Antonio Martínez-Richa

A solid-state 13C NMR analysis of ambers

Abstract Colombian copalite samples (found at 73°5′W 6°19′N in the Cimitarra quadrangle), and commercial amber samples from Chiapas (Mexico), Dominican Republic and Poland (Baltic) were studied using solid-state 13C CP-MAS Nuclear Magnetic Resonance (NMR). NMR spectra for Colombian amber are shown and compared to those reported for the resin from the genus of the African species Hymenaea verrucosa and other Hymenaea trees, for which a close resemblance is observed. NMR spectral features of the other studied ambers are also discussed. A physical aging experiment was performed on a Colombian sample at 65°C. Differences in the solid-state NMR and IR spectra are explained in terms of changes in chemical cross-linking, isomerization and morphology.

Effects of molecular geometry on liquid crystalline phase behaviour: isotropic-nematic transition

The effects of range and geometry of a simple attractive square-well on the phase diagram of hard ellipsoids and hard spherocylinders is systematically studied using a simple van der Waals type theory. The orientational single particle distribution function is approximated using the Onsager trial function. The quantitative errors introduced by this are thought to be considerably smaller than the use of the van der Waals approximation, which has been shown to give qualitatively correct phase diagrams for similar models. The phase diagrams obtained for hard ellipsoids and hard spherocylinders of aspect ratios ranging between 3 and 10 with a variety of square-well attractions are found to fall into three general types. The first type shows liquid-vapour coexistence and an isotropic-nematic transition, which meet at a liquid-vapour-nematic triple point. The second type shows a marked widening of the isotropic-nematic biphasic region which pre-empts the liquid-vapour coexistence. The final phase diagram shows a strong destabilization of the nematic phase with respect to the isotropic, which results in a shift of the phase transition to higher densities and pressures as the temperature is lowered.

Semi-continuous Heterophase Polymerization of n-Butyl Methacrylate: Effect of Monomer Feeding Rate

The effect of monomer feeding rate on particle size, molar masses, glass transition and tacticity of poly(n-butyl methacrylate) (PBMA) nanoparticles synthesized by semi-continuous heterophase polymerization under monomer-starved conditions is reported. Three feed rates were examined. Highly monomer-starved conditions at the two slower addition rates were confirmed by the low amount of residual monomer throughout the reaction and by the fact that the instantaneous polymerization and feeding rates became similar at later stages of the reaction. Under these conditions, polymer particles in the nanometer range (30 to 35 nm) were obtained. Glass transition temperatures are substantially higher than those reported for commercial PBMA. Polymers tacticity was determined by 13C-NMR spectroscopy. NMR measurements confirm that the syndiotactic content of the PBMA synthesized here is larger than those of the commercial ones made by free-radical polymerization. Molar masses are much lower than those expected from termination by chain transfer to monomer, which is the typical termination mechanism in microemulsion polymerization.

Self-diffracting effects in hybrid materials

The authors experimentally demonstrate self-diffraction effect in a hybrid material. Patterns at different light intensity levels were obtained. A dependence of the number of diffraction rings with the intensity of laser is observed.

Carbon-13 CP-MAS nuclear magnetic resonance studies of teas

13C CP-MAS NMR spectra of green and black tea were obtained and assigned based on the solid-state NMR spectra of tropolone, (+)-catechin hydrate, gallic acid, caffeine and flavone derivatives. The peak shape and chemical shifts observed for carbonyl carbons in CP-MAS spectra of teas indicate the existence of different chemical species, mainly free phenollic acids and ester derivatives of flavonoids. The peak patterns allow to establish differences between both teas.

Application of 29Si and 27Al magic angle spinning nuclear magnetic resonance to studies of the building materials of historical monuments.

We report the application of 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) studies on building stones from historical monuments to obtain direct information about the degree of degradation and to observe the changes in the consolidated material after treatment with tetraethoxysilane (TEOS). Using the data obtained from deteriorated materials, a diagnostic laboratory, a suitable treatment and recommendations for building conservation may infer. A case study is presented using stones from Guanajuato City Main Church (Central Mexico). X-ray diffraction patterns to characterize the species present in the stones agree with the solid state NMR results.

Keto-Enol Tautomerism of Dimedone Studied by Dynamic NMR

The keto-enol tautomeric equilibrium of dimedone has been investigated by high-resolution 13C NMR spectroscopy. Kinetic information of the solution keto-enol tautomerism for dimedone in DMSO, in the temperature range of 25–85 °C, was derived from line shape measurements in a 75-MHz spectrometer. A value of 3.43 Kcal/mol was found for the Arrhenius activation energy Ea and of 1.07 × 106 s−1 for the pre-exponential factor A. With the use of the observed chemical shifts in the high-resolution 13C-NMR spectra of dimedone in the solid state, an estimate coalescence temperature of 240 K for dimedone in DMSO was obtained by extrapolation of the experimental curve. The estimated free energy of activation at the coalescence temperature, ΔGc≠, is 10.8 Kcal/mol. Finally, the 13C spin-lattice relaxation times, T1(13C), in solid dimedone were measured as a function of temperature in the range of 25 to 90 °C. The data are discussed in terms of the different motional environments that result from the geometric restrictions imposed by hydrogen bonding in the crystal structure.

Quadrupolar moment calculations and mesomorphic character of model dimeric liquid crystals

Abstract Model dimeric compounds CH3ue5f8(CH2)nCOOue5f8Rue5f8OOCue5f8(CH2)pue5f8COOue5f8Rue5f8OOC(CH2)nue5f8CH3, where (R=ue5f8C6H4ue5f8C(CH3)ue605HCue5f8C6H4ue5f8), of semiflexible liquid crystal polymers (LCP) have been studied by molecular modeling techniques. First, a conformational analysis of the molecules was made in order to find the lowest energy conformation structure. In a second stage, geometry optimization calculations of these conformers were performed at the quantum mechanics semiempirical PM3 level. Some molecular parameters (electric dipolar and quadrupolar moments) were obtained at the improved ab initio Hartree–Fock HF/6-31G(d)//PM3 level and are related with the mesomorphic character of these systems. It is found that the trace of the quadrupole moment tensor correlates with the spacer (ue5f8(CH2)p-linkage) length, and this can be an indication of the trend that this magnitude exhibits in aromatic liquid crystal systems.

Nonlinear optical characterization of ionics liquids

In this work, we show results about the nonlinear optical characterization for four ionic liquids (ILs), namely 1-buthyl-3- methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-ethyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)imide ([EMIM][TF2N]), 1-ethyl-3-methylimidazolium trifluoroacetate ([EMIM][CF3COO]),1-buthyl-3-methylimidazolium trifluoroacetate ([BMIM][CF3COO]), using z-scan technique.

Variation of intrinsic viscosity in the hydrolysis of hydroxyethylcellulose, and its relationship with resistance to enzymatic degradation

A mathematical model for the quantitative analysis of kinetic data obtained in the hydrolysis of cellulose ethers is described. The model is applied to solution viscosity measurements obtained during the enzymatic hydrolysis of O-(2-hydroxyethyl)cellulose (HEC). Data analysis involves the derivation of the intrinsic viscosity dependence on the enzymatic hydrolysis time, from the analytical expressions of the Martin equation. The scope of the method and its relationship to the resistance to enzymatic degradation of cellulose derivatives are discussed. The importance of the substitution pattern (represented by the degree of substitution) and molecular size (end-to-end distance derived from the Flory-Fox equation), on the biodegradability of HEC is demonstrated.