2,5-Dimethoxy-1,4-bis[2-(2,4-dimethoxyphenyl) ethenyl]benzene studied by quantum chemical calculations and single crystal X-ray diffraction

The configurational isomers of 2,5-dimethoxy-1,4-bis-[2-(2,4-dimethoxyphenyl)ethenyl]benzene have been investigated by ab initio (4-21G level) and MOPAC-AM1 semiempirical methods. The calculations were guided by and compared with single crystal X-ray results of the trans,trans-isomer determined at 298 K and 103 K. The barriers of rotation of the latter isomer in the free and the solid state were enumerated. A comparison with related molecules, in particular 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene, is made. The positioning of substituents is shown to be of importance for ring rotational movements and NMR shifts, as well as for UV /Vis spectroscopy. © 1996 by John Wiley & Sons, Inc.     

On the use of common effective core potentials in density functional calculations. I. Test calculations on transition-metal carbonyls

The performance of effective core potentials adjusted at the Hartree-Fock level but applied in density functional calculations has been tested in a set of calculations using various basis sets and/or core potentials. Test molecules have been the first-row transition-metal carbonyls Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ and the second-row carbonyls Mo(CO)₆, Ru(CO)₅, and Pd(CO)₄. Only “small-core” potentials have been used, and these are able to reproduce molecular structures and bond energies from all-electron calculations. Relativistic effects have been estimated for the second-row carbonyls by using quasi-relativistic core potentials. © 1996 John Wiley & Sons, Inc.     

“Titration” polymerization of monovinylacetylene

A polymer consisting of a saturated carbon backbone with pendent acetylenic groups was prepared from monovinylacetylene. A titration was performed between the monomer and tertiary butyllithium, its lithiating agent. The charge transfer complex formed between the solvent THF and the tertiary butyllithium was used as an indicator of the unreacted butyllithium. Hence, a stoichiometric quantity of tertiary butyllithium was added dropwise to a solution of monovinylacetylene in THF to form lithiovinylacetylene. The addition of a slight excess of butyllithium led to the polymerization of the lithiated monomer. The obtained polymer was reprotonated and characterized. This polymerization was evaluated as a possible route to synthesize poly(vinylacetylene) with processable molecular weights, for its application as a potential carbon fiber precursor. © 1996 John Wiley & Sons, Inc.     

Nonlinear optical polymers. Second harmonic generation in corona-poled thin films

The synthesis and second harmonic coefficients, d_3,1 and d_3,3 as well as the related susceptibilities χ⁽²⁾_zzz of five series of (NLO-dye methacrylate)-(methyl methacrylate) copolymers were investigated. The NLO-chromophores bound covalently to the polymer backbone were 5-(2,2-dicyanovinyl)-or 4-(2-cyano-2-methoxycarbonyl)vinyl-1-piperidino-2-thiophene (P₁ and P₂), 4-nitro-4′-alkoxy-stilbene (P₃), 4-nitro-3′-methoxy-4′-alkoxystilbene (P₄) and 4-nitro-4′-alkoxy-α-cyano stilbene (P₅). The second order nonlinear optical properties of corona-poled aligned thin polymer films, using a needle electrode in order to induce noncentrosymmetry, were evaluated. Nonlinear susceptibilities, χ⁽²⁾_zzz, of the films were derived from the analysis of full-angle Maker fringe patterns at 1064 nm, χ⁽²⁾_zzz values as high as 1.98×10⁻⁷ esu for P₂ copolymers and of 1.19×10⁻⁷ esu for P₃ copolymers could be achieved.     

Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50–300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles (‘holes’). The value of the critical matrix ligament thickness (ID_c) uniquely depends on the molecular structure: at an increasing network density, ID_c increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency.     

Antimicrobial Polyketide Metabolites from Penicillium bissettii and P. glabrum

Screening of several fungi from the New Zealand International Collection of Microorganisms from Plants identified two strains of Penicillium, P. bissettii and P. glabrum, which exhibited antimicrobial activity against Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus. Further investigation into the natural products of the fungi, through extraction and fractionation, led to the isolation of five known polyketide metabolites, penicillic acid (1), citromycetin (2), penialdin A (3), penialdin F (4), and myxotrichin B (5). Semi-synthetic derivatization of 1 led to the discovery of a novel dihydro (1a) derivative that provided evidence for the existence of the much-speculated open-chained form of 1. Upon investigation of the antimicrobial activities of the natural products and derivatives, both penicillic acid (1) and penialdin F (4) were found to inhibit the growth of Methicillin-resistant S. aureus. Penialdin F (4) was also found to have some inhibitory activity against Mycobacterium abscessus and M. marinum along with citromycetin (2).     

Monte Carlo simulation of the chain length distribution in pulsed-laser polymerization experiments in microemulsion

The Monte Carlo method has been used for numerically simulating pulsed-laser polymerization (PLP) in microemulsion, in order to establish if a shift from inflection point to peak maximum as the best measure of the propagation rate constant, k_p, will occur theoretically. Termination is assumed to be instantaneous in the simulations as droplet sizes can be very small in microemulsions. From the results of the simulations it is found that instantaneous termination indeed causes the peak maximum to become the best measure of k_p. From these results it can be deduced that in bulk it is not simply the Poisson-broadening that causes the peak maximum to yield an overestimation of k_p. This overestimation is rather caused by the fact that the termination rate is finite leading to an asymmetrical peak in the molecular weight distribution. In combination with broadening this yields the inflection point to be the best measure of k_p in the bulk.     

NMR measurement of the surface dynamics of poly(dimethylsiloxane) adsorbed on silica gel

CPMAS-DD ¹³C NMR spectroscopy was used to examine the mobility of poly(dimethylsiloxane) adsorbed on silica gel (PDMS/SiO₂) at submonolayer coverages. The spin-lattice relaxation time in the rotating frame (T_1ρH) decreased linearly with increasing loading. This is consistent with a decrease in the mobility of the polymer segments as the loading is increased. The decrease in mobility results from interpolymer interference. We propose a model that explains these results in terms of a surface intrinsic viscosity that incorporates the polymer-polymer interactions on the surface.     

Sturmian Ground States in Classical Lattice–Gas Models

Journal of Statistical Physics - We construct for the first time examples of non-frustrated, two-body, infinite-range, one-dimensional classical lattice–gas models without periodic...