Synthesis and properties of crosslinked polyvinylformamide and polyvinylamine hydrogels in conjunction with silica particles

Polyvinylamine hydrogels with silica particles encapsulated (PVAm/silica) were produced by a two‐step synthesis. In the first step, polyvinylformamide/silica (PVFA/silica) hybrids were synthesized from vinylformamide (VFA) and 1,3‐divinylimidazolidin‐2‐one (1,3‐bisvinylethyleneurea, BVU), as the crosslinker, by radical copolymerization in silica/water suspensions using different compositions of VFA/BVU. The target product PVAm/silica was obtained by acidic hydrolysis of the PVFA/silica hydrogels in a second step. The chemical structures of both hydrogels, PVFA/silica and PVAm/silica, respectively, were revealed by solid‐state ¹³C(¹H) cross‐polarity/magic‐angle spinning NMR spectroscopy. Both hydrogels swelled significantly in water. The swelling capacity of the two systems was characterized by the correlation length ξ (or hydrodynamic blob size) of the network meshes with small‐angle neutron scattering experiments. ξ is significantly larger for PVAm/silica than for PVFA/silica, which corresponds to the observed higher swelling capacity of this polyelectrolyte material. Furthermore, the swelling behavior of the hybrid hydrogels was quantitatively described in terms of free swell capacity, centrifuge‐retention capacity, adsorption against pressure, and free swell rate as compared with values of the corresponding copolymer hydrogels. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3144–3152, 2002     

Recent approaches towards the asymmetric synthesis of alpha,alpha-disubstituted alpha-amino acids

The class of alpha,alpha-disubstituted alpha-amino acids has gained considerable attention in the past decades and continues doing so. The ongoing interest in biological and chemical properties of the substance class has inspired the development of many new methodologies for their asymmetric construction, which have not found their way into the general focus of organic chemistry yet. The aim of this review is to provide an overview of the developments in the field since 1998.     

Zeolite synthesis: an energetic perspective

Taking |D(H(2)O)(x)|[AlSiO(4)] based materials (where D is Li, Na, K, Rb or Cs) as an archetypal aluminosilicate system, we use accurate density functional theory calculations to demonstrate how the substitution of silicon cations in silica, with pairs of aluminium and (alkali metal) cations, changes the energetic ordering of different competing structure-types. For large alkali metal cations we further show that the formation of porous aluminosilicate structures, the so-called zeolites, is energetically favored. These findings unequivocally demonstrate that zeolites can be energetic preferred reaction products, rather than being kinetically determined, and that the size of the (hydrated) cations in the pore, be it inorganic or organic, is critical for directing zeolite synthesis.     

The pH dependence of phenolphthalein: A 13c NMR study

The pH dependence of the ¹³C NMR spectra of phenolphthalein has been measured and interpreted. The spectrum of the neutral compound 1 is compared with the spectra of the dianion 4, the trianion5 and the carbocation7. At the first time spectroscopic evidence for the carbinol 6 is given.     

Glycan side chains on naturally presented MHC class II ligands

The molecular characterization of unknown naturally presented major histocompatibility complex (MHC) class II glycopeptides carrying complex glycans has so far not been achieved, reflecting the different fragmentation characteristics of sugars and peptides in mass spectrometric analysis. Human leukocyte antigen (HLA)-DR-bound peptides were isolated by affinity purification, separated via high performance liquid chromatography and analyzed by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. We were able to identify two naturally processed MHC class II ligands, CD53(122-136) and CD53(121-136), carrying complex N-linked glycan side chains by a combination of in-source and collision-induced fragmentation on a quadrupole time-of-flight tandem mass spectrometer.     

Reactivation pathway of the hydrogenase H‐cluster: Density functional theory study

This work puts forth a reaction pathway for the reactivation of exogenous ligand inhibited H‐cluster, the active site of Fe‐only hydrogenases. The H‐cluster is a dimetal complex, Fe–Fe, with the metal centers bridged by di(thiomethyl)amine. Exogenous ligands, H₂O, and OH^?, are bound to the distal iron (Fe_d). Density functional theory (DFT) calculations on the native and ruthenium‐modified H‐cluster have been performed using the B3LYP functional with 6‐31+G** and 6‐311+G** basis sets. We have ascertained that there is a thermodynamically favorable pathway for the reactivation of the OH^? inhibited H‐cluster, which proceeds by an initial protonation of the Fe_d–OH^? complex. The proposed reaction pathway has all its intermediate reactions ensue exothermically. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Relaxation processes in semidilute solutions of polymers in liquid crystal solvents

We investigate the relaxation phenomena in a polymer (polystyrene)/liquid crystal (4-cyano-4'-n-octyl-biphenyl) system, in its homogeneous isotropic phase near the isotropic-isotropic, isotropic-nematic, and isotropic-smectic coexistence curve, using both polarized and depolarized photon correlation spectroscopy (PCS). We study this system for different polystyrene molecular weights (4750, 12 500, and 65 000 g/mol), different compositions (50, 40, 30, and 10% polystyrene (PS) by weight), and different temperatures close to phase boundaries. First of all, we determine the phase diagrams of this system for the different molecular weights. The shape of the phase diagrams strongly depends on the molecular weight. However, in all cases, at low temperatures, these systems separate into an almost pure liquid crystalline (LC) phase and polystyrene-rich phase. PCS measurements show that the relaxation processes in the homogeneous phase are not affected by the proximity of the nematic, or smectic, boundaries (even at a temperature of 0.1 degrees C above the phase separation in two phases). In polarized PCS experiments, we always see three relaxation processes well separated in time: one, very fast, with a relaxation time of the order of 10(-5) s; a second one with a relaxation time within the range 10(-2)-10(-3) s; and a last one, very slow, with a relaxation time of the order of 1 s. Both the fast and slow modes are independent of the wave vector magnitude, while the intermediate relaxation process is diffusive. In depolarized PCS experiments, the intermediate mode disappears and only the fast and slow relaxation processes remain, and they are independent of the magnitude of the wave vector. The diffusive mode is the classical diffusive mode, which is associated with the diffusion of polymer chains in all polymer solutions. The fast mode is due to the rotational diffusion of 4-cyano-4'-n-octyl-biphenyl (8CB) molecules close to polystyrene chains (transient network). Finally, we assign the slowest mode to reorientational processes of small aggregates of PS chains that are not dissolved in 8CB.     

XUV free-electron laser desorption of NO from graphite (0001)

We report results of femtosecond laser induced desorption of NO from highly oriented pyrolytic graphite using XUV photon energies of hν = 38 eV and hν = 57 eV. Femtosecond pulses with a pulse energy of up to 40 μJ and about 30 fs duration generated at FLASH are applied. The desorbed molecules are detected with rovibrational state selectivity by (1 + 1) REMPI in the A(2)Σ(+) ← X(2)Π γ-bands around λ = 226 nm. A nonlinear desorption yield of neutral NO is observed with an exponent of m = 1.4 ± 0.2. At a fluence of about 4 mJ/cm(2) a desorption cross section of σ(1) = (1.1 ± 0.4) × 10(-17) cm(2) is observed, accompanied with a lower one of σ(2) = (2.6 ± 0.3) × 10(-19) cm(2) observable at higher total fluence. A nonthermal rovibrational population distribution is observed with an average rotational energy of = 38.6 meV (311 cm(-1)), a vibrational energy of = 136 meV (1097 cm(-1)) and an electronic energy of = 3.9 meV (31 cm(-1)).     

Oxide- and zeolite-supported molecular metal complexes and clusters: physical characterization and determination of structure, bonding, and metal oxidation state

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.