion of sulfanilamide 1 into azanion 2 results in strong frequency decreases of the asymmetrical and symmetrical vibrations of the SO 2 group: predicted 122 cm Ϫ1 , measured 140 cm Ϫ1 in solution (total values), as well as in other essential IR spectral changes. According to the computations, the strongest changes in the steric structure take place at and next to the azanionic center. The new (azanionic) charge is distributed, as follows: 0.08e Ϫ , 0.23e Ϫ , and 0.12e Ϫ are delocalized over the amino, phenylene, and sulfonic groups, respectively, and 0.57e Ϫ of it remain localized at the azanionic center.

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ABSTRACT:The structure and infrared spectra of formonitrile (hydrogen cyanide) and acetonitrile, their anions, and radicals (45 species in all) have been studied by density functional theory (DFT) B3LYP 6-31ϩϩG(d,p) calculations. The results obtained have been compared with the literature and original experimental data. The zero-point vibrational energy (ZPVE)-corrected deprotonation energies of the studied (and other) nitriles correlate fairly well (R ϭ 0.94) with their pK a values, measured in dimethyl sulfoxide (DMSO). The theoretical cyano stretching frequencies C'N of the species studied have been found to correlate satisfactorily (R ϭ 0.97) with the experimental ones. The latter correlation can be used for a theoretical estimation of C'N of many nitriles, their anions, and radicals. The geometry parameters for the molecules, calculated by the same theoretical method, practically coincide with those determined experimentally in the gas state.

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