Multiphonon vibrational relaxation in liquids: should it lead to an exponential-gap law?

The profound differences between solids and liquids notwithstanding, high-frequency vibrational energy relaxation in liquids seems to be well described by assuming that the excess energy is being transferred into discrete overtones of some fundamental intermolecular vibrations-precisely the way it is in crystalline solids. In a solid-state context, this kind of analysis can be used to justify the observation that relaxation rates fall off exponentially with the energy being transferred. Liquids, however, have a substantial degree of disorder, causing their relevant intermolecular spectra to have correspondingly diffuse band edges and large bandwidths. It is therefore not at all obvious what should become of this exponential-gap-law phenomenology. We show in this paper how near exponential-gap-law behavior can still be derived for vibrational energy relaxation in liquids. To do so, we take advantage of the simple dynamics that the high-frequency relaxation has when it is launched from an individual instantaneous configuration. Interestingly, the physically relevant region turns out not to be true asymptotic limit of our formalism, but for realistic liquid parameters the behavior in the physical regime differs only slightly from an exact exponential-gap law and is strikingly independent of the details of the intermolecular spectra.     

The stereoselective synthesis of novel macrolide antibacterial agents via an intramolecular 1,3-dipolar cycloaddition of azomethine ylide

An intramolecular 1,3-dipolar cycloaddition of azomethine ylide, generated in situ via the reaction of C12-glycinate derivative of macrolide with formaldehyde, provided a novel tricyclic macrolide. The high stereoselectivity of this [2+3] reaction was achieved by introducing a suitable directing group at C-6 position of macrolide.     

An ESR study of some group IVB free radical adducts of diethylketomalonate

The formation of the compound RSnX(acac)₂ (acac = 2,4-pentanedionato) by reaction of bis(2,4-pentanedionato)tin(II) on a halide RX with R = CH₃, C₂H₅, C₄H₉, C₆H₅, CH₂I, (C₆H₅)₃SnCH₂, (C₂H₅)₃SnCH₂ and X = I, Br has been studied by polarography. At 25°C, it is in fact an equilibrium whose constant has been measured. The intermediate formation of the ion-pair [RSn(acac)₂⁺X^?] has allowed us to explain the experimental results.     

Synthesis and polymerisation of α,ω-bis(3-pyrrolyl)alkanes

The synthesis, characterisation and polymerisation studies of a homologous series of α,ω-bis(pyrrolyl)alkanes are described. These α,ω-bis(pyrrolyl)alkanes were produced using Friedel-Crafts acylation followed by reduction of the carbonyl group using Red-Al^®. Chemical polymerisation of the resultant dimers using FeCl₃ produced poly(α,ω-bis(pyrrolyl)alkane) films, which were characterised by SEM, FTIR and tested for conductivity.     

Reaction Engineering Aspects of α-l,4-D-Glucan Phosphorylase Catalysis

Some important process properties of α-l,4-D-ghican phosphorylases isolated from the bacteriumCorynebacterium callunae and potato tubers (Solatium tuberosum) were compared. Apart from minor differences in their stability and specificity (represented by the maximum degree of maltodextrin conversion) and a 10-fold higher affinity of the plant phosphorylase for maltodextrin (K _M of 1.3 g/L at 300 mM of orthophosphate), the performances of both enzymes in a continuous ultrafiltration membrane reactor were almost identical. Product synthesis was carried out over a time course of 300–400 h in the presence or absence of auxiliary pullulanase (increasing the accessibility of the glucan substrate for phosphorolytic attack up to 15–20%). The effect of varied dilution rate and reaction temperature on the resulting productivities was quantitated, and a maximum operational temperature of 40°C was identified.     

Dihydrobenzoxazines and tetrahydroquinoxalines by a tandem reduction‐reductive amination reaction

A tandem reduction‐reductive amination reaction has been applied to the synthesis of 3,4‐dihydro‐2H‐1,4‐benzoxazines and 1‐acetyl‐1,2,3,4‐tetrahydroquinoxalines. The nitroketones required for the benzoxazine ring closures were prepared by (A) alkylation of the anion derived from 2‐nitrophenol with an allylic halide or (B) nucleophilic aromatic substitution of an allylic alkoxide on 2‐fluoro‐1‐nitrobenzene followed by ozonolysis. Precursors for the quinoxalines were prepared by alkylation of the anion of 2‐nitroacetanilide with an allylic halide followed by ozonolysis. Catalytic hydrogenation of the nitroketones using 5% palladium‐on‐carbon in methanol then gave the target heterocycles by a reduction‐reductive amination sequence. The N‐methyl derivatives for both ring systems were easily prepared by adding 5‐10 equivalents of aqueous formaldehyde prior to the reduction. The dihydrobenzoxazines were isolated in high yield following purification by chromatographic methods; tetrahydroquinoxalines were isolated in a similar manner and possessed differentiated functionality on the two nitrogens.     

On-line determination of organophosphorus pesticides in water by solid-phase microextraction and gas chromatography with thermionic-selective detection

This paper describes the extraction of 49 organophosphorus pesti-cides (OPPs) from water samples using solid-phase microextraction (SPME). Three fibers, including a 15-μm XAD-coated fiber, a 85-μm polyacrylate-coated fiber, and a 30-μm polydimethylsilox-ane-coated fiber (PDMS), were evaluated here. The effects of stirring and the addition of NaCl to the sample were examined for the polyacrylate-coated fiber. The precision of the technique was examined for all three fibers and the extraction kinetics were investigated using the XAD- and polyacrylate-coated fibers. With some exceptions, the XAD- and polyacrylate-coated fibers performed better than the PDMS-coated fiber. The superiority of the XAD-nd polyacrylate-coated fiber. The superiority of the XAD- and polyacrylate-coated fibers over the PDMS-coated fibers can be attribuibuted to the aromatic functionalities of the XAD and the polar functionalities in the polyacrylate. The relatively high percent RSDs indicate that the SPME technique needs to be further refined before it can be used for anything other than screening. A more effective form of agitation than mechanical stirring may be neccessary to reduce variability and achieve a faster equilibrium between the sample and the SPME fiber.