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The squarate anion and their coordination compounds with Li+, Na+ and K+ are studied in gas phase and aqueous solution using ab initio quantum chemical methods and a sequential Monte Carlo/quantum mechanical procedure. The infrared and Raman spectra were calculated and the vibrational modes assigned at the second order Møller–Plesset perturbation (MP2) level of theory, employing standard split-valence basis set with inclusion of polarization and diffuse functions (6-31G(d), 6-31+G(d), 6-311+G(d), 6-311+G(2d), 6-311+G(2df)) on the O and C atoms. The vibrational analysis showed an important role played by the polarization functions on the low frequency vibrations. The solvent and counter-ions effects on the electronic spectrum are analyzed showing that both should be included in the calculation in order to reproduce the observed UV spectrum. This conclusion supports our previous analysis on the oxocarbon series.

The structure and spectroscopic properties of the deltate anion are calculated in the gas phase using ab initio quantum chemical methods and in aqueous solution through a sequential Monte Carlo-quantum mechanical procedure. The effects of the solvent and counter-ions on the electronic spectrum are analyzed, showing that both should be included in the calculation in order to reproduce the observed UV spectrum. For the smallest cyclic oxocarbon, the deltate anion, the calculated electronic transitions were 254 and 246 nm considering the [Li₂(C₃O₃)(H₂O)₂₀] species. This is in accordance with the expected behavior for the oxocarbon series, predicting absorption bands close to 200 nm for the deltate anion.

Oligotiophenes derivatives present potential applicability for the construction of electronic devices. In this work we analyzed the structure–property relationship for a class of cyanothiophene compounds using ab initio [Hartree–Fock (HF) and second-order Møller–Plesset (MP2)] and density functional theory (DFT) theoretical approaches. The energy gap between the valence and conduction bands was evaluated using distinct methodologies, being the results compared with experimental data. At the DFT level using the HF optimized geometries (DFT//HF) the average deviation was 0.22 eV (SVWN//HF) and 0.23 eV (BLYP//HF). The results from the configuration interaction implemented in the semiempirical ZINDO/S-CI program were also in good agreement with experiment, being the average deviation found to be 0.20 eV when the HF optimized geometries were used. In general, the energy gap for the cyanothiophene derivatives was lower than the values for the unsubstituted parent compounds, showing that these molecules should be considered as a good prototype to develop new organic conductor polymers. We believe this study may be useful to predict electronic properties that are related with the conductivity of organic materials. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Ab initio methods were used in conjunction with Monte Carlo simulation to analyze the structure and spectroscopic properties of the croconate ion in the gas phase and in aqueous solution. The infrared and Raman spectra were calculated and band assignments were made showing a good agreement with experiment. The electronic spectrum of the croconate ion was calculated in the gas phase and in aqueous solution, using a sequential Monte Carlo/quantum mechanical approach, taking into account the solvent and counter ion effects. The electronic spectrum for the free croconate ion in aqueous solution showed two transitions at 479 and 468 nm when the first solvation shell is considered. These transitions were not sensitive to additional solvent molecules beyond the first solvation shell. The experimental electronic spectrum was only reproduced when the combined effects of the solvent and counter ion were taken into account. The calculated spectrum for the cis-[Li₂(C₅O₅)(H₂O)₂₁] complex showed two transitions at 383 and 365 nm, in agreement with the experimental observations of 372 and 351 nm. These results strongly suggest that in order to reproduce the experimental electronic spectrum of the oxocarbons in solution, we must take into account the combined effects of the solvent and the counter ions. A new proposal for the interaction of Li+ with the croconate anion in solution, based on the theoretical electronic spectra, is also discussed.

The tautomerism in Sudan III bis-azo dye has been analyzed using ab initio Hartee–Fock [HF] and second-order Møller–Plesset perturbatim theory [MP2] and density functional (B3LYP) methods. Gas-phase and solution calculations were performed to investigate the solvent effect on the azo (OH)→hydrazone (NH) tautomeric equilibrium. The azo (OH) tautomer was found to be preferred in gas phase at the HF level of theory. The inclusion of the electronic correlation (MP2) shifted the equilibrium toward the hydrazone (NH) form. The NH isomer was also found to be more favorable in the gas phase according to the B3LYP results. In solution the equilibrium is shifted toward the NH tautomer as the dielectric constant of the medium increases. The energy barrier for the intramolecular proton transference was calculated and the value found suggested a strong hydrogen bond. The B3LYP and MP2 activation Gibbs free energies were very close and much lower than the HF value. The ultraviolet/visible electronic spectra for the minima and transition state (TS) structures were calculated and compared with the experimental data. The theoretical band positions obtained considering the TS geometry were found to best agree with the experimental data. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1076–1086, 2000

Anhydrotetracycline (AHTC) is the major toxic decomposition product of the antibiotic tetracycline with a characteristic electronic spectrum used for identification purposes. In the present work, the electronic spectra of the fully deprotonated forms (L2−) of the free AHTC molecule, the main species involved in the metal complexation reaction, have been analysed using the spectroscopic version of the INDO/S method. The polarization and specific solute–solvent interaction were analysed using the standard SCRF continuum model and the supermolecule approach. The theoretical band spectra were simulated by fitting the calculated frequency and oscillator strength values to a Lorentzian type function and directly compared with the experimental aqueous solution spectrum. For the L2−(10-O) deprotonated molecule, the theoretical spectrum in the near UV region showed three absorption bands located at 215, 261 and 336 nm, which are in very good agreement with the observed values: 220, 268 and 335 nm. All transitions were assigned as π→π* of the BCD ring chromophore with contributions of the A ring for the absorption at 261 nm. The same transitions were located at 222 and 261 nm for the L2−(11-O) and the shoulder at 236 nm is not observed.

The potential energy surface (PES) for the cyclooctane molecule was comprehensively investigated at the Hartree–Fock (HF) level of theory employing the 3–21G, 6–31G, and 6–31G* basis sets. Six distinct true minimum energy structures (named B, BB, BC, CROWN, TBC, and TCC1), characterized through harmonic frequency analysis, were located on the multidimensional PES. Two transition state structures were also located on the PES for the cyclooctane molecule. Electron correlation effects were accounted for using the Møller–Plesset second-order perturbation theory (MP2) approach. The predicted global minimum energy structure on the ab initio PES for the cyclooctane molecule is the BC conformer. A gas phase electron diffraction study at 300 K suggested a conformational mixture while an NMR study in solution at 161.5 K predicted the BC conformer as the predominant form. The equilibrium constants reported in the present study, which were evaluated from the ab initio calculated total Gibbs free energy change values, were in good agreement with both experimental investigations. The ab initio results showed that the low temperature condition significantly favored the BC conformer while above room temperature both BC and CROWN structures can coexist. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 524–534, 1998