The maximum hardness and minimum polarizability principles as the basis for the study of reaction profiles

New and useful aspects of chemical reactivity as described by reactivity indexes and used in connection with the maximum hardness and minimum polarizability principles (MHP and MPP, respectively) are discussed and illustrated for two classical reactions in organic chemistry. They include the Beckmann rearrangement and the condensation reactions of -amino acids. The MPP appears as a more general rule than the MHP. Another relevant result is related to the usefulness of both empirical reactivity rules to predict the most probable reaction mechanism among two different pathways displaying very close values in activation energy (competitive pathways). This is illustrated for the condensation reaction of a series of -amino acids: while the accepted stepwise route follows both the MHP and MPP rules, the alternative concerted channel does not, yet its associated activation energy is slightly lower than that corresponding to the nonconcerted reaction mechanism.From the Proceedings of the 28th Congresco de Quimicos Teóricos de Expresión Latina (QUITEL 2002)     

Structural Characterization of Cadmium Hexacyanometallates(II) and Related Complexes

Cadmium hexacyanometallates (II) crystallize in the same monoclinic cell (space group C⁵ _2h–P2_1/n) reported for the analogs of manganous complexes. In that monoclinic framework, two outer cations (M = Mn²⁺ or Cd²⁺) of neighboring structural units M₂[L^II(CN)₆], (L = Fe^II, Ru^II, Os^II), are linked by two common water molecules. On heating above 100°C, these water bridges are removed. In dehydrated complexes, the outer cation (M) remains linked to only three CN ligands. The loss of aquo-ligands induces a remarkable -back donation from the inner cation (L) to CN ligands, which was studied using IR, Raman, and Mössbauer spectroscopies.     

Non-hyperbolic surfaces having all ideal triangles with finite area

We construct examples ofC ³ compact surfaces of non-positive curvature having non-Anosov geodesic flows and satisfying the following property: there existsL>0 such that the area of every ideal triangle in the universal covering of the surface is bounded above byL.Partially supported by CNPq of Brazilian Government     

Additive of substituent effects on the chemical shift of the formyl proton in disubstituted benzaldehydes

An additivity relationship of substituent effects on the formyl proton chemical shift has been tested with ten disubstituted benzaldehydes. In most cases it is found that the formyl proton chemical shift, extrapolated to infinite dilution in nitromethane, is in good agreement with that calculated using the additivity relationship.     

Synthesis and structure of new substituted 2-dicyanomethylene-1,2-dihydropyridines

The synthesis of substituted 2-dicyanomethylene-1,2-dihydropyridines 3b-c and 2-dicyanomethylene-2,3,4,5-tetrahydropyridine 4 is accomplished from malononitrile dimer and propenenitriles 2 . All structures were determined by nmr techniques; however to put our structural assignment for 6-amino-3-cyano-5-ethoxycarbonyl-2-dicyanomethylene-1,2-dihydropyridine 3d on a firm and unequivocal basis we decided to perform X-ray diffraction analysis of this compound.     

Spiroborates derived from catecholborane and ephedrine type aminoalcohols - model compounds for the study of intramolecular N→B coordination by dynamic 1H-, 11B- and 13C-NMR.

A convenient synthesis of the title compounds is reported. These serve as models to study intramolecular N→B coordination by means of dynamic NMR spectroscopy (¹H, ¹¹B, ¹³C). Steric interactions between substituents at the boroxazolidine ring (C(5)Ph, C(4)Me, NMe) determine the stability of the N→B bond and the nitrogen configuration.     

A model for the charge capacity of 1T–TiS2 intercalated with Li

The variation of the charge capacity of a double-layer 1T–TiS² model system upon increasing intercalation with lithium is examined at a molecular level. The charge capacity is modeled through the global hardness index of density functional theory. The results obtained qualitatively reproduce the experimental trend observed in the voltage–composition variation curve in this system. Comparison with previous theoretical models show that the present approach may give a more complete information about the double-layer capacitance, since it takes into account the lithium ionicity and the host–guest specific interactions. © 1995 John Wiley & Sons, Inc.     

The formate anion: the harmonic and anharmonic force fields

Harmonic and anharmonic symmetry force constant matrices have been obtained for six isotopically substituted species of the formate anion. The valence force fields derived from the above matrices were used to recalculate the vibrational frequencies. These force fields are compared with others previously reported and obtained via other methods.     

Noticeable Quantities of Functional Compounds and Antioxidant Activities Remain after Cooking of Colored Fleshed Potatoes Native from Southern Chile

The effect of cooking on the concentrations of phenolic compounds and antioxidant activities in 33 colored-fleshed potatoes genotypes was evaluated. The phenolic profiles, concentrations, and antioxidant activity were evaluated with a liquid chromatography diode array detector coupled to a mass spectrometer with an electrospray ionization interface (HPLC-DAD-ESI-MS/MS). Eleven anthocyanins were detected; in the case of red-fleshed genotypes, these were mainly acyl-glycosides derivatives of pelargonidin, whereas, in purple-fleshed genotypes, acyl-glycosides derivatives of petunidin were the most important. In the case of the purple-fleshed genotypes, the most important compound was petunidin-3-coumaroylrutinoside-5-glucoside. Concentrations of total anthocyanins varied between 1.21 g kg⁻¹ in fresh and 1.05 g kg⁻¹ in cooked potato and the decreases due to cooking ranged between 3% and 59%. The genotypes that showed the highest levels of total phenols also presented the highest levels of antioxidant activity. These results are of relevance because they suggest anthocyanins are important contributors to the antioxidant activity of these potato genotypes, which is significant even after the drastic process of cooking.