Adding water to sugar: a spectroscopic and computational study of alpha- and beta-phenylxyloside in the gas phase

The gas phase structures of phenyl alpha- and beta-d-xylopyranoside (alpha- and beta-pXyl) and their mono-hydrates have been investigated using a combination of resonant two-photon ionization (R2PI), ultra-violet hole-burning and resonant infrared ion dip spectroscopy, coupled with density functional theory (DFT) and ab initio computation. The hole-burning experiments indicate the population of a single conformer only, in each of the two anomers. Their experimental and calculated infrared spectra are both consistent with a conformational assignment corresponding to the computed global minimum configuration. All three OH groups are oriented towards the oxygen atom (O1) on the anomeric carbon atom to form an all trans(ttt) counter-clockwise chain of hydrogen bonds. The mono-hydrates, alpha- and beta-pXyl(H(2)O) each populate two distinct structures in the molecular beam environment, with the water molecule inserted between OH4 and OH3 or between OH3 and OH2 in alpha-pXyl(H2O), and between OH2 and O1 in either of two alternative orientations, in beta-pXyl(H2O). In all of the mono-hydrated xyloside complexes, the water molecule inserts into the weakest link of the sugar molecules' hydrogen-bonded chain of hydroxy groups, creating a single extended chain, strengthened by co-operativity. The all-trans configuration of the xylose moiety is retained and the mono-hydrate structures correspond to those calculated to lie at the lowest relative energies.     

Hydrogen bonding in polymers: III further infrared temperature studies of polyamides

Fourier transform infrared temperature studies of two semi-crystalline polyamides, nylon 12 and a terpolymer of nylon 6/66/610 are presented. The results obtained are consistent with those recently published from similar studies of an amorphous polyamide and nylon 11 and further bolster our general interpretation of the infrared spectra of polyamides.     

FTIR studies of calcium and sodium ionomers derived from an ethylene–methacrylic acid copolymer

A systematic study of the sodium and calcium salts of an ethylene–methacrylic acid copolymer is reported. Fourier-transform infrared spectroscopy (in the midinfrared region) is applied to the characterization of structural changes as a function of temperature and time of annealing. In the spectra of calcium ionomers, bands associated with carboxylate dimers are identified and assignments of specific spectral features to multiplets and clusters are discussed. The spectroscopic changes observed in the spectra of sodium ionomers differ somewhat from their calcium counterparts in that a single infrared band attributed to isolated carboxylate groups is observed. Assignments of specific bands to multiplets and clusters can, however, be made in a manner consistent with the interpretation of the spectra of calcium ionomers.     

Substrate interactions, effects of symmetry breaking, and convection in a 2D horizontally shaken granular system

We describe experiments on a horizontally shaken [x = Asin(omegat)] single layer of hard spheres rolling on a nearly horizontal surface. We identify a novel substrate-mediated convective flow which occurs when the system is tilted slightly so that the weak gravitational force, g-->(eff), acting on the particles is not parallel to the driving direction. As the shaking amplitude is increased, the system progresses through four regimes: solid-flat, solid-inclined, convective, and disordered. The control parameter is the driving velocity, Aomega, rather than the usual Aomega(2) of vertically shaken 3D systems. At the onset of convection, the critical velocity is V(c) approximately sqrt[2g(eff)d].     

Intramolecular hydrogen bonding and calixarene-like structures in p-cresol/formaldehyde resins

The nature of the strong hydrogen bonds found in p-cresol/formaldehyde (PCF) resins, compared to ordinary phenolic compounds, is studied. The evidence from FTIR spectroscopy indicates that this strong interaction is due to intramolecular hydrogen bonding from “calixarene-like” structures. The formation of this structure in PCF is enabled by its “linear” (all-ortho-linkage) structure, which is not present in branched resins. Additionally, a transition is observed at around 175 to 200°C where the intramolecular hydrogen bonded structure is lost. This structure cannot be recovered upon cooling or annealing due to restrictions on conformational rotations that are coupled to a new pattern of intermolecular hydrogen bonding. However, the structure is reformed by dissolving the resin in solution and casting new films.     

Towards aflatoxins: a formal synthesis of aflatoxin B2

The development of a formal synthesis of aflatoxin B2 is described, which utilizes a Dötz benzannulation reaction as a key step.