Carbohydrate pyrolysis mechanisms from isotopic labeling: Part 2. The pyrolysis of d-glucose: General disconnective analysis and the formation of C1 and C2 carbonyl compounds by electrocyclic fragmentation mechanisms

The flash pyrolysis of d-glucose was investigated by the use of isotopic labeling with ¹³C, in conjunction with GC/MS. Co-pyrolysis of uniformly labeled and unlabeled d-glucose established the extent of unimolecular formation of each of the pyrolysis products. A complete set of singly labeled d-glucose isotopologs was used to determine the origin of specific carbons within each of the pyrolysis products. The Cyclic Grob 1,3-diol fragmentation and the tandem alkaline pinacol rearrangement/retro-aldol fragmentation (TAPRRAF) discovered from the pyrolysis of glycerin were applied to the analysis of pyrolytic fragmentation pathways for d-glucose. These mechanisms provide means of initial carbon–carbon bond breakage, and are consistent with the high proportion of carbon-unimolecularity observed for many of the volatile low-molecular weight products of the reaction. These and other reactions, including the retro-aldol fragmentation, carbonyl migration, dehydration, ene-reaction, retro-Claisen cleavage, hydrolysis, or alcoholysis were applied conceptually to the initial fragments resulting from either mechanism to ascertain the ultimate fate of the carbons of d-glucose. The “predicted” results were then compared with labeling patterns observed by experiment. The most promising rationalizations provided by this exercise are presented herein, for the formation of five C₁ and C₂ carbonyl-containing pyrolysis products: formaldehyde, formic acid, acetaldehyde, glycolaldehyde and acetic acid.     

(Pd+Pr)/zeolite catalyst for the synthesis of C2?C4 hydrocarbons from CO and H2

The activity of (Pd+Pr)/ZSM-5 catalyst is an order of magnitude higher compared to unmodified Pd/ZSM-5. Its selectivity towards C₂–C₄ hydrocarbons containing olefins is about 50 wt. %.

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(Pd+Pr)/CBK - Pd/CBK, C₂–C₄ , , j50% .

     

Reactions of C2(a 3πu) with CH4, C2H2, C2H4, C2H6, and O2 using multiphoton UV excimer laser photolysis

C₂(a ³π_u) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and < 10^?6(x10^?10cm³s^?1) are reported for reactions with C₂H₄, C₂H₂, O₂, C₂H₆, and CH₄, respectively at 298 K. C₂(a ³π_u) fragments are generated by multiphoton ArF excimer laser photodissociation at C₂H₂, and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C₂(a ³π_u) → (X ¹σ⁺_g) quenching channel. C₂ + C₂H₂ represents the one possible exception to the reactive channel.     

CO(υ)高振动激发态向C2H2的振动传能研究

用时间分辨富里叶红外发射谱研究了高振动激发态ＣＯ向Ｃ２Ｈ２的传能，得到了ＣＯ（ｖ＝１－３）各振动态布居及其随时间的变化。利用微分法解出弛豫微分方程组，获得ＣＯ（ｖ＝１－３）向Ｃ２Ｈ２的传能速率常数分别为：２．０±０．１，６．０±０．２和９．４±０．８（１０＾－１３ｃｍ＾３·ｍｏｌｅｃｕｌｅ＾－１·ｓ＾－１）。传能速率随着振动量子数的增加而迅速增加。ＣＯ的振动能应向Ｃ２Ｈ２的对称伸缩模ｖ２近共振Ｖ－     

The geometry, vibrational frequencies, thermochemistry, quadrupole moments and electronic structure of C2Na2: Comparison with C2Li2, C2H2, C2F2 and C2Cl2

The electronic structure of Na₂C₂ is studied using ab initio electronic structure methods and is compared to the companion molecule Li₂C₂. Both the linear D_∞h and planar structures are minima on the ground state potential surface with the planar D_2h conformation being the lowest energy form, similar to Li₂C₂. At the CCSD(t) level the planar form is more stable that the linear by 11.2 kcal/mol as compared with 7.34 kcal/mol for Li₂C₂. Both molecules are significantly ionic. The vibrational frequencies, atomization energy at 0 K, D₀, and the standard enthalpy of formation, are calculated and compared to those of Li₂C₂ as well as HCCH, FCCF and ClCCCl. We find D₀ and to be 331.1 and 84.92 kcal/mol for Li₂C₂ and 298.3 and 93.25 kcal/mol for Na₂C₂. We calibrate these by calculating the same quantities for HCCH, FCCF and ClCCCl.     

Culcitosides C2 and C3 from the starfishCulcita novaeguineae

Two steroid glycosides not previously described have been isolated from the digestive system of the starfishCulcite novaeguiniae, and these have been called culcitosides C₂ and C₃. With the aid of chemical and spectral methods, the chemical structure of C₂ has been established as 24-methyl-5-cholestane-3,4,6,8,15,16,28-heptaol 28-O-[O-(2,4-di-O-methyl--D-xylopyranosyl)-(1 2)--L-arabinofuranoside, and that of C₃ as its 4-deoxy analogue.Pacific Ocean Institute of Bioorganic Chemistry of the Far Eastern Scientific Center of the USSR Academy of Sciences, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 592–596, September–October, 1986.     

Endohedral complexes of C58 cage with H2 and CO

Ab initio Hartree–Fock (HF) calculations were carried out to determine the structures and energies of the endohedral complexes of C₅₈ cage with H₂ or CO. It was demonstrated that the formation of these complexes is endothermic with destabilization energies of 3.3 kcal/mol for H₂ and 18.6 kcal/mol for CO. Furthermore, the H₂ and CO molecules have different orientations in the C₅₈ cage, namely, the orientation of the molecular axis of the former is normal to the face of the 7-member ring, while that of the latter is parallel to that face. In addition, the H–H bond of the H₂ molecule is shortened inside the cage, whereas the length of the C–O bond remains unchanged.     

Vapour-liquid equilibrium data for the systems C2H6 + N2, C2H4 + N2, C3H8 + N2, and C3H6 + N2

High pressure vapour-liquid equilibrium data for the C₂H₆ + N₂, C₂H₄ + N₂, C₃H₈ + N₂, and C₃H₆ + N₂ systems are presented. The data are obtained isothermally in the range from 200 K to 290 K. For each point of data, temperature, pressure and liquid and vapour phase mole fractions are measured.Values for the vapour phase mole fractions are calculated from the obtained pressure, temperature and liquid phase mole fractions. The calculated values are compared with the experimental results, and it is found that the average mean deviation between calculated and experimental mole fractions is less than 0.009 for the systems considered in this work.     

Semiempirical molecular dynamics studies of C60/C70 fullerene oxides: C60O, C60O2 and C70O

The spectral analysis indicates that all isomers of C₆₀O, C₇₀O and C₆₀O₂ have an epoxide-like structure (an oxygen atom bridging across a C–C bond). According to the geometrical structure analysis, there are two isomers of fullerene monoxide C₆₀O (the 5,6 bond and the 6,6 bond), eight isomers of fullerene monoxide C₇₀O and eight isomers of fullerene dioxide C₆₀O₂. In order to simulate the real reaction conditions at 300 K, the calculation of the different isomers of C₆₀O, C₆₀O₂ and C₇₀O fullerene oxides was carried out using the semiempirical molecular dynamics method with two different approaches: (a) consideration of the geometries and thermodynamic stabilities, and (b) consideration of the ozonolysis mechanism. According to the semiempirical molecular dynamic calculation analysis, the probable product of this ozonolysis reaction is C₆₀O with oxygen bridging over the 6–6 bond (C_2v). The most probable product in this reaction contains oxygen bridging across in the upper part of C₇₀ (6–6 bond in C₇₀O-2 or C₇₀O-4) an epoxide-like structure. C₆₀O₂-1, C₆₀O₂-3 and C₆₀O₂-5 are the most probable products for the fullerene dioxides. All of these reaction products are consistent with the experimental results. It is confirmed that the calculation results with the semiempirical molecular dynamics method are close to the experimental work. The semiempirical molecular dynamics method can offer both the reaction temperature effect by molecular dynamics and electronic structure, dipole moment by quantum chemistry calculation.     

Multiconfiguration self-consistent wavefunctions for CH4, C2H4 and C2H6

The results of several MC SCF calculations on CH₄, C₂H₄ and C₂H₆ with minimun bases of Slater type AO's are reported. The computing method is a quadratically convergent process. Better final energies are obtained if localized MO's are used.     

C3 epimerization and selective C2C3 bond fission of alkyl chrysanthemate

Treatment of alkyl chrysanthemate with Lewis acid leads to C₃ epimerization, while protonic acid treatment gives rise to selective C₂C₃ bond cleavage. The latter method is successfully applied to the synthesis of optically active tetrahydrolavandulol.     

Comparison of theoretical methods for assessing the heat of formation of C1 and C2 chlorofluorocarbons and hydrochlorofluorocarbons

The heat of formation of a number of key C₁ and C₂ chlorofluorocarbons and hydrochlorofluorocarbons have been calculated by G3, G3MP2, G3MP2B3 and G3B3 methods. Based on the results of the atomization approach, it was found that the errors are approximately dependent upon the number of C-F or C-Cl bonds. Moreover, the bond additive correction (BAC) procedure and isodesmic reactions approach improved the accuracy and decreased these system errors significantly. The extended comparison between the BAC procedure and isodesmic reaction approaches had been made; the latter yielded the best results and showed broader applicability.     

Zur Trennung von H2, O2, N2, NO, CO, N2O, CO2, C2H6, C2H4, C2H2, NO2(N2O4) und Feuchtigkeit

Ohne Zusammenfassung     

Enthalpies of formation of nitrobuckminsterfullerenes: Extrapolation to C60(NO2)60

As with polynitrocubanes, new high-energy materials will be the likely result of bonding multiple nitro groups to a buckminsterfullerene (C₆₀) backbone. Here, we present a series of semi-empirical calculations on nitrofullerenes of varying nitro content. Enough nitrofullerene molecules were calculated to allow an extrapolation to pernitrobuckminsterfullerene, or C₆₀(NO₂)₆₀.     

Thermodesorption of C2?C3 olefins from the surface of molybdenum sulfide

The interaction of ethylene and propylene with the surface of MoS₂ has been investigated by means of a thermodesorption technique. Two types of adsorption sites on the surface of MoS₂ have been detected. From one type of sites the olefin is desorbed without changes, whereas from the other in the form of conversion products. Propylene desorption has been shown to be accompanied with carbon deposition on the active sites.

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MoS₂. , , . , .

     

Signature of electronic correlation in multi-electron emission from C60

Contrasting single to double-particle transition probabilities induced by a one-particle perturbation yields information on the strength of (two-particle) correlation in a system. Here, we calculate single and double ionization probabilities for a C₆₀ molecule upon the absorption of a single photon. We employ the Hartree–Fock and the frozen-core approximations for single-particle states and the random-phase approximation for the screened Coulomb interaction. Energy and angular dependencies of total and differential cross-sections are analyzed and the signature of the geometry of the fullerene and its charge-density response are pointed out.     

A theoretical study of the reaction of SiH2 with C2H2 and C2D2

Potential energy surfaces of the reaction of SiH₂ and C₂H₂ (and C₂D₂) have been calculated by means of ab initio molecular orbital theory at the QCISD/6-311G++(2df, 2p)//MP2/6-31G(d, p) level with corrections for the triple excitations to the QCISD energies. The barrier heights for the two reaction channels of the adduct, thus calculated, were further utilized for the dynamical calculation of the rate constants in the framework of quantum statistical Rice-Ramsperger-Kassel theory. Contributions of the rate constants of the various pathways to the total rate constant (K_T) for the disappearance of the reactants are critically examined and compared with experiment. The pressure dependence of K_T(C₂H₂) is primarily due to the formation of silirene. K_T(C₂D₂) is consistently higher than K_T(C₂H₂). The standard heat of formation of silirene is predicted to be 72.1 ± 3 kcal/mol. Rearrangement of silirene to vinylsilylene requires an activation energy smaller than that to silylacetylene.     

Elucidation of biochemical formation of C1/C2-(bromo-/chloro)-hydrocarbons

Summary To elucidate possible biochemical pathways of the formation of halogenated C₁/C₂-hydrocarbons we investigated the product pattern of the reaction of chloroperoxidase (CPO) with natural substances including the haloform reaction as a second step. The analysis of the reaction products was performed by HRGC-ECD. We report here first results. As substrates we used acetic acid and acetone while KBr and NaCl served as halogen sources. The system acetic acid/KBr/CPO formed tribromomethane as the main product, with dibromochloromethane (CHBr₂Cl) and 1,1,2,2-tetrachloroethane as by-products. Incubation with acetone resulted in the formation of CHBr₃ as the main product, with CHBr₂Cl, CH₂Br-CH₂Br, CHCl₂-CHCl₂, CHCl₂-CCl₃ and trichloroethene as side products, while the reaction of acetone with NaCl and CPO yielded chloroform as a main product.Results partly presented at the ANAKON '91 in Baden-Baden, April 22–24, 1991     

Carbohydrate pyrolysis mechanisms from isotopic labeling: Part 3. The Pyrolysis of d-glucose: Formation of C3 and C4 carbonyl compounds and a cyclopentenedione isomer by electrocyclic fragmentation mechanisms

The flash pyrolysis of d-glucose was investigated by the use of ¹³C labeling, in conjunction with GC/MS. Co-pyrolysis of uniformly labeled and unlabeled d-glucose established the extent of unimolecular formation of each of the pyrolysis products. A complete set of singly labeled d-glucose isotopologs was used to determine the origin of specific carbons within each of the pyrolysis products. The results were compared with the expected labeling patterns that arise when the cyclic Grob 1,3-diol fragmentation and the tandem alkaline pinacol rearrangement/retro-aldol fragmentation (TAPRRAF) discovered from the pyrolysis of glycerin are used to initiate breakage of the six-carbon chain of d-glucose. The most promising rationalizations provided by this exercise are presented herein, for the formation of six C₃ and eight C₄ acyclic carbonyl-containing pyrolysis products, and for 3-cyclopentene-1,2-dione.