Theoretical study of the electronic structure of CnS (n=1-6) thiocumulenes

Linear sulfur-carbon chains C(n)S (n=1-6) of astronomical interest were examined by means of several theoretical methods. The three smallest compounds of the series were chosen to evaluate the performance of several computational models, including Hartree-Fock theory, density functional theory with the Becke's three parameter exchange functional and the correlation functional of Lee, Yang, and Parr (B3LYP), and electron-correlated methods (second-order Moller-Plesset perturbation method (MP2), configuration interaction method including single and double excitations (CISD), and quadratic configuration interaction method including single and double excitations (QCISD) in combination with a large variety of basis sets. The systematic comparison between the experiment and theory indicates that the B3LYP/6-311G** method can be considered suitable for the study of the electronic structures of the C(n)S compounds. The electronic ground states of the C(n)S molecules alternate between 1Sigma and 3Sigma for odd and even values of n, respectively. The B3LYP/6-311G** wave functions for these electronic ground states were analyzed by means of the atoms in molecules (AIM) and natural bond orbital (NBO) methods. Both approaches suggest that the electronic structures for the singlet and triplet compounds must be considered separately. According to the NBO method, singlet compounds can be properly represented by acetylenic structures with alternating single and triple bonds (S[triple bond]C-C[triple bond]C...). However, triplet compounds are better described by means of double bond-double bond cumulenic structures (S=C=C=C=C...) as a consequence of the average between different alpha and beta electronic densities. AIM delocalization indexes and NBO interactions between localized orbitals also indicate that these structures are strongly pi delocalized. Finally, the different singlet and triplet structures proposed provide a consistent explanation for the geometries, dipole moments, and spin-density values of the C(n)S compounds studied.     

Enzyme promiscuity: first protein-catalyzed Morita-Baylis-Hillman reaction

The Morita-Baylis-Hillman reaction of cyclohexenone and p-nitro benzaldehyde is catalyzed by carrier proteins such as serum albumins or enzymes such as certain lipases, conversion of up to 35% and enantioselectivities of up to 19% being observed.     

Molecular mechanics (MM2) and conformational analysis of compounds with N?C?O units. Parametrization of the force field and anomeric effect

A molecular mechanics force field was developed for systems bearing the N? C? O unit on the basis of 6-31G* and 4-21G “ab initio” calculations with full optimization of the geometry and experimental heats of formation. The parameters used, which implicitly included the anomeric effect, provided good geometric and energetic results, both for the compounds on which the parametrization was based and for others on which the validity of the predictions was checked.     

The first total synthesis of the (±)-17-methyl-trans-4,5-methyleneoctadecanoic acid and related analogs with antileishmanial activity

The first total synthesis of the marine cyclopropane fatty acid (±)-17-methyl-trans-4,5-methyleneoctadecanoic acid was accomplished in eight steps and in 9.1% overall yield starting from 1-bromo-12-methyltridecane. The cis analog (±)-17-methyl-cis-4,5-methyleneoctadecanoic acid was also synthesized but in seven steps and in 16.4% overall yield. With the two isomeric cyclopropane fatty acids at hand it was possible to unequivocally corroborate the trans relative configuration of the naturally occurring fatty acid by gas chromatographic co-elution of the corresponding methyl esters. The cis isomer was cytotoxic to Leishmania donovani promastigotes with an IC₅₀ of 300.2 ± 4.2 μM.     

PRESSURE-VELOCITY EQUILIBRIUM HYDRODYNAMIC MODELS

This article describes mathematical models for phase separated mixtures of materials that are in pressure and velocity equilibrium but not necessarily temperature equilibrium. General conditions for constitutive models for such mixtures that exhibit a single mixture sound speed are discussed and specific examples are described.     

Railway Train-Track Dynamics for Wheelflats with Improved Contact Models

A variety of methods have been proposed to calculate the dynamic response caused by a railway vehicle affected by a wheelflat. Most of the sophisticated procedures evaluate the elastic properties of the wheel-rail contact by means of the Hertz model. However, the hypotheses that must be satisfied in order to apply the Hertzian contact model are not fulfilled when the wheel-rail contact occurs in the area of wheel affected by the flat. This gives rise to deviations in the results of the dynamic model compared to the real situation. With the objective of analysing the influence of the elastic wheel-rail contact model, a procedure was developed to determine the dynamic response caused by a geometric irregularity (in rail or wheel) by means of Hertzian and non-Hertzian contact models. Results of the wheelflat impact simulations given by both types of contact model have been compared in this work.     

A molecular mechanics study of conformational trends in simple alcohols and ethers. Part I: Geometric trends

We have modified the MM2(80) molecular mechanics force field for alcohols and ethers to reproduce geometric trends predicted by ab initio calculations with the 4-21G basis set. The results obtained with the modified force field are consistent with experimental evidence.     

Molecular mechanics of peroxides. II. Cyclic compounds

The conformations of various cyclic peroxides have been determined using a molecular mechanics force field developed by the authors and previously applied to linear peroxides. Comparison of the results with those of experimental and ab initio studies shows that this force field may be employed without correction for cyclic compounds.