Fourier transforms infrared spectra and structure of triformylmethane. A density functional theoretical study

Validation of potential energy distribution by VEDA in vibrational assignment some of β-diketones; comparison of theoretical predictions and experimental vibration shifts upon deutration

The harmonic vibrational frequencies of the cis-enol forms of some of β-diketones with different substitution in beta position, vis. H, CH3, and Ph ring, as the symmetric and asymmetric molecules, were calculated using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level of theory. The results of DFT calculations were used to obtain the potential energy distribution (PED) by VEDA software. The PED results compared with the Gauss View animation, as our reassignments, and the experimental IR shifts upon deuteration of hydrogen in the OH and CHα. According to our study, the PED contributions, Gauss View animation and observed shifts show similar results for most of the bands which are not coupled with the OH and/or CHα bending, such as asymmetric and symmetric CH₃ stretching and in-plane deformations, CH₃ rocking vibrations and 8a, 19b, 9a, 15, 18a, and 12 motions of the phenyl ring. The largest discrepancies were observed in the 1700-1000 cm^-1 region, likely due to the coupling with the OH and CHα in-plane bending vibrations, such as νaC = C-C = Ο, νsC = C-C = Ο and δOH. Furthermore, the calculated PED contributions by VEDA software do not well define the vibrational contributions to those groups in the molecule that are directly involved in the intramolecular hydrogen bond and the observed failure of the VEDA procedure is possibly due to inappropriateness of the default options.

Theoretical investigation of solvent effect on the keto–enol tautomerization of pentane-2,4-dione and a comparison between experimental data and theoretical calculations

The equilibrium constants of stable keto and enol forms of pentane-2,4-dione (known as acetylacetone) are estimated, using the second-order Møller−Plesset (MP2), density functional theory (B3LYP and M06-2X), composite methods (G4, G3, G3B3, CBS-QB3, and G3MP2B3), double-hybrid density functional theory (B2PLYP), and long-range corrected (LC) hybrid functional (ωB97X-D). These methods are integrated with the PCM, CPCM, and SMD models to elucidate the effect of solvent on thermodynamic parameters. The reported measured enol contents in the solutions and gas phase are utilized to benchmark the predictions of different quantum mechanical methods for the keto–enol equilibrium in acetylacetone. In this study, we calculated the enol content in 16 acetylacetone solutions and in the gas phase. Among the applied methods, the MP2 level and the B3LYP level underestimate and overestimate, respectively, the enol content of acetylacetone in the gas phase and solutions. The G3B3 and G3MP2B3 levels give reasonable agreement with the measured data. The best results obtained by calculations at the B2PLYP/6-31+G(2d,p) and CBS-QB3 levels, with mean absolute errors (MAE) relative to experiments of 2.30 and 5.45 and root mean square deviation (RMSD) errors of 0.78 and 1.66, respectively. According to our calculations, one enol and two keto forms (Ket1 and Ket2) coexist in polar solutions. The effect of solvent was more pronounced on the structure and stability of the Ket2 tautomer than others. The strength of the intramolecular hydrogen bond in the enol form of acetylacetone is almost independent of the solvent polarity.

C-H...O hydrogen bond in chloroform-triformylmethane complex: blue-shifted or red-shifted?

B3LYP/6-311+G** level of theory is used to investigate the C-H...O hydrogen bond formed by chloroform and two conformers of triformylmethane (TFM), i.e. cis-TFM (concerned with C1 configuration) and trans-TFM (concerned with C2 and C3 configurations). Polarized continuum model (PCM) is used to study the solvent (chloroform) effect on this hydrogen bond. The C3 configuration is more stable than the C1 configuration whether the absolute energy or the stabilization energy is concerned. For the C1 and C2 configurations this hydrogen bond is of blue-shifted type both in gas phase and in chloroform solution. For the C3 configuration this hydrogen bond is of red-shifted type in gas phase but turns into blue-shifted type in chloroform solution instead. It's inappropriate to simply designate this hydrogen bond as blue-shifted type or red-shifted type.

Fourier transform infrared and Raman spectra, vibrational assignment and density functional theory calculations of naphthazarin

FT Raman and FTIR spectra of Naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) and its deuterated analogue are recorded. Comparison between the spectra obtained by two techniques, a series of density functional theory (DFT) calculations and the spectral behavior upon deuteration were used for the assignment of the vibrational spectra of this compound. The calculated vibrational frequencies by the B3LYP, B3PW91, G96LYP, G96P86, and MPWLYP density functionals are generally consistent with the observed spectra. Infrared and Raman vibrational transitions predicted by B3LYP/6-311++G** are reported for the titled compound and its deuterated analogous and the assignments are discussed. All experimental and theoretical results support a relatively weak hydrogen bond in naphthazarin (NZ), compared with that in the enol form of normal beta-diketones. The observed nuOH/nuOD and gammaOH/gammaOD appear at about 3060/2220 and 790/560 cm(-1), respectively, which are consistent with the calculated hydrogen bond geometry and proton chemical shift results. Two bands at about 350 and 290 cm(-1) are assigned to the O...O stretching modes belong to A1 and B2 species, respectively.