Propagation of single-cycle terahertz pulses in random media

We describe what are to our knowledge the first measurements of the propagation of coherent, single-cycle pulses of terahertz radiation in a scattering medium. By measuring the transmission as a function of the length L of the medium, we extract the scattering mean free path l(s)(omega) over a broad bandwidth. We observe variations in l(s) ranging over nearly 2 orders of magnitude and covering the entire thin sample regime from L/l(s)<1 to L/l(s)~10 . We also observe scattering-induced dispersive effects, which can be attributed to the additional path traveled by photons scattered at small angles.     

Thermal degradation study of phenolphthalein polycarbonate

Phenolphthalein polycarbonate underwent complicated thermal degradation which included random scission, rearrangement, hydrolysis, Friedel-Crafts acylation, and cross-linking. The carbonate group and lactone ring were both susceptible to thermal deterioration. Kinetic parameters were determined from the dynamic TGA thermograms. During early stages of degradation the measured reaction order was nearly 1, which suggested a random chain scission mechanism. The measured activation energy was 42.6 kcal/mol, compared with 41.2 kcal/mol calculated from isothermal aging. The Arrhenius preexponential constant was 3.09 × 10¹¹ min^?1. Below 80% weight residue the plot of fractional weight against 1/T revealed that complicated reactions with different activation energies occurred simultaneously and resulted in a final overlap of TGA curves for different heating rates indicative of cross-linking and a lower preexponential constant. The reaction order changed and kept increasing in the last stages of degradation. Pyrolysis of this polymer was performed at 350°C under vacuum, followed by GC-mass spectroscopic identification of products. The volatile products (17.5%) contained CO₂, CO, O₂, H₂O, phenol, fluorenone, diphenyl carbonate, xanthone, anthraquinone, 2-hydroxylanthraquinone, 2-benzoxyanthraquinone, phenolphthalein, and trace amounts of benzoxyphenol and hydroquinone; the other 82.5% of products was insoluble gel. Functional group changes were examined by Fourier transform infrared spectroscopy (FT-IR). Lactone, carbonate, and aromatic absorptions decreased during degradation. Increasing absorptions at 1739, 1728, 1280–1200, and 1138–1075 cm^?1 were believed to result from aromatic ester (1728 cm^?1) and phenyl aromatic ester (1739 cm^?1) cross-linkages ortho to the aromatic ether group (increases at 1155 cm^?1 and 1280–1200 cm^?1). Existence of 2-hydroxyanthraquinone and xanthone contained in the crosslinked polymer matrix were also detected. Mechanisms for random scission, rearrangement, Friedel-Crafts acylation, hydrolysis, and cross-linking were suggested.     

Blends of nylon‐6 with phenol‐containing polymers

The miscibility of nylon‐6 with poly(4‐vinylphenol) (PVPh) or poly(1‐hydroxy‐2,6‐methylphenylene) (p‐Cl‐novolac) was studied with differential scanning calorimetry and small‐angle X‐ray scattering techniques. Both PVPh and p‐Cl‐novolac are miscible with nylon‐6 at the molecular level. The presence of the phenolic polymers affects the crystallization of nylon‐6 and suppresses its melting point. PVPh increases the long space order in crystalline nylon‐6 because it increases the thickness of the amorphous layers. In contrast, a small quantity of p‐Cl‐novolac tends to decrease the long space order. It seems that p‐Cl‐novolac distributed in the amorphous regions introduces more order in these regions and makes the amorphous layers thinner. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 841–850, 2001     

Observation and mechanistic study of facile C-O bond formation between a well-defined aryl-copper(III) complex and oxygen nucleophiles

A well-defined macrocyclic aryl–Cu(III) complex (2) reacts readily with a variety of oxygen nucleophiles, including carboxylic acids, phenols and alcohols, under mild conditions to form the corresponding aryl esters, biaryl ethers and alkyl aryl ethers. The relationship between these reactions and catalytic C-O coupling methods is demonstrated by the reaction of the macrocyclic aryl–Br species with acetic acid and p-fluorophenol in the presence of 10 mol% Cu(I). An aryl-Cu(III)-Br species 2(Br) was observed as an intermediate in the catalytic reaction. Investigation of the stoichiometric C-O bond-forming reactions revealed nucleophile-dependent changes in the mechanism. The reaction of 2 with carboxylic acids revealed a positive correlation between the log(k(obs)) and the pK(a) of the nucleophile (less-acidic nucleophiles react more rapidly), whereas a negative correlation was observed with most phenols (more-acidic phenols react more rapidly). The latter trend resembles previous observations with nitrogen nucleophiles. With carboxylic acids and acidic phenols, UV-visible spectroscopic data support the formation of a ground-state adduct between 2 and the oxygen nucleophile. Collectively, kinetic and spectroscopic data support a unified mechanism for aryl-O coupling from the Cu(III) complex, consisting of nucleophile coordination to the Cu(III) center, deprotonation of the coordinated nucleophile, and C-O (or C-N) reductive elimination from Cu(III).     

Preparation of Sec-Alkyl Isouronium Salts

Eight examples of sec-alkyl isouronium salts were readily prepared from sec-alcohols, cynamide, and methanesulfonic acid (or CF₃SO₃H or HBF₄).     

Synthesis of 2,6-disubstituted dihydropyrans via an efficient BiBr3-initiated three component, one-pot cascade

The rapid synthesis of cis-2,6-disubstituted dihydropyrans is achieved in a three-component, one-pot cascade reaction. BiBr₃-mediated addition of ketene silyl acetals or silyl enol ethers to β,γ-unsaturated cis-4-trimethylsilyl-3-butenal provides a Mukaiyama aldol adduct containing a vinylsilane moiety tethered to a silyl ether. Addition of a second aldehyde initiates a domino sequence involving intermolecular addition followed by an intramolecular silyl-modified Sakurai (ISMS) reaction. Isolated yields of this one-pot reaction vary from 44 to 80% and all compounds were isolated as the cis-diastereomers (10 examples).     

Identification of Acepromazine and Its Metabolites in Horse Plasma and Urine by LC–MS/MS and Accurate Mass Measurement

Acepromazine maleate (Sedalin^?) was administered orally to six thoroughbred horses at a dose of 0.15?mg?kg^?1. Urine and blood samples were collected up to 412?h post-administration. Plasma and urine were hydrolysed; plasma samples were then processed using liquid–liquid extraction and urine samples using solid-phase extraction. A sensitive tandem mass spectrometric method was developed in this study, achieving a lower limit of quantification for acepromazine of 10?pg?mL^?1 in plasma and 100?pg?mL^?1 in urine. Acepromazine, hydroxyethylpromazine, hydroxyacepromazine, hydroxyethylpromazine sulphoxide, hydroxyethylhydroxypromazine, dihydroxyacepromazine and dihydroxyhydroxyethylpromazine were detected in the post-administration samples. The parent drug and its metabolites were identified using a combination of UPLC–MS/MS and accurate mass measurement. Separation of the structural isomers hydroxyethylpromazine sulphoxide and hydroxyethylhydroxypromazine was another significant outcome of this work and demonstrated the advantages to be gained from investing in chromatographic method development.     

Characterization of single- and double-stranded DNA on gold surfaces

Single- and double-stranded deoxy ribonucleic acid (DNA) molecules attached to self-assembled monolayers (SAMs) on gold surfaces were characterized by a number of optical and electronic spectroscopic techniques. The DNA-modified gold surfaces were prepared through the self-assembly of 6-mercapto-1-hexanol and 5'-C(6)H(12)SH -modified single-stranded DNA (ssDNA). Upon hybridization of the surface-bound probe ssDNA with its complimentary target, formation of double-stranded DNA (dsDNA) on the gold surface is observed and in a competing process, probe ssDNA is desorbed from the gold surface. The competition between hybridization of ssDNA with its complimentary target and ssDNA probe desorption from the gold surface has been investigated in this paper using X-ray photoelectron spectroscopy, chronocoulometry, fluorescence, and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS). The formation of dsDNA on the surface was identified by PM-IRRAS by a dsDNA IR signature at approximately 1678 cm(-)(1) that was confirmed by density functional theory calculations of the nucleotides and the nucleotides' base pairs. The presence of dsDNA through the specific DNA hybridization was additionally confirmed by atomic force microscopy through colloidal gold nanoparticle labeling of the target ssDNA. Using these methods, strand loss was observed even for DNA hybridization performed at 25 degrees C for the DNA monolayers studied here consisting of attachment to the gold surfaces by single Au-S bonds. This finding has significant consequence for the application of SAM technology in the detection of oligonucleotide hybridization on gold surfaces.     

Observations of crystallization and melting in poly(ethylene oxide)/poly(methyl methacrylate) blends by hot-stage atomic-force microscopy

The binary blend of poly(ethylene oxide)/atactic poly(methyl methacrylate) is examined using hot-stage atomic-force microscopy (AFM) in conjunction with differential scanning calorimetry and optical microscopy. It was found possible to follow in real time the melting process, which reveals itself to be nonuniform. This effect is ascribed to the presence of lamellae having different thicknesses. The crystallization process of poly(ethylene oxide) from the miscible melt is also followed in real time by AFM, affording detailed images of the impingement of adjacent spherulites and direct observation of lamellar growth and subsequent polymer solidification in the interlamellar space.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2643–2651, 1998