On Raman optical activity sign-switching between the ground and excited states leading to an unusual resonance ROA induced chirality

Raman optical activity (ROA) spectra recorded for a chiral naphthalene diimide derivative (nBu-NDI–BINAM) dissolved in a series of solvents exhibit strong solute to solvent induced chirality with: (1) dominating bands of solvents, (2) nBu-NDI–BINAM resonance ROA (RROA) bands which are barely visible, (3) monosignate RROA Solvent spectra with an unexpected sign concordant with that of the ECD band of the resonant electronic state, (4) bisignate RROA bands for a few solvents, and (5) superposition of non-resonant and resonant ROA bands of the chiral solvents. The unusual ROA enhancement was explained in terms of resonance energy transfer with resonant Raman emission. The surprising RROA sign-switching was found to be due to specific conformational equilibria where one solute conformer dominates in the ground and the other in the first excited singlet state, and, the signs of the related ECD bands of these two conformers are opposite.

Unusual solute to solvent induced chirality in ROA comes from specific conformer equilibria in the ground and the excited states.     

Synthesis of two glycolipid antigens of the causative agent of Lyme disease

As a prelude to development of a human vaccine against Lyme disease, the first chemical synthesis of glycolipid antigens of Borrelia burkholderi is reported. First, cholesteryl β-d-galactopyranoside was synthesized and was converted to partially protected congeners having the HO-6 group of the galactose moiety unprotected. Treatment of these intermediates with palmitic and oleic acid, respectively, under dehydrative conditions followed by removal of the protecting groups afforded cholesteryl 6-O-palmitoyl/oleoyl-β-d-galactopyranosides that were identical to the glycolipids isolated from B. burkholderi.     

Interferences in ultratrace speciation of organolead and organotin by gas chromatography with atomic spectrometric detection

Interferences in trace and ultratrace speciation analysis of organotin and organolead compounds in various samples by gas chromatography coupled with atomic absorption spectrometry (AAS) and/or microwave induced plasma atomic emission spectrometry (MIP AES) are investigated. Particular attention is given to the effects of matrix co-extractives and reagents impurities introduced during sample preparation. Their influence on the detection limits is discussed in terms of baseline noise level, blank value, formation of artefacts and signal suppression. Loss of column resolution during the analysis of some matrices is observed.     

Determination of selenium in human tissues by atomic absorption spectrometry

A simple method is described for the determination of selenium in human tissues without the use of perchloric acid. Digestion with nitric and sulphuric acids is followed by hydride generation and atomic absorption spectrometry. Results for NBS bovine liver and IAEA horse kidney reference materials were in good agreement with assigned concentrations, as was also achieved with the perchloric acid digestion. Recovery of added selenium was >90%, and the relative standard deviation was 5.5% for within-batch and 6.9% for between-batch analyses. The values of selenium in heart tissue were 0.9–1.3 μg g^?1 dry weight.     

H-bonding dependent structures of (NH4+)3H+(SO4 2-)2. Mechanisms of phase transitions

The role of different H-bonds in phases II, III, IV, and V of triammonium hydrogen disulfate, (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), has been studied by X-ray diffraction and (1)H solid-state MAS NMR. The proper space group for phase II is C2/c, for phases III and IV is P2/n, and for phase V is P onemacr;. The structures of phases III and IV seem to be the same. The hydrogen atom participating in the O(-)-H(+).O(-) H-bond in phase II of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) at room temperature is split at two positions around the center of the crucial O(-)-H(+).O(-) H-bonding, joining two SO(4)(2)(-) tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The NH(4)(+) ions participate mainly in bifurcated H-bonds with two oxygen atoms from sulfate anions. On cooling, the major contribution of the bifurcated H-bond becomes stronger, whereas the minor one becomes weaker. This is coupled with rotation of sulfate ions. In all the phases of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), some additional, weak but significant, reflections are observed. They are located between the layers of the reciprocal lattice, suggesting possible modulation of the host (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) structure(s). According to (1)H MAS NMR obtained for phases II and III, the nature of the acidic proton disorder is dynamic, and localization of the proton takes place in a broader range of temperatures, as can be expected from the X-ray diffraction data.     

Solvation of Cobalt(II) Ion in N,N-Dimethylformamide–Methanol Mixed Solvent: Calorimetry and VIS Spectroscopy Studies

The enthalpies of solution of Co(II) and Na(I) trifluoromethanesulfonates (triflates) in N,N-dimethylformamide (DMF)–methanol (MeOH) mixtures have been measured over the whole range of solvent composition. From these data the enthalpies of transfer of Co(II) and triflate ions from methanol to the mixed solvent have been determined usingliterature values of the enthalpies of transfer of the Na⁺ ion. The results have been analyzed by means of the theory of preferential solvation. The analysis revealed the preference of DMF for solvating the Co(II) ion in the MeOH-rich region of solvent composition and the lack of preference of any component in the DMF-rich region. Visible absorption spectra of the Co(II) ion in DMF–MeOH mixtures have been also measured in the whole range of solvent composition and analyzed using the partial least-squares method. The mean composition of the solvation sphere of the Co(II) ion versus solvent composition has been determined on the basis of both analyses. The results were found to be consistent with each other and with those obtained previously from FT-IR spectra.     

4,4′-Dipyridyl and 2,2′-dipyridyl complexes of rare-earth perchlorates

4,4-Dipyridyl and 2,2-dipyridyl complexes of rare-earth perchlorates of the formulaLn(4-dipy)₈(ClO₄)₃HClO₄ · 4H₂O (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 4-dipy=4,4-dipyridyl) andLn(2-dipy)₃(ClO₄)₃ · 6H₂O (Ln=Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y, 2-dipy = 2,2-dipyridyl) have been synthesized. The IR spectra of these compounds and other physical properties are discussed.

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4,4-Dipyridyl- und 2,2-Dipyridylkomplexe von Seltenerdmetallperchloraten

Zusammenfassung Es wurden 4,4-Dipyridylkomplexe des TypsLn(4-dipy)₈(ClO₄)₃HClO₄ · · 4 H₂O mitLn=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Y und 2,2-Dipyridylkomplexe des TypsLn(2-dipy)₃(ClO₄)₃ · 6 H₂O mitLn=Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu und Y dargestellt. Die IR-Spektren und andere physikalische Eigenschaften werden diskutiert.

     

The membrane free sonoelectroanalytical determination of trace levels of lead and cadmium in human saliva

In this paper, square wave anodic stripping voltammetry (SWASV), synergistically coupled with an ultrasonically enhanced preconcentration step has been shown to yield a quantitative determination of lead and cadmium in human saliva at a membrane free in situ plated mercury thin film glassy carbon electrode. The sensitivity was facilitated by acoustic streaming which promoted efficient mass transport to the electrode thus reducing sampling times. Cavitation was responsible for cleaning and activating the electrode surface, this was essential in order to obtain a reproducible and representative signal. In silent conditions electrode fouling leading to fluctuations in the baseline current and signal depression, precluded accurate quantitative analyses. The results presented herein provide an extension to the proof of concept given in the authors' earlier work, with the analysis of lead in human saliva as opposed to artificial saliva reported. We also address the hitherto unreported detection and determination of cadmium in this medium. Results for both were independently verified by inductively coupled plasma-mass spectroscopy (ICP-MS). Close agreement between lead concentration determined by sono-SWASV and independent and blind ICP-MS is reported for human saliva samples having a total lead content of 0.92 microg L(-1) and 5 microg L(-1) with a detection limit of 0.5 microg L(-1). Microaddition calibration data for cadmium additions of 0.0125 microg L(-1) to samples spiked with 2.5 microg L(-1) and 5.0 microg L(-1) (reflecting levels in workers occupationally exposed) exhibited close agreement with the known total cadmium in the samples. A detection limit of 1 microg L(-1) cadmium in saliva has been established.     

Activation volume measurement for C[bond]H activation. Evidence for associative benzene substitution at a platinum(II) center

The reaction of the platinum(II) methyl cation [(N-N)Pt(CH(3))(solv)](+) (N-N = ArN[double bond]C(Me)C(Me)[double bond]NAr, Ar = 2,6-(CH(3))(2)C(6)H(3), solv = H(2)O (1a) or TFE = CF(3)CH(2)OH (1b)) with benzene in TFE/H(2)O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C(6)H(5))(solv)](+) (2). High-pressure kinetic studies were performed to resolve the mechanism for the entrance of benzene into the coordination sphere. The pressure dependence of the overall second-order rate constant for the reaction resulted in Delta V(++) = -(14.3 +/- 0.6) cm(3) mol(-1). Since the overall second order rate constant k = K(eq)k(2), Delta V(++) = Delta V degrees (K(eq)) + Delta V(++)(k(2)). The thermodynamic parameters for the equilibrium constant between 1a and 1b, K(eq) = [1b][H(2)O]/[1a][TFE] = 8.4 x 10(-4) at 25 degrees C, were found to be Delta H degrees = 13.6 +/- 0.5 kJ mol(-1), Delta S degrees = -10.4 +/- 1.4 J K(-1) mol(-1), and Delta V degrees = -4.8 +/- 0.7 cm(3) mol(-1). Thus DeltaV(++)(k(2)) for the activation of benzene by the TFE solvento complex equals -9.5 +/- 1.3 cm(3) mol(-1). This significantly negative activation volume, along with the negative activation entropy for the coordination of benzene, clearly supports the operation of an associative mechanism.