Dalva E. C. Ferreira

Quantum mechanical/effective fragment potential (QM/EFP) study of phosphate monoester aminolysis in aqueous solution.

In this work, we investigated the P-O bond cleavage of dianionic p-nitrophenyl phosphate monoester (p-NPP), with attack of aqueous methylamine on phosphorus, by using the hybrid quantum mechanical/effective fragment potential (QM/EFP) approach. We explored the structures, energetic properties, reaction mechanism, and charge distribution along the entire reaction coordinate. Our B3LYP/6-31++G(d,p)/EFP results show that the cleavage of the P-O bond of p-NPP proceeds through a concerted mechanism with an activation energy of about 26 kcal/mol and an activation free energy at 39 degrees C of 23.6 kcal/mol, in good agreement with the experimental value of 27 kcal/mol. The reaction involves a trigonal bipyramidal (TBP) transition state with less initial bonding to the nucleophile than to the leaving group. The extent of the P-O(nitrophenolate) bond dissociation at the transition state, with methylamine as the nucleophile, is less than that with OH(-). The computed charge distribution along the reaction coordinate is consistent with a progressive charge transfer from the nitrogen atom of methylamine to the phosphate unit along the reaction coordinate. A new strategy to build the initial EFP cluster, by running a classical Monte Carlo simulation and analyzing the center-of-mass radial pair distribution function, was also used.

Ab Initio Highly Correlated Conformational Analysis of 1,2-Difluorethane and 1,2-Dichloroethane.

Temperature-dependent conformational population calculations for anti and gauche forms of 1,2-dichloroethane and 1,2-difluorethane were carried out at a highly correlated level of theory (MP4(SDTQ) and CCSD(T)) employing good quality basis sets (6-311++G(3df,3pd) and aug-cc-pVQZ) for the determination of gas relative conformational energies, making use of the statistical thermodynamics formalism for the evaluation of the thermal energy correction at the MP2/6-311++G(3df,3pd) and MP2/aug-cc-pVTZ levels. In addition to the standard calculation of thermodynamic partition functions, a treatment of the lowest-frequency vibrational mode as hindered rotation and anharmonic correction to vibrational frequencies was also included. We found a good agreement between ab initio calculated conformational population values and experimental gas-phase electron diffraction data for the 1,2-dicloroethane. However, for the 1,2-difluorethane species, a reasonable agreement with the experimental anti/gauche population ratio obtained from the analysis of gas-phase far-infrared (50-370 cm(-1)) and low-frequency Raman (70-300 cm(-1)) spectra was not obtained. The results reported here indicate that, for 1,2-difluorethane, and probably other substituted alkanes where the gauche effect is of relevance, a more appropriated treatment of the low-frequency modes must be pursued in order to reproduce experimental conformational population data.

Molecular properties of coordination compounds of the croconate ion with first-row sivalent transition metals: a quantum mechanical study

In this paper we report geometries, magnetic and vibrational spectroscopic properties of croconate complexes [M(C5O5)(H2O)4] (M= Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) calculated at the Density Functional Theory level. The ground state of all complexes was found to be of high spin, in accordance with the experimental proposal. The calculated structures and vibrational frequencies were also in agreement with experiment, even though comparisons were made with the solid state structure. The calculated nonlinear optical (NLO) properties were for all the compounds analyzed, using the Time Dependent Hartree-Fock (TDHF) method within the static approach, suggesting that these transition metal complexes might be considered as lead molecules to the development of novel based-molecular materials.

A THEORETICAL INVESTIGATION OF STRUCTURAL, SPECTROSCOPIC AND THERMODYNAMIC PROPERTIES OF CYCLODECANE

The torsion potential energy surface for cyclodecane has been examined using molecular mechanics and quantum chemical methods. The boat-chair-boat (BCB) conformer is predicted as the most stable structure at all levels of theory employed, in agreement with low temperature experiments. We found 13 low-energy conformers that cover a range of ~10 kcal mol-1. The relative abundance of the main isomers (named BCB, TBC, TBCC and TCCC) as a function of the temperature is discussed. In addition, energy calculations at high correlated levels of theory (MP4(SDTQ) and CCSD(T)) were for the first time performed for these conformations. In general, the predicted Gibbs population is in fairly good agreement with the experimental data. The simulated IR, VCD and 13C NMR spectra were obtained showing good accordance with the observed data, providing important features for conformational analysis of cyclodecane.