Intramolecular hydrogen isotope effect in the pyrolysis of diethyl carbonate-1,1,1,2,2-d5

The thermal unimolecular decomposition of diethyl carbonate-1,1,1,2,2-d₅ has been examined in the high-pressure-limiting region. The observed chemistry is consistent with a simple, competitive two-channel model: The intramolecular isotope effect k_H/k_D has been determined, and the relative Arrhenius parameters for the two channels are given by over the temperature range of 540–620 K. These Arrhenius parameters predict an isotope effect k_H/k_D = 5.4 at 300 K.     

IR laser pyrolysis and the isotopic labelling of organometallic compounds

A homogeneous gas phase pyrolysis technique involving SF₆ sensitization and a continuous wave (CW) CO₂ laser, is described for preparing small quantities (～ 100 mg) of organometallic compounds. Several reactions have been successfully carried out and the synthesis of Os(CO)₅ from H₂Os(CO)₄ and CO is discussed in detail. With IR laser pyrolysis, room temperature reactors and sub-atmospheric pressures can be used for reactions which normally require high pressures and temperatures.     

Intramolecular stable carbon isotopic analysis of archaeal glycosyl tetraether lipids

Glycolipids are prominent constituents in the membranes of cells from all domains of life. For example, diglycosyl‐glycerol dibiphytanyl glycerol tetraethers (2Gly‐GDGTs) are associated with methanotrophic ANME‐1 archaea and heterotrophic benthic archaea, two archaeal groups of global biogeochemical importance. The hydrophobic biphytane moieties of 2Gly‐GDGTs from these two uncultivated archaeal groups exhibit distinct carbon isotopic compositions. To explore whether the isotopic compositions of the sugar headgroups provide additional information on the metabolism of their producers, we developed a procedure to analyze the δ¹³C values of glycosidic headgroups. Successful determination was achieved by (1) monitoring the contamination from free sugars during lipid extraction and preparation, (2) optimizing the hydrolytic conditions for glycolipids, and (3) derivatizing the resulting sugars into aldononitrile acetate derivatives, which are stable enough to withstand a subsequent column purification step. First results of δ¹³C values of sugars cleaved from 2Gly‐GDGTs in two marine sediment samples, one containing predominantly ANME‐1 archaea and the other benthic archaea, were obtained and compared with the δ¹³C values of the corresponding biphytanes. In both samples the dominant sugar headgroups were enriched in ¹³C relative to the corresponding major biphytane. This ¹³C enrichment was significantly larger in the putative major glycolipids from ANME‐1 archaea (～15‰) than in those from benthic archaea (<7‰). This method opens a new analytical window for the examination of carbon isotopic relationships between sugars and lipids in uncultivated organisms. Copyright © 2010 John Wiley & Sons, Ltd.     

Carbohydrate pyrolysis mechanisms from isotopic labeling: Part 1: The pyrolysis of glycerin: Discovery of competing fragmentation mechanisms affording acetaldehyde and formaldehyde and the implications for carbohydrate pyrolysis

The flash pyrolysis of glycerin was investigated by the use of isotopic labeling with ¹³C, in conjunction with GC/MS analysis of the products. The formation of acetaldehyde and acrolein was shown to occur by unimolecular reaction. Acetaldehyde was found to be produced by at least two competing mechanisms. Mechanism “A” delivered C-2 of glycerin to the carbonyl group of acetaldehyde, whereas mechanism “B” delivered C-2 of glycerin to the methyl group of acetaldehyde. Formaldehyde was exclusively derived from C-1 or C-3 by either mechanism. The partition between the two mechanistic paths was found to be influenced by the presence of potassium salts and acids, but not by the presence of benzoyl peroxide or galvanoxyl. Mechanism “A” is postulated to be a concerted cyclic version of the Grob Fragmentation, proceeding through an intermediate which is hydrogen-bonded between the 1- and 3-hydroxyl groups to simultaneously generate enol-acetaldehyde, formaldehyde and H₂O. Mechanism “B” as favored by the presence of alkali is postulated to involve hydrogen-bonding between adjacent hydroxyl groups and to be an alkaline version of the pinacol rearrangement followed by retro-aldol fragmentation of the intermediary 3-oxopropoxide anion. These mechanistic classes are both fundamentally important, not only for their effect on glycerin, but for being able to provide numerous means of initial pyrolytic carbon–carbon bond breakage along carbohydrate carbon-chains, given the numerous 1,2,3-triol interactions that are possible. Further nomenclature is introduced to refine the distinctions among isotopomers and isotopologs (ipsomer, ipsolog, naturalomer, “nominal” isotopolog, ubiquilog), as extensions of a concept previously adopted by IUPAC.     

Carbohydrate pyrolysis mechanisms from isotopic labeling: Part 1: The pyrolysis of glycerin: Discovery of competing fragmentation mechanisms affording acetaldehyde and formaldehyde and the implications for carbohydrate pyrolysis

The flash pyrolysis of glycerin was investigated by the use of isotopic labeling with ¹³C, in conjunction with GC/MS analysis of the products. The formation of acetaldehyde and acrolein was shown to occur by unimolecular reaction. Acetaldehyde was found to be produced by at least two competing mechanisms. Mechanism “A” delivered C-2 of glycerin to the carbonyl group of acetaldehyde, whereas mechanism “B” delivered C-2 of glycerin to the methyl group of acetaldehyde. Formaldehyde was exclusively derived from C-1 or C-3 by either mechanism. The partition between the two mechanistic paths was found to be influenced by the presence of potassium salts and acids, but not by the presence of benzoyl peroxide or galvanoxyl. Mechanism “A” is postulated to be a concerted cyclic version of the Grob Fragmentation, proceeding through an intermediate which is hydrogen-bonded between the 1- and 3-hydroxyl groups to simultaneously generate enol-acetaldehyde, formaldehyde and H₂O. Mechanism “B” as favored by the presence of alkali is postulated to involve hydrogen-bonding between adjacent hydroxyl groups and to be an alkaline version of the pinacol rearrangement followed by retro-aldol fragmentation of the intermediary 3-oxopropoxide anion. These mechanistic classes are both fundamentally important, not only for their effect on glycerin, but for being able to provide numerous means of initial pyrolytic carbon–carbon bond breakage along carbohydrate carbon-chains, given the numerous 1,2,3-triol interactions that are possible. Further nomenclature is introduced to refine the distinctions among isotopomers and isotopologs (ipsomer, ipsolog, naturalomer, “nominal” isotopolog, ubiquilog), as extensions of a concept previously adopted by IUPAC.     

Intermolecular methyl transfer on pyrolysis of carpronium chloride

The methyl transfer occurring in the production of methyl N,N-dimethyl-γ-aminobutyrate by pyrolysis of carpronium chloride was examined by means of pyrolysis gas chromatography mass spectrometry with the aid of some deuterated compounds. The mass spectra of methyl N,N-dimethyl-γ-aminobutyrate, produced from deuterated derivatives of carpronium chloride, showed inter alia, characteristic molecular ion peaks which indicated that the methyl of the trimethylammonium group transfers and displaces the methyl of the carbomethoxy group of the tertiary amino compound. The results show that an intermolecular methyl transfer occurs in part on pyrolysis of carpronium chloride, to form methyl N,N-dimethyl-γ-aminobutyrate in which the methyl oxygen is replaced by a methyl from the nitrogen of the original compound. The mechanism presented involves the bimolecular reaction between zwitterionic intermediates formed by ionic O-demethylation of carpronium chloride.     

The effect of N2O on the isotopic composition of air-CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.     

Intramolecular cyclization of 1-allyl- and 1-methallyl-6-amino-2-thiouracils

Intramolecular cyclization of 1-allyl- and 1-methallyl-6-amino-2-thiouracils afforded 5-amino-2,3-dihydrothiazolo[3,2-a]pyrimidin-4-ones, whose structures were proved by chemical transformations. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1694–1696, July, 2005.     

Intramolecular d10-d10 interactions in heterometallic clusters of the transition metals

Weak attractive interactions between closed shell metal ions have been increasingly studied in the last few years and are generally designated as metallophilic interactions. They are best evidenced in the solid state where structural data obtained by X-ray diffraction provide precise information about the distance between the metals involved. The strength of such metal-metal interactions has been compared to that of hydrogen bonding (ca. 7-11 kcal mol(-1)) and is clearly sufficient to bring about novel bonding and structural features and confer interesting physical properties such as luminescence, polychromism, magnetism or one-dimensional electrical conductivity. The Cu(I)-Cu(I), Ag(I)-Ag(I) and Au(I)-Au(I) interactions have been increasingly observed and the latter have certainly been the most studied. Early qualitative analyses of the aurophilic attraction focused on Au-Au bonding originating from 6s, 6p and 5d orbital mixing. Numerous theoretical studies on metallophilic interactions continue to be carried out at various levels of sophistication which take into account relativistic and correlation effects to describe these van der Waals-type interactions. In this critical review, we would like to focus on the synthesis and structures of heterometallic clusters of the transition metals in which intra- rather than intermolecular d(10)-d(10) interactions are at work, in order to limit the role of packing effects. We wish to provide the reader with a comparative overview of the metal core structures resulting from or favoring metallophilic interactions but do not intend to provide a comprehensive coverage of the literature. We will first examine heterometallic clusters displaying homometallic and then heterometallic d(10)-d(10) interactions. Although the focus of this review is on d(10)-d(10) interactions involving metals from the group 11, we shall also briefly examine for comparison some complexes displaying intramolecular d(10)-d(10) interactions involving metals from other groups (188 references).     

氯化钙催化纤维素热裂解动力学研究

用差示热重分析仪对氯化钙纤维素热裂解动力学催化影响进行了研究。结果表明，氯化钙对焦炭的形成具有强烈的促进效果，使热裂解最终残留物产率从5％提升到10％以上，氯化钙的存在影响到热失重初始阶段活性纤维素的生成，使热重曲线向低温侧移动，并在低温段产生了小的失重速率峰。通过热重分析发现，氯化钙催化条件下纤维素热裂解动力学参数被分为了三段，分别对应于活性纤维素的生成、炭化和活性纤维素转化为挥发分产物三个区间，并依次成为整体失重过程的控制步骤。结合Broido-Shafizadeh机理分析以及与纯纤维素热裂解动力学参数的对比，氯化钙对这三个主要反应步骤都产生了促进效果，其中以催化焦炭的生成最为明显，在促进焦炭化的过程中，降低了气体产物的生成比率。     

Intramolecular heterocyclization with accompanying exoiminization in the reaction of ammonia with 2-chloroperfluoro-1-cyclohexene-1-thiocyanate

The reaction of ammonia with 2-chloroperfluoro-1-cyclohexene-1-thiocyanate leads to 2-amino-7-iminoperfluoro-4,5,6-trihydrobenzo-1,3-thiazole (II) due to intramolecular heterocyclization in the Thorpe reaction with the concurrent exoiminization. Analysis of the IR spectra of (II) in CCl₄ and in the solid phase indicated that molecules of (II) in the crystal form hydrogen-bonded centrosymmetric dimers (D), which are in equilibrium with free molecules of (II) in solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2366–2370, October, 1991.The authors express their gratitude to D. I. Zagorevskii for taking the mass spectrum of (II) and to L. S. German for a useful discussion of this work.     

Kinetics of isotopic exchange between chloramine-B and radioactive chloride

The kinetics of chlorine isotope exchange between chloramine-B /CAB/ and chloride has been studied using ion-exchange separation and tracer technique. McKay's plot are linear. The exchange reaction is fast in acidic medium, very slow in neutral medium and does not take place in alkaline medium. In the acidic range the exchange is maximum at pH 3.3. The rate of exchange decreases at pH >3.3 and <3.3. The order with respect to CAB and chloride is unity. The order with respect to [H⁺] is unity at pH>5. Addition of neutral salt or parent compound has no effect on the rate of exchange. Activation energy and activation entropy for this exchange reaction have been calculated.     

Intramolecular amidoselenation leading to lactams by the reaction of N-alkylalkenamides with benzeneselenenyl chloride

The reaction of N-alkylalkenamides (1)-(4) with benzeneselenenyl chloride affords γ- or δ-lactams (7)-(10) in good to excellent yields through the intramolecular attack of the nitrogen atom of the amide group on episelenonium ion intermediates.     

Generation and rearrangement of a 1-norbornene in pyrolysis of 1-sila-2-oxynorbornanes

Gas phase pyrolysis of 1-sila-2-oxynorbornenes yields a nortricyclene which is proposed to arise from a β-elimination to form a 1-norbornene, which isomerizes to a 2-norbornacylidene which undergoes γ-CH insertion.