Showing papers on "Chemical decomposition published in 1988"

Adsorption and decomposition of aliphatic alcohols on titania

Abstract: Etude, par thermodesorption, de l'adsorption puis de la decomposition chimique du methanol, ethanol, propanol-1 et propanol-2 sur le dioxyde de titane

Pathways for carboxylic acid decomposition on titania

Abstract: Etude de l'adsorption puis de la decomposition chimique de l'acide formique, acetique et propionique sur le dioxyde de titane

A calorimetric study of naturally occurring gas hydrates

Abstract: Mesure calorimetrique des enthalpies de dissociation, capacites calorifiques de 2 hydrates naturels de gaz

Defect formation in SiO2/Si(100) by metal diffusion and reaction

Abstract: The decomposition of SiO2 films on Si(100) during ultrahigh vacuum anneal is found to be strongly enhanced by monolayer amounts of impurities deposited on the SiO2 surface. s‐ and p‐band elements initiate decomposition via formation of volatile suboxides by surface reaction, whereas most transition metals decompose the oxide via laterally inhomogeneous growth of voids in the oxide. Transition metals need to diffuse to the SiO2/Si interface to enhance oxide decomposition via formation of volatile SiO. It is inferred that transition metal particles should be efficient in creating electrical defects in gate oxide layers.

Reactions of ethanethiol on Mo(110): formation and decomposition of a surface alkyl thiolate

Abstract: The reactions of ethanethiol (C/sub 2/H/sub 5/SH) on Mo(110) under ultrahigh vacuum have been investigated by temperature-programmed reaction, X-ray photoelectron, and high-resolution electron energy loss spectroscopies. Electron energy loss spectroscopy indicates that the S-H bond in ethanethiol dissociates below 120 K to form a surface ethyl thiolate (C/sub 2/H/sub 5/S). At low coverages the ethyl thiolate decomposes to atomic carbon, atomic sulfur, and gaseous H/sub 2/, with decomposition complete below 350 K. At high coverages, the surface thiolate decomposes during temperature-programmed reaction via three competing pathways: hydrogenolysis at 300 K to gaseous ethane, dehydrogenation at 340 K to gaseous ethylene, and decomposition to surface carbon, surface sulfur, and gaseous dihydrogen. Notably, the presence of surface atomic sulfur is not necessary for selective formation on clean Mo(110); the thiolate remains intact up to the temperature of hydrogenolysis onset. The last pathway proceeds via a hydrocarbon fragment(s) which decomposes at 570 K to gaseous H/sub 2/ and atomic carbon. At saturation, /approximately/ 75% of all irreversibly chemisorbed ethanethiol forms gaseous hydrocarbons. The coverage-dependent kinetics for ethanethiol decomposition are discussed in terms of electronic and site-blocking effects.

Mechanism of ozone decomposition in water. The role of termination

Abstract: The radical-chain mechanism of ozone decomposition of ozone decomposition in water is suggested. Addition of an extra chain termination step, i.e., reaction between superoxide and ozonide radicals, allowed the author to account for various reaction kinetic orders with respect to ozone and hydroxide ion concentrations reported in the literature, in particular the transition from 1.5 to 1 kinetic order for (O/sub 3/) at neutral pH. The rate constant of the proposed new termination reaction has been estimated to be between 2 x 10/sup 2/ and 1 x 10/sup 4/ dm/sup 3/.mol/sup -1/.s/sup -1/.

Quantum chemical studies of the products of decomposition of anticancer (2-haloethyl)nitrosoureas under physiological conditions

Abstract: Les intermediaires plausibles examines sont des composes hydroxydiazoiques (R-N=N-OH), des diazoniums (R-N 2 + ), et des sels de diazoates (R-N=N-O − )

High-pressure kinetic study of formation and dissociation of first- and second-row d10 divalent metal ion complexes with bipyridine in aqueous solution: a cation size dependent reaction mechanism

Abstract: Constantes de vitesse de formation et de dissociation de Zn(H 2 O) 4 bpy 2+ et Cd(H 2 O) 4 bpy 2+

Cobalt basic salts as inorganic precursors of cobalt oxides and cobalt metal: Thermal behaviour dependence on experimental conditions

Abstract: The influence of heating rates and environment conditions on the thermal behaviour of two cobalt basic salts,β-Co2(OH)3Cl and Co5(OH)85Cl15 · 25H2O, have been studied The processes were followed using thermogravimetric and differential thermal analysis, X-ray diffraction, and infrared spectroscopy When decomposition reactions are carried out in a nitrogen atmosphere, Co3O4 is always formed in quantities that seem to depend on the heating rates When decomposition processes are carried out in an X-ray high-temperature diffraction chamber, pure cobalt is obtained at 750° C as the final product Green cobalt hydroxychloride, the composition of which determined from chemical analysis seems to correspond to the formula Co5(OH)85Cl15 · 25H2O or alternatively 4Co(OH)2 CoCl15(OH)05 · 25H2O, has been isolated as a stable compound and the evolution of this material during the ageing process has been followed by X-ray diffraction, electron microscopy and BET surface-area determination

Photosensitized oxidation of phytol in seawater

Abstract: In synthetic seawater containing traces of anthraquinone, phytol is photo-oxidized rapidly by solar radiation. The photochemical processes lead mainly to the formation of 2,6,10-trimethyltridecane, 6,10,14-trimethyl-2-pentadecanone, pristanal and phytenal. In order to explain the formation of these different compounds, mechanisms which first involve abstraction reactions of hydrogen atoms in an allylic position are proposed. These results suggest a non-negligible photodegration of the free phytol in seawater before its incorporation into the marine sediments.

Scanning kinetic spectroscopy and temperature-programmed desorption studies of the adsorption and decomposition of hydrogen cyanide on the nickel(111) surface

Abstract: Etude experimentale entre 84 et 1000 K. Influence du degre de recouvrement sur la decomposition et le mecanisme d'adsorption

Theoretical Study of the Decomposition of Five-Coordinate Silicon Anions

Abstract: We have performed a theoretical study of the decomposition of the five-coordinate silicon anions (siliconates) H 3(CH3)SiOHand (CH3) 4SiOH-. These ions can be formed from hydroxide ion attack on methylsilane or tetramethylsilane, respectively. Both MNDO and ab initio calculations show that removal of methane from the five-coordinate structure is likely, via a transfer of the proton from the OH group to an adjacent methyl substituent. This process we term anionic dissociative proton transfer, because the transition state is one in which the methyl group being removed is almost completely dissociated to methide ion before the proton transfer takes place. This conclusion is in agreement with earlier experimental work by DePuy, Bierbaum, and Damrauer, which indicated a correlation between the rates for these reactions and the gas-phase acidities of the products. Detailed comparisons among the MNDO calculations, the ab initio calculations, and available experimental data indicate that great caution should be exercised in using MNDO calculations to draw even semiquantitative conclusions about these silicon-containing anion systems without resorting to reliable ab initio results on similar systems. The chemistry of silicon and silicon compounds has become increasingly important to the creation of many new materials, including catalysts, polymers, ceramics, glasses, and composites. We have embarked on a theoretical study of the mechanisms of elementary silicon reactions, of which this paper is the third in the series. In previous papers we discussed a comparison of fluorine atom and fluoride ion attack on methane and silane1 and the stabilities of some small five-coordinate siliconates.2 In this paper we continue the discussions of five-coordinate siliconates and in particular the decomposition of siliconates substituted by both methyl and hydroxyl groups. Our consideration of these systems was spurred by the gas-phase studies conducted by DePuy, Bierbaum, and Damrauer of the attack of hydroxide ion onto various silanes and their subsequent decomposition.3 Our eventual goal is to perform theoretical calculations for rather large molecular systems involved in the polymerization of silanols to form silica. At present, available ab initio molecular orbital methods are much too time-consuming to attack directly the systems of interest. Therefore, we depend heavily on Dewar's MNDO method to calculate properties for the large molecular systems involved in these processes.4 We have found, however, that because of known and suspected deficiencies of MNDO, it is necessary to carry out reliable ab initio calculations on smaller model systems whenever possible to verify and calibrate the MNDO results. In particular, this combination of techniques has uncovered and quantified several deficiencies in MNDO calculations on silicon compounds, including a tendency to overestimate heats of formation of small anions and certain transition states1 and an inability to predict the correct energy ordering of isomers of five-coordinate silicon complexes.2 Of great importance in this paper is the determination of the usefulness of MNDO in determining the properties and reactivities of five-coordinate siliconates, which appear to be quite prevalent in reactions of anions with tetrahedral silicon compounds, as discussed by Dewar and Healy in an earlier paper on MNDO calculations. 5 There have been only a few published theoretical works on silicon chemistry using semiempirical techniques,5•6 but recently quite a number of ab initio studies of small silicon systems have appeared.7-1 8 Thus, we consider in this paper the formation and decomposition of siliconates containing both methyl and hydroxyl groups. There are a variety of possible decomposition routes, including simple removal of any of the ligands to form the negative ion of that ligand and the resultant silane and interaction of various ligands to produce a variety of leaving groups. Of particular interest are interactions of the proton of the hydroxyl group with other ligands t Bolling Air Force Base. !North Dakota State University. 0002-7863/88/1510-3056$01.50/0 to form neutral leaving groups of the form XH, in which X is a ligand other than hydroxyl. These processes have been invoked in explaining the gas-phase data of DePuy, Bierbaum, and Damrauer. 3 Calculations on compounds related to those of interest here have been considered in a recent paper by Sheldon, Hayes, Bowie, and DePuy (SHBD). 16 The calculations performed by these authors at the self-consistent field (SCF) level with the 6-31G basis set predict that the addition of hydroxide ion to methylsilane to form the five-coordinated silicon anion is downhill by 45.7 kcal moi-1• Elimination of ethane from methoxymethylsilane and methane from methoxydimethylsilane is reported to occur with large barriers. It should be noted, however, that these calculations on hypervalent silicon anions were performed without either d or diffuse functions in the basis set or correlation corrections for the computation of energetics. Since no actual transition states were obtained by SHBD, the reported barriers may be particularly unreliable. Calculations The semiempirical calculations were performed with the MNDO method developed by Dewar and co-workers. The silicon parameters were obtained from a later communication by Dewar. 19 All stationary (1) Davis, L. P.; Burggraf, L. W.; Gordon, M.S.; Baldridge, K. K. J. Am. Chern. Soc. 1985, 107, 4415-4419. (2) Gordon, M. S.; Davis, L. P.; Burggraf, L. W.; Damrauer, R. J. Am. Chern. Soc. 1986, 108, 7889-7893. (3) DePuy, C. H.; Bierbaum, V. M.; Damrauer, R. J. Am. Chern. Soc. 1984, 106, 4051-4053. (4) Dewar, M. J. S.; Thiel, W. J. Am. Chern. Soc. 1977, 99, 4899-4907. (5) Dewar, M. J. S.; Healy, E. Organometal/ics 1982, I, 1705-1708. (6) Verwoerd, W. S. J. Comput. Chern. 1982, 3, 445-450. (7) Gordon, M.S.; George, C. J. Am. Chern. Soc. 1984, 106, 609-611. (8) Gordon, M. S. J. Am. Chern. Soc. 1984, 106, 4054-4055. (9) O'Keeffe, M.; Gibbs, G. V. J. Chern. Phys. 1984, 81, 876-879. (10) Binkley, J. S. J. Am. Chern. Soc. 1984, 106, 603-609. (11) Brandemark, U.; Siegbahn, P. E. M. Theor. Chim. Acta 1984, 66, 233-243. (12) Baird, N.C. Can. J. Chern. 1985, 63, 71-76. (13) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M. B.; Apeloig, Y.; Chandrasekhar, J.; Schleyer, P. J. Am. Chern. Soc. 1986, 108, 26Q-269. (14) O'Keeffe, M. J. Am. Chern. Soc. 1986, 108, 4341-4343. (15) Sheldon, J. C.; Hayes, R.N.; Bowie, J. H. J. Am. Chern. Soc. 1984, 106,7711-7715. (16) Sheldon, J. C.; Hayes, R.N.; Bowie, J. H.; DePuy, C. H. J. Chern. Soc., Perkin Trans. 2 1987, 275-280. ( 17) Gordon, M.S. In Structure and Energetics; Liebman, J., Greenburg, A., Eds.; VCR: New York, 1987; Vol. II, Chapter 4. (18) Baldridge, K. K.; Boatz, J. A.; Koseki, S.; Gordon, M.S. Annu. Rev. Phys. Chern., in press. © 1988 American Chemical Society Decomposition of Five-Coordinate Silicon Anions points on the potential surface were optimized by procedures supplied with the MOPAC package of programs available through QCPE.20 The ab initio geometries were predicted at the 6-31G(d)2 SCF level for the monomethylsiliconate (H3SiMeoH-) and its reactions and at the 3-21G* 22 SCF level for the tetramethylsiliconate and its reactions. For energy comparisons on the monomethylsiliconate system, single-point calculations were performed with second-order perturbation theory23 and the 6-31 +G(d) basis set.24 For the tetramethylsiliconate system MP2/6-31 +G(d), energy differences were estimated by assuming correlation effects obtained from MP2/3-21G* and polarization effects resulting from RHF /6-31 +G(d) energies are additive.2s To reduce the computational effort involved in the geometry predictions for the tetramethylsiliconate, all methyl groups were assumed to be staggered and C, symmetry was assumed within the methyl groups. Since no such assumptions were made for fragments of the siliconate, fragmentation energies will be calculated to be slightly too exoergic. Because of the restrictions placed on the structures of the tetramethylsiliconates, no force fields were calculated for these species. Energy differences reported for the ab initio calculations correspond to enthalpies (or energies) calculated at 0 K but corrected for zero-point vibrational energies for the case of the monomethylsiliconate and energies at 0 K for the case of the tetramethylsiliconate (no zero-point correction). In the case of MNDO, the differences correspond to enthalpy differences at 298 K, since the output is given as standard heats of formation at 298 K. We estimate that reconciliation of these two quantities (correcting for the temperature difference) would amount to at most a correction of 1-3 kcal mol-1• Thus, the differences in both these quantities will be reported as energy differences, keeping in mind that they refer to different temperatures for the two methods. Cartesian force constant calculations were carried out on suspected transition states. Transition states were verified by diagonalization of the force constant matrix in order to determine the presence of one, and only one, negative eigenvalue. Selected transition states were then further characterized by performing intrinsic reaction coordinate (IRC) calculations to determine the set of reactants and products connected by that transition state. For the ab initio transition states, the method developed by Ishida, Murokuma, and Komornicki26 as implemented by Schmidt, Gordon, and Dupuis27 was employed. This method calculates the path of steepest descent along the potential surface from the transition state forward to products and backward to reactants. For the MNDO transition states, the approximate intrinsic reaction-coordinate method developed by Stewart, Davis, and Burggraf and implemented as an option in MOPAC was used. 28 This method follows the path of steepest descent via a classical trajectory whose kinetic energy is compl

Role of free radicals and reaction-diffusion phenomena in the autocatalytic permanganate-oxalate reaction

Abstract: Les radicaux libres produits pendant la decomposition des complexes oxalato de Mn(III), reagissent ulterieurement avec les complexes de Mn(III), en faisant la reaction en presence de Ce(IV)