Methane dissociative adsorption on the Pt(111) surface over the 300-500 K temperature and 1-10 Torr pressure ranges

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.     

A stereoselective 3-endo-alkyl-5,6-dehydroisoquinuclidine synthesis.

Cycloaddition of N-carbethoxy-2-alkyl-1,2-dihydropyridines $\text{4}$ with phenylvinylsulfone $\text{5}$ provides adducts $\text{6}$, which upon desulfonylation afford stereoselectively N-carbethoxy-3-$\text{endo}$- alkyl-5,6-dehydroisoquinuclidines $\text{1}$.     

A calix(4)arene pyridine derivative and its monomeric component: structural and thermodynamic aspects of their complexation with metal cations

The interaction of a calix(4)arene derivative, namely 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra[2-(4-pyridyl)methoxy]calix(4)arene, 1a, and its monomeric component, p-tert-butylphenoxy-4-pyridine, 1b, with metal cations has been investigated in acetonitrile and methanol. (1)H NMR measurements carried out in CD(3)CN show the primary role played by the pyridyl nitrogens in their complexation with metal cations. Conductance measurements demonstrated that for all cations (except mercury) the composition of the metal ion complexes of 1a is 1:1 (ligand:metal cation). However, 1a hosts two mercury cations per unit of ligand. For the monomer 1b, complexes of 2:1 (ligand:metal cation) stoichiometries are formed with the exception of Pb(2+) (1:1 composition). The thermodynamics of complexation of these systems are reported in acetonitrile. Data in methanol are limited to stability constant values for mercury(II) and these ligands. This paper demonstrates for the first time that thermodynamic data for the complexation of the monomeric component of the ligand and metal cations contribute significantly to the interpretation of systems involving cation-calixarene interactions in solution.     

A kinetic and product study of the Cl + HO2 reaction

Absolute rate data and product branching ratios for the reactions Cl + HO2 --> HCl + O2 (k1a) and Cl + HO2 --> OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10(-11) exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10(-11) exp[(123 +/- 15)/T] cm3 molecule(-1) s(-1), respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10(-11) cm3 molecule(-1) s(-1). Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10(-11) exp[-(708 +/- 29)/T] cm3 molecule(-1) s(-1). These expressions for k1a and k1b are valid for 226 K < or = T < or = 336 and 256 K < or = T < or = 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K < or = T < or = 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K < or = T < or = 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10(-11) exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10(-11) exp[-(733 +/- 41)/T] cm3 molecule(-1) s(-1), respectively. These expressions for k1a and k1b are valid for 256 K < or = T < or = 296 and 226 K < or = T < or = 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10(-11) exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10(-11) exp[-(801 +/- 94)/T] cm3 molecule(-1) s(-1), respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 --> OH for detection purposes.     

Preparation of 8-(arylsulfanyl)adenines with diazonium salts under mild, aerobic conditions

8-(Arylsulfanyl)adenines 11 were prepared in up to 75% yield by reacting the 8-thionoadenine 6 (acetic acid 3-(6-amino-8-thioxo-7,8-dihydropurin-9-yl)propyl ester) with benzenediazonium tetrafluoroborates in DMSO. Benzenediazonium ions carrying an electron-withdrawing substituent gave the highest yields. The reaction proceeded smoothly at room temperature without any base and could be performed under air atmosphere. The extremely mild conditions are compatible with a wide range of functional groups.     

Ring closure to beta-turn mimics via copper-catalyzed azide/alkyne cycloadditions

[Structure: see text]. Copper-catalyzed azide alkyne cycloadditions of the linear substrates 1 were used to form the cyclic derivatives 2. Computational, NMR, and CD analyses of these compounds indicate that their most favorable conformational states include type I and type II beta-turn conformations. Selectivity for the dimeric products 6 in these cyclization reactions is discussed.     

Autocatalytic inactivation of cytochrome p-450 and chloroperoxidase by 1-aminobenzotriazole and other aryne precursors

Cytochrome P-450, as reported previously is inactivated during catalytic turnover of 1-aminobenzotriazole due to alkylation of its prosthetic heme group. NMR analysis of the heme adduct after removal of the iron atom identifies it unequivocally as a derivative of protoporphyrin IX in which two of the nitrogens are bridged by a benzene ring. Cytochrome P-450 destructive activity is retained by analogues with Me or Ac substituents on the exocyclic N but is lost when the N itself is removed or is replaced by a hydroxyl or nitro function. Prosthetic heme alkylation also occurs with 1-amino-1H-naphtho (2,3-d)triazole, the analogue with one additional benzene ring. In vivo studies suggest that 1-aminobenzotriazole is relatively nontoxic. Catalytic turnover of 1-aminobenzotriazole by chloro-peroxidase results in the formation of phenol and in inactivation of the enzyme. The phenol obtained in deuterated water incorporates one deuterium into the aromatic ring. The data indicate that benzyne, formed by enzymatic oxidation of 1-aminobenzotriazole, is responsible for inactivation of cytochrome P-450 and chloroperoxidase.     

Polyethers derived from bisphenols and highly fluorinated aromatics

A series of fluorinated aromatic polyethers was synthesized via aromatic nucleophilic substitution of highly fluorinated aromatics (1,2,4,5-tetrafluorobenzene, hexafluorobenzene, and decafluorobiphenyl) with bisphenol AF or bisphenol A. Polymerization with 1,2,4,5-tetrafluorobenzene was not observed, and polymerization of hexafluorobenzene with bisphenol proceeded only if the potassium carbonate–bisphenol ratio was carefully controlled. The polymer condensed from decafluorobiphenyl and bisphenol AF was prepared in 77% yield with an inherent viscosity of 1.01 dL/g. The polymer prepared from the condensation of decafluorobiphenyl with bisphenol A was obtained in 48% yield with an inherent viscosity of 0.28 dL/g. These polymers were very soluble in common organic solvents, formed clear, colorless films, and were thermally stable (> 450°C by TGA). The fully fluorinated polymer exhibited low water uptake (0.3%) and dielectric constant (2.17). © 1992 John Wiley & Sons, Inc.     

Steam distillation with resin extraction for isolation and concentration of organic compounds from aqueous samples

Steam distillation is combined with solid-phase resin extraction using a simple apparatus and interface. A concentration factor of at least 100 is obtained. The method is effective for the isolation and concentration of organic compounds from complex aqueous samples ar μg l^?1 levels. Factors affecting the recovery of model compounds are studied; it is shown that the recovery of many compounds is improved markedly by disconnecting the condenser. Under proper conditions, compounds with boiling points in excess of 400°C are recovered quantitatively in only 24 mill.     

Photo-oxidation of metal carbynes

Photo-oxidation of the metal carbyne complexes (η-C⁵H₅)L₁L₂MCR (MMo, W; L₁, L₂CO, P(OMe)₃; R ≡ phenyl, cyclopropyl) generates radical that are extremely reactive. Depending on the reaction conditions, either the oxidized species exhibit the ligand exchange and atom abstraction processes typical of metal radicals, or the carbyne ligands undergo a series of highly unusual rearrangement to yield organic products. For the cyclopropyl carbynes (η⁵-C₅H₅)[P(OMe)₃](CO)MC(c-C₃H₅), photo-oxidation in the absence of added ligands results in the formation of cyclopentenone. When substituents are present on the cyclopropyl ring, single regioisomers or stereoisomers of the resulting cyclopentenones are produced.