Gas chromatographic/mass spectrometric determination of 3-methoxy-1,2-propanediol and cyclic diglycerols, by-products of technical glycerol, in wine: interlaboratory study

The aim of the present study was to provide the official wine control authorities with an internationally validated method for the determination of 3-methoxy-1,2-propanediol (3-MPD) and cyclic diglycerols (CycDs)-both of which are recognized as impurities of technical glycerol-in different types of wine. Because glycerol gives a sweet flavor to wine and contributes to its full-body taste, an economic incentive is to add glycerol to a wine to mask its poor quality. Furthermore, it is known that glycerol, depending on whether it is produced from triglycerides or petrochemicals, may contain considerable amounts of 3-MPD in the first case or CycDs in the second. However, because these compounds are not natural wine components, it is possible to detect glycerol added to wine illegally by determining the above-mentioned by-products. To this end, one of the published methods was adopted, modified, and tested in a collaborative study. The method is based on gas chromatographic/mass spectrometric analysis of diethyl ether extracts after salting out with potassium carbonate. The interlaboratory study for the determination of 3-MPD and CycDs in wine was performed in 11 laboratories in 4 countries. Wine samples were prepared and sent to participants as 5 blind duplicate test materials and 1 single test material. The concentrations covered ranges of 0.1-0.8 mg/L for 3-MPD and 0.5-1.5 mg/L for CycDs. The precision of the method was within the range predicted by the Horwitz equation. HORRAT values obtained for 3-MPD ranged from 0.8 to 1.7, and those obtained for CycDs ranged from 0.9 to 1.3. Average recoveries were 104 and 109%, respectively.     

Reductive activation of nitrate reductases

Protein film voltammetry of Paracoccus pantotrophus respiratory nitrate reductase (NarGH) and Synechococcus elongatus assimilatory nitrate reductase (NarB) shows that reductive activation of these enzymes may be required before steady state catalysis is observed. For NarGH complementary spectroscopic studies suggest a structural context for the activation. Catalytic protein film voltammetry at a range of temperatures has allowed quantitation of the activation energies for nitrate reduction. For NarGH with an operating potential of ca. 0.05 V the activation energy of ca. 35 kJ mol-1 is over twice that measured for NarB whose operating potential is ca. -0.35 V.     

Bi(III) halides as efficient catalysts for the O-acylative cleavage of tetrahydrofurans: an expeditious entry to tetralins

The mild (DCM/20 °C), quantitative, regioselective, O-acylative cleavage of tetrahydrofurans using organic acid halides with catalytic Bi(III) halides is reported. X-ray crystallography is used to rationalise the failure of the reaction in the case of certain crowded acid chlorides, and a useful aspect of chemoselectivity is revealed. The synthetic potential of this reaction is illustrated with a highly efficient O-acylative cleavage/intramolecular alkylation approach to tetralins.     

Nonadiabatic alignment of asymmetric top molecules: rotational revivals

The rotational revival structure of asymmetric top molecules, following irradiation by an intense picosecond laser pulse, is explored theoretically and experimentally. Numerically we solve nonperturbatively for the rotational dynamics of a general asymmetric top subject to a linearly polarized intense pulse, and analyze the dependence of the dynamical alignment on the field and system parameters. Experimentally we use time-resolved photofragment imaging to measure the alignment of two molecules with different asymmetry, iodobenzene, and iodopentafluorobenzene. Our numerical results explain the experimental observations and generalize them to other molecules. The rotational revival structure of asymmetric tops differs qualitatively from the intensively studied linear top case. Potentially it provides valuable structural information about molecules.     

Pentalene complexes of group 7 metal carbonyls: an organometallic mixed-valence system with very large metal-metal electronic coupling

The bimetallic complexes [M(CO)(3)](2)(mu:eta(5):eta(5)-Pn) (Pn = pentalene, C(8)H(6); M = Mn, Re) have been synthesized and characterized crystallographically; the Mn compound was isolated as solely the anti-isomer, while the Re analogue was formed as a mixture of anti- and syn-isomers. [Mn(CO)(3)](2)(mu:eta(5):eta(5)-Pn) may be reduced chemically to its mono- and dianions; the mixed-valence Mn(I)/Mn(0) monoanion is shown by ESR, vibrational, and electronic spectroscopies to be a Robin-Day class III system with an exceptionally large electronic coupling between the metal centers.     

Influence of structural changes on the ion selectivities of magnesium ionophores based on malonic acid diamides

In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992     

Hydroxy peroxy nitrites and nitrates from OH initiated reactions of isoprene

The reaction of hydroxy peroxy radicals (RO(2)) with NO represents one of the most crucial tropospheric processes, leading to terrestrial ozone formation or NO(x)() removal and chain termination. We investigate the formation of hydroxy peroxy nitrites (ROONO) and nitrates (RONO(2)) from the OH-isoprene reactions using DFT and ab initio theories and variational RRKM/master equation (vRRKM/ME) formalism. The binding energies of ROONO from NO addition to RO(2) are determined to be in the range of 20-22 kcal mol(-)(1), and the bond dissociation energies of ROONO to form an alkoxy radical (RO) and NO(2) range from 6 to 9 kcal mol(-)(1). Isomerization of ROONO to RONO(2) is exothermic by 22-28 kcal mol(-)(1). The entrance and exit channels of the RO(2)-NO reaction are found to be barrierless, and the rate constants to form ROONO are calculated to be 3 x 10(-)(12) to 2 x 10(-)(11) cm(3) molecule(-)(1) s(-)(1) using the canonical variational transition state theory. The vRRKM/ME analysis reveals negligible stabilization of excited ROONO and provides an assessment of ROONO isomerization to RONO(2).     

Optimized ensemble Monte Carlo simulations of dense Lennard-Jones fluids

We apply the recently developed adaptive ensemble optimization technique to simulate dense Lennard-Jones fluids and a particle-solvent model by broad-histogram Monte Carlo techniques. Equilibration of the simulated fluid is improved by sampling an optimized histogram in radial coordinates that shifts statistical weight towards the entropic barriers between the shells of the liquid. Interstitial states in the vicinity of these barriers are identified with unprecedented accuracy by sharp signatures in the quickly converging histogram and measurements of the local diffusivity. The radial distribution function and potential of mean force are calculated to high precision.     

Magnetic coupling and intermetallic electron transfer in the heterodinuclear bioctahedral complexes MW(III)Cl(9)(n-) (M = V(II), Cr(III), Mn(IV)): tweaking the balance between ferromagnetism and antiferromagnetism

Density functional theory (DFT) calculations have been used to investigate the effect of intermetallic electron transfer on the mode of magnetic coupling in the face-shared bimetallic complexes MWCl(9)(n-) (M = V, Cr, Mn; all with a nominal d(3) valence electronic configuration on each metal atom). These calculations illustrate a simple rule: when the oxidation state of M is lower than that of W, antiferromagnetic coupling is preferred, while ferromagnetism (via crossed exchange pathways) is favored when M has the higher oxidation state. This underlying trend in intermetallic interactions is seen to depend on the interplay among ligand field splitting, spin polarization splitting of alpha- and beta-spin orbitals, and the relative energies of the M and W valence d orbitals, and is mirrored in the results seen in a wider survey of mixed-metal, face-shared complexes.     

Vergleich der Me?genauigkeit der Szintillations-Z?hlung 3H- und/oder 14C-markierter Proben durch externe bzw. interne Standardisierung und Anwendung auf die Sauerstoff-Kolben-Verbrennung

Zusammenfassung Nach Verbrennung im Sauerstoffkolben zeigen die Szintillationsproben häufig über Stunden eine ansteigende Zählrate. Auch danach unterscheiden sich identisch präparierte Proben noch statistisch signifikant stärker, als aufgrund von Wäge- und Pipettierfehlern zu erwarten ist. Die zeitliche Inkonstanz und die Unterschiede von Probe zu Probe lassen sich durch Löschkorrekturkurven (Abhängigkeit der Zählausbeute und des externen Standardkanalverhältnisses von der Sauerstoffkonzentration) weitgehend eliminieren. Die so erhaltenen dpm-Werte haben geringere Standardabweichungen als die durch interne Standardisierung erhaltenen. Das Verfahren der externen Standardisierung wird einer Fehleranalyse für einzel- und doppelmarkierte Proben unterworfen. Ein Toluol/Methanol/Phenäthylamin-System absorbiert Kohlendioxid und Wasser quantitativ. Für einzelmarkierte Proben können mit diesem Gemisch für ³H 25% und für ¹⁴C 85% Zählausbeute erhalten werden. Bei ³H/¹⁴C-Markierung werden bei einer Kanaloptimierung nach Klein und Eisler noch 15% Zählausbeute für ³H neben 10% für ¹⁴C erhalten.

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Comparison of errors in liquid scintillation counting after quench correction by external or internal standardisation applied for oxygen flask combustion of ³H- and/or ¹⁴C-labelled samples

After oxygen flask combustion scintillation samples often show an increase in counting rates for several hours. Even after such a time identically prepared samples are statistically significantly more different as one should expect by weighing and pipetting errors. The inconstant cpm values during the first hours and the differences between the samples can mostly be eliminated by quench correction curves (dependence of counting efficiency and external channel ratio upon oxygen concentration). The dpm values achieved according to this method show smaller standard deviations than by internal standardisation. The procedure of the external standardisation was examined for error propagation. A system of toluene/methanol/phenethylamine absorbs carbon dioxide and water quantitatively. With this mixture the following counting efficiencies are achieved: 25% for ³H, 85% for ¹⁴C and for samples containing ³H and ¹⁴C 15% for ³H and 10% for ¹⁴C if the ³H channel is adjusted according to Klein and Eisler.

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Symbole und Abkürzungen ESKV Externes Standard-Kanal-Verhältnis - ESKV-ESKV₀ - ESKV₀ Beginn der Gültigkeit der Löschfunktion, stärkst gelöschte Probe - ZA auf 1,0 bezogene Zählausbeute - S Standardabweichung - cpm Impulsrate (min^–1) - dpm Zerfallsrate (min^–1) - A bzw. B nieder- bzw. höherenergetisches Nuklid - (A) Meßkanal A - k,l,m,n Eichfaktoren