Einführung von Carboxylgruppen in vernetztes Polystyrol

Ohne Zusammenfassung     

Electronic and vibrational second-order nonlinear optical properties of protein secondary structural motifs

A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.     

Conformational Study of Naringenin in the Isolated and Solvated States by Semiempirical and Ab Initio Methods

Naringenin is a natural widespread flavanone occurring in different foodstuffs that presents several important biological activities. Although its properties are well documented, its mechanisms of action are still controversial. The present article reports a conformational analysis of naringenin, using the semiempirical AM1 and ab initio methods, at the Hartree–Fock level of theory. The 3-21G, 3-21G*, 6-31G, and 6-31G** basis sets were used. The electron correlation effects were included through the Møller–Plesset second-order perturbation theory. The solvation of naringenin has been investigated through the standard SCRF, the supermolecule (SM), and the combined SM/SCRF models. The results have shown that there are two degenerate forms of naringenin, differing mainly by the orientation of a hydroxyl group (C4—OH). The energy barrier for the interconversion between them is ca. 6 kcal.mol^–1, suggesting some conjugation between the -system of the aromatic B ring and the hydroxyl group (C4—OH).     

Energy transfer between 1,3-dimethylxanthine and europium(III) in aqueous solution

Luminescence of 1,3-dimethylxanthine in solution is impaired owing to collisional deactivation by solvent molecules and matrix interferences. Energy absorbed by 1,3-dimethylxanthine has been transferred to trivalent europium which emits narrow-band radiation in a region distant from background interferences. The enhanced luminescence of trivalent europium was utilized for the determination of 1,3-dimethylxanthine in buffered aqueous solution. An analytically useful range from 1.1 × 10^?5?5.0 × 10^?4 M and a detection limit of 1.1 × 10^?5 M were obtained. The methodology for the analytical procedure was determined.     

An experimental test of the Bernstein—Levine theory of branching ratios

The first test of the information-theoretic approach to branching ratios has been made for the reaction: F + HD å HF2 (V′, R′) + D å DF3(V′, R′) + H.The vibrational (V′) and rotational (R′) product energy-distributions for both branches of this reaction have been obtained by the infrared chemiluminescence technique, and have been used in the calculation of an information-theoretic branching ratio, Γ_HF/DF = 1.41 ± 0.18. This is in excellent agreement with the experimentally measured branching ratio of 1.45. However, results from classical trajectory calculations raise a question as to the significance of this agreement. Classical trajectory calculations (on various energy-surfaces) predict an increase in Γ with reagent J. The information-theoretic analysis applied to the product energy-distributions from these trajectory calculations leads to a qualitatively different result. As a possible alternative to the information-theoretic view, simple kinematic features are noted which could account for Γ > 1, as well as for the significant differences in product energy-distribution. On this alternative view, the two features are not indisolubly linked — the extent to which they appear in conjunction will depend on the nature of the energy surface.