Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one bond (1)J(C-H) NMR coupling constants in cyclohexane,six-membered heterocycles,and cyclohexanone derivatives

Cyclohexane (1), oxygen-, sulfur-, and/or nitrogen-containing six-membered heterocycles 2-5, cyclohexanone (6), and cyclohexanone derivatives 7-16 were studied theoretically [B3LYP/6-31G(d,p) and PP/IGLO-III//B3LYP/6-31G(d,p) methods] to determine the structural (in particular C-H bond distances) and spectroscopic (specifically, one bond (1)J(C-H) NMR coupling constants) consequences of stereoelectronic hyperconjugative effects. The results confirm the importance of n(X) --> sigma*(C-H)(app) (where X = O, N), sigma(C-H)(ax) --> pi*(C=O), sigma(S-C) --> sigma*(C-H)(app), sigma(C-S)-->sigma*(C-H)(app), beta-n(O) --> sigma*(C-H), and sigma(C-H) --> sigma*(C-H)(app) hyperconjugation, as advanced in previous theoretical models. Calculated r(C-H) bond lengths and (1)J(C-H) coupling constants for C-H bonds participating in more than one hyperconjugative interaction show additivity of the effects.     

Water-soluble gadofullerenes: toward high-relaxivity, pH-responsive MRI contrast agents

The water-soluble endohedral gadofullerene derivatives, Gd@C(60)(OH)(x) and Gd@C(60)[C(COOH)(2)](10), have been characterized with regard to their MRI contrast agent properties. Water-proton relaxivities have been measured in aqueous solution at variable temperature (278-335 K), and for the first time for gadofullerenes, relaxivities as a function of magnetic field (5 x 10(-4) to 9.4 T; NMRD profiles) are also reported. Both compounds show relaxivity maxima at high magnetic fields (30-60 MHz) with a maximum relaxivity of 10.4 mM(-1) s(-1) for Gd@C(60)[C(COOH)(2)](10) and 38.5 mM(-1) s(-1) for Gd@C(60)(OH)(x) at 299 K. Variable-temperature, transverse and longitudinal (17)O relaxation rates, and chemical shifts have been measured at three magnetic fields (B = 1.41, 4.7, and 9.4 T), and the results point exclusively to an outer sphere relaxation mechanism. The NMRD profiles have been analyzed in terms of slow rotational motion with a long rotational correlation time calculated to be tau(R)(298) = 2.6 ns. The proton exchange rate obtained for Gd@C(60)[C(COOH)(2)](10) is k(ex)(298) = 1.4 x 10(7) s(-1) which is consistent with the exchange rate previously determined for malonic acid. The proton relaxivities for both gadofullerene derivatives increase strongly with decreasing pH (pH: 3-12). This behavior results from a pH-dependent aggregation of Gd@C(60)(OH)(x) and Gd@C(60)[C(COOH)(2)](10), which has been characterized by dynamic light scattering measurements. The pH dependency of the proton relaxivities makes these gadofullerene derivatives prime candidates for pH-responsive MRI contrast agent applications.     

Partial dewetting of polyethylene thin films on rough silicon dioxide surfaces

The effect of roughness on the dewetting behavior of polyethylene thin films on silicon dioxide substrates is presented. Smooth and rough silicon dioxide substrates of 0.3 and 3.2-3.9 nm root-mean-square roughness were prepared by thermal oxidation of silicon wafers and plasma-enhanced chemical vapor deposition on silicon wafers, respectively. Polymer thin films of approximately 80 nm thickness were deposited by spin-coating on these substrates. Subsequent dewetting and crystallization of the polyethylene were observed by hot-stage optical microscopy in reflection mode. During heating, the polymer films melt and dewet on both substrates. Further observations after cooling indicate that, whereas complete dewetting occurs on the smooth substrate surface, partial dewetting occurs for the polymer film on the rough surface. The average thickness of the residual film on the rough surface was determined by ellipsometry to be a few nanometers, and the spatial distribution of the polymer in the cavities of the rough surface could be obtained by X-ray reflectometry. The residual film originates from the impregnation of the porous surface by the polymer fluid, leading to the observed partial dewetting behavior. This new type of partial dewetting should have important practical consequences, as most real surfaces exhibit significant roughness.     

Treatment methods for the determination of delta2H and delta18O of hair keratin by continuous-flow isotope-ratio mass spectrometry

The structural proteins that comprise approximately 90% of animal hair have the potential to record environmentally and physiologically determined variation in delta2H and delta18O values of body water. Broad, systematic, geospatial variation in stable hydrogen and oxygen isotopes of environmental water and the capacity for rapid, precise measurement via methods such as high-temperature conversion elemental analyzer/isotope ratio mass spectrometry (TC/EA-IRMS) make these isotope systems particularly well suited for applications requiring the geolocation of hair samples. In order for such applications to be successful, however, methods must exist for the accurate determination of hair delta2H and delta18O values reflecting the primary products of biosynthesis. Here, we present the results of experiments designed to examine two potential inaccuracies affecting delta2H and delta18O measurements of hair: the contribution of non-biologic hydrogen and oxygen to samples in the form of sorbed molecular water, and the exchange of hydroxyl-bound hydrogen between hair keratin and ambient water vapor. We show that rapid sorption of molecular water from the atmosphere can have a substantial effect on measured delta2H and delta18O values of hair (comprising approximately 7.7% of the measured isotopic signal for H and up to approximately 10.6% for O), but that this contribution can be effectively removed through vacuum-drying of samples for 6 days. Hydrogen exchange between hair keratin and ambient vapor is also rapid (reaching equilibrium within 3-4 days), with 9-16% of the total hydrogen available for exchange at room temperature. Based on the results of these experiments, we outline a recommended sample treatment procedure for routine measurement of delta2H and delta18O in mammal hair.     

Hydrogenation of benzene using aqueous solution of polyoxometalates reduced by CO over gold catalysts

Aqueous polyoxometalate (H3PMo12O40) solution reduced by CO with liquid water using gold nanoparticle catalysts at room temperature, which contains protons in liquid water and electrons associated with the reduced polyoxometalate, can produce gaseous H2 or can hydrogenate benzene over an electrochemical cell consisting of a simple carbon anode, a proton-exchange membrane, and a Pt- or Rh-based cathode. In the present cell, H2 can be produced from the reduced H3PMo12O40 solution at voltages that are lower by about 1.15 V compared to water electrolysis.     

4-Amino-2,6-dimethyl-5-oxo-3-thioxo-2,3,4,5-tetrahydro-1,2,4-triazine palladium(II) complexes

Summary The reaction of [PdCl₂(PhCN)₂] with 4-amino-2,6-dimethyl-5-oxo-3-thioxo-2,3,4,5-tetrahydro-1,2,4-triazine (L) yields [{PdClL(-Cl)}₂]·2H₂O and [PdCl₂L₂]·4H₂O complexes, which react with KBr in Me₂Co to give the corresponding bromo-complexes [{PdBrL(-Br)}₂] and [PdBr₂L₂]. When the chloro-complexes are treated with AgClO₄ in EtOH, [Pd(H₂O)₂L](ClO₄)₂ and [Pd(H₂O)₂L₂](ClO₄)₂ are formed. I.r., visible, and¹H-n.m.r. spectra have been recorded and facilitate assignments.     

The Synthesis of β,γ- and α,β-Unsaturated Aldehydes via Polyene Epoxides

A substitute for the Darzens glycidic ester synthesis for converting unsaturated ketones or aldehydes into the homologated β,γ- or α,β-unsaturated aldehydes employing sulfur ylides is described. The carbonyl group is converted into the unsaturated oxirane which is then rearranged to the new aldehyde. High yields of isomerically pure aldehydes are available by this method and the process is of practical importance in the conversion of β-ionone into the β-C₁₄-aldehyde, a key intermediate in the Isler synthesis of vitamin A. The efficient preparation of α- and β-cyclocitral by the novel process is also described.     

Total Synthesis of All Eight Stereoisomers of α-Tocopheryl Acetate. Determination of their diastereoisomeric and enantiomeric purity by gas chromatography

All eight stereoisomers of α-tocopheryl acetate have been synthesized in a state of high chemical and stereoisomeric purity. Key chiral side-chain intermediates were prepared from (+)-(S)-3-hydroxy-2-methylpropanoic acid. New routes to (2R, 4′ RS, 8′ RS)-α-tocopheryl acetate, a mixture of four diastereoisomers, were also developed. A sensitive gas chromatographic method was developed to determine the diastereoisomeric and enantiomeric purity of α-tocopherol samples as the methyl ethers. It was established for the first time that naturally occurring α-tocopherol is essentially a single enantiomer (2 R, 4′ R, 8′ R), synthetic all-rac-α-tocopherol an equimolar mixture of four racemates, and that natural (E)-(7 R, 11 R)-phytol is diastereoisomerically and enantiomerically homogeneous.     

PRIMARY PHOTOCHEMICAL PRODUCTS OF DITHIOGLYCOLIC ACID IN AQUEOUS SOLUTION

Abstract— The photochemistry of dithioglycolic acid at 254 nm was investigated in deaerated aqueous solutions in the pH range 1.4–7.3. Initial yields of the primary photochemical products H₂S,–SH and aldehyde-(probably glyoxylic acid) were determined. The complex pH dependence of these simultaneously formed first stable products is interpreted in terms of the ground-state ionic equilibria, and in addition pH-dependent processes occurring in the excited state and labile intermediate sequence.
It is suggested that the photochemical mechanism involves two parallel pathways: one a hydrolytic splitting of S - S leading to H₂S formation (unaffected by the presence of O₂ or isopropanol), the other a free radical mechanism via C–S breakage, which is affected by the presence of O₂ or isopropanol.     

2D LC Separation and Determination of Bradykinin in Rat Muscle Tissue Dialysate with On-Line SPE-HILIC-SPE-RP-MS

A 2D liquid chromatography (LC) system using hydrophilic interaction chromatography (HILIC) and reversed phase columns has been employed for comprehensive (LC × LC) separation of rat muscle tissue micro-dialysate. Incorporation of an on-line reverse-phase solid phase extraction (SPE) enrichment column in front of the first dimension enabled aqueous samples with high salt concentrations to be injected directly without compromising the chromatographic performance of the HILIC column. Since the SPE enrichment column allowed injection of large sample volumes (e.g. 450 μL), a capillary HILIC column (inner diameter 0.3 mm) could be employed instead of a larger column which is often used in the first dimension to load sufficient amounts of sample. The two chromatographic dimensions were connected using a column selector system with 18, 1.0 mm I.D. C₁₈ “transition” SPE columns. A PLRP C₁₈ column was used in the second dimension. The 2D LC system’s performance was evaluated with a tryptic digest mixture of three model proteins. Good trapping accuracy (HILIC→transition SPE→RP recovery >95%) and repeatability (within-and between day retention time RSDs of first and second dimension chromatography >1%) was achieved. A dialysis sample of rat muscle tissue was separated with the 2D system, revealing complexity and large differences in concentrations of the various compounds present, factors which could potentially interfere with the quantification and monitoring of two target analytes, arg-bradykinin and bradykinin. Subsequently, “Heart-cut” 2D LC-electrospray–mass spectrometry (ESI–MS) with post-column on-line standard injection was employed to monitor arg-bradykinin and bradykinin levels as a function of various muscle conditions. The method’s quantification precision was RSD = 3.4% for bradykinin.