2680-01-5Relevant academic research and scientific papers
Coupling reactions in aldehydes adsorbed on V(100) single-crystal surfaces
Shen, Min,Zaera, Francisco
experimental part, p. 8708 - 8713 (2009/10/23)
The thermal chemistry of formaldehyde on vanadium (100) single-crystal surfaces was characterized under ultrahigh vacuum (UHV) conditions by using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) in combination with isotope-labeling experiments. Particular emphasis was placed on establishing a mechanism for the formation of ethylene, which was observed to desorb in two temperature regimes, at 290 and 540 K. The low-temperature reaction was determined to occur via the coupling of methylene groups formed on the surface upon dissociation of the C-O bond in adsorbed formaldehyde. The high-temperature ethylene, on the other hand, was proven to require the prior formation of a diolate, -OCH2CH2O-, intermediate. This chemistry was shown to be quite general, also occuring in cross-coupling mode between two different coadsorbed aldehydes.
H and D atom addition to ethylene on Cu(100): Absence of ethyl H/D shift and decomposition
Yang, Michael X.,Bent, Brian E.
, p. 822 - 832 (2007/10/03)
The addition of gas phase H and D atoms to unsaturated hydrocarbons physisorbed on metal surfaces is a viable synthetic route to partially-deuterated alkyl groups on the surface (Jenks, C. J.; Xi, M.; Yang, M. X.; Bent, B. E. J. Phys. Chem. 1994, 98, 2152-2157). Because these processes are exothermic by ～60 kcal/ mol, the possibility of alkyl decomposition and/or rearrangement prior to thermal accommodation with the surface (a possibility not explicitly addressed in prior studies) should be considered. In the studies here, these decomposition and rearrangement possibilities have been investigated by studying H and D atom addition to variously-deuterated ethylenes physisorbed on a Cu(100) surface. Ethyl decomposition by C-H, C-D, or C-C bond scission has been addressed and shown not to occur by comparison with results from previous studies of the surface species that would be formed by these bond scission processes. H/D shift between the two carbons of the ethyl groups has been addressed by heating the surface to induce β-hydrogen or β-deuterium elimination. The resulting alkene product ratios are compared with those for β-elimination from selectively-deuterated ethyl groups formed by an independent route, i.e., the dissociative adsorption of a labeled bromoethane. The results show that the extent of H/D shift, if it occurs at all, is H/kD) for β-hydrogen/β-deuterium elimination is 9.5 ± 0.4 at ～260 K on Cu(100). No secondary isotope effect of D for H substitution at the α-carbon is detected to within the experimental uncertainty. These results demonstrate, at least for a Cu-(100) surface, the feasibility of synthesizing selectively-labeled surface alkyl groups from H and D atom addition to alkenes.
Hydrocarbon Activation by Gas-Phase Lanthanide Cations: Interaction of Pr+, Eu+, and Gd+ with Small Alkanes, Cycloalkanes, and Alkenes
Schilling, J. Bruce,Beauchamp, J. L.
, p. 15 - 24 (2007/10/02)
We describe ion beam studies of the interaction of gas-phase lanthanide ions, praseodymium (Pr+), europium (Eu+), and gadolinium (Gd+), with small alkanes, cycloalkanes, alkenes, and several oxygen-containing compounds.Only Gd+ is seen to activate C-H and C-C bonds of alkanes.The ground-state electronic configuration of Gd+ (4f75d16s1) is different from those of Pr+ (4f36s1) and Eu+ (4f76s1), leading to the conclusion that the f electrons play little part in the metal ion reactivity.Gd+ can be thought of as having two valence electrons, and indeed it reacts similarly to Sc+ and the other group 3 metal ions Y+ and La+, yielding products corresponding to elimination of hydrogen, alkanes, and alkenes.The elimination of neutral alkenes in the reaction of Gd+ with alkanes results in the formation of metal dialkyl or hydrido-alkyl complexes.This finding leads to estimates for the sum of two Gd+ ? bond dissociation energies of between 110 and 130 kcal/mol.Gd+ and Pr+ react readily with alkenes, yielding mostly dehydrogenation products along with smaller amounts of C-C bond cleavage products.Reactions of Gd+ and Pr+ with oxyen-containing species such as nitric oxide, formaldehyde, acetaldehyde, and acetone yield primarily the metal oxide ions and provide a lower limit for D(M+-O) of 179 kcal/mol, in good agreement with literature values of D(Pr+-O) = 188.4 +/- 5.2 kcal/mol and D(Gd+-O) = 181.0 +/- 4.4 kcal/mol.In keeping with the strong metal ? bonds, Gd+ is also seen to readily react with formaldehyde to eliminate CO and form GdH2+.
Thermal Reactions of Pyrrolidine at Elevated Temperatures. Studies with a Single-Pulse Shock Tube
Lifshitz, Assa,Bidani, Menashe,Agranat, Ahuva,Suslensky, Aya
, p. 6043 - 6048 (2007/10/02)
The thermal decomposition of pyrrolidine was studied behind reflected shocks in a single-pulse shock tube over the temperature range 900-1400 K and overall densities of ca. 3 x 1E-5 mol/cm3.Under these conditions the following reaction products were found in the postshock mixtures: H2, CH4, C2H4, C2H6, C2H2, C3H6, CH2=C=CH2, CH3CCH, CH2=CHCH2CH3, HCN, CH3CN, CH2=CHCN, C2H5CN, and small quantities of pyrrole and butadiene.Studies with a mixture of pyrrolidine and pyrrolidine-d9 show that C2H4 and C3H6 are produced by direct ring cleavage with the following rate parameters: pyrrolidine -> C2H4 + (CH2)2-NH, k = 3.42 x 1E16 exp(-75.2 x 1E3/RT) s-1 and pyrrolidine -> C3H6 + CH2=NH, k = 1.35 x 1E16 exp(-80.4 x 1E3/RT) s-1, where R is expressed in units of cal/(K mol).Hydrogen cyanide has the highest concentration among the nitrogen-containing products, followed by acetonitrile.No reaction products resulting from pyrrolidine isomerization were observed.
Gas-Phase Studies of Alkane Oxidation by Transition-Metal Oxides. Selective Oxidation by CrO+
Kang, H.,Beauchamp, J. L.
, p. 7502 - 7509 (2007/10/02)
The gas-phase reactions of CrO+ with alkanes have been studied by using ion beam reactive scattering techniques.CrO+ undergoes facile reactions with alkanes larger than methane.CrO+ selectively oxidizes ethane to form ethanol.In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur.In reactions with cyclic alkanes, cyclopropane and cyclobutane yield products characterictic of C-C bond cleavage.In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H2O.A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D0(CrO+-H) = 89 +/- 5 kcal/mol-1.This bond energy has implications for the reaction mechanisms of CrO+ with alaknes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr+-O bond as an initial step.This is supported by an examination of the reactions of Cr+ with alcohols.
Mechanistic and Kinetic Study of Alkane Activation by Ti+ and V+ in the Gas Phase. Lifetimes of Reaction Intermediates
Tolbert, M. A.,Beauchamp, J. L.
, p. 7509 - 7517 (2007/10/02)
The reactions of Ti+ and V+ with several deuterium-labeled alkanes are studied by using an ion beam apparatus.The dominant reactions observed for both of these metal ions are single and double dehydrogenations.Alkane loss recations are also observed for Ti+ but may be due to electronically excited states.The dehydrogenation mechanisms are investigated by using partially deuterated alkenes.The results are consistent with 1,2-eliminations for both V+ and Ti+, where deuterium scrambling may occur in the latter case.It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti+ with n-butane.Although the dehydrogenation reactions of V+ and Ti+ appear to be similar to those of Ru+ and Rh+, there are some important differences in the reactivity of V+.Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V+ in comparison to Ti+, Ru+, and Rh+.Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti+ and V+ interact with n-butane.The measured upper limits to the unimolecular decomposition rates are 1.47 X 105 s-1 and 1.23 X 107 s-1 for V+ and Ti+ respectively.Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d10 is employed as the neutral reactant.The slower rate for V+ is suggested to arise from the inability of V+ to form two strong ? bonds due to the 3d4 electronic configuration of the ground-state ion.This renders C-H bond insertion energetically much less favorable for V+ than for the other metal ions and limits the excitation energy of reaction intermediates.
Collisional Energy Transfer in the Two-Channel Thermal Decomposition of Bromoethane-1,1,2,2-d4
Nguyen, Tam T.,King, Keiieth D.,Gilbert, Robert G.
, p. 494 - 498 (2007/10/02)
The two-channel thermal decomposition of CHD2CD2Br (products HBr + C2D4, DBr + CHDCD2), along with the decomposition of CH3CH2Br, has been studied by using the technique of very low-pressure pyrolysis (VLPP).Rate coefficients were obtained at pressures both so low that only gas/wall collisions occur (over the temperature range 950-1200 K) and dilute in various bath gases (pressures up to 10 Pa) over the range 1000-1070 K.Fitting these data by solution of the appropriate reaction-diffusion integrodifferential master equation yields the gas/wall collisional efficiency, the extrapolated high-pressure rate parameters, and the gas/gas collisional energy transfer probability function, P(E,E').The extrapolated high-pressure rate coefficients are as follows: for CHD2CD2Br, 1013.20 exp(-227.4 kJ mol-1/RT) s-1 (HBr elimination), 1013.15 exp(-230.2 kJ mol-1/RT) s-1 (DBr elimination), and 1013.6 exp(-221 kJ mol-1/RT) s-1 for CH3CH2Br, in good agreement with those obtained by other methods.Gas/wall collision efficiencies (the wall being seasoned quartz) are ca. 0.6 at 1200 K, ca. 0.8 at 1000 K (with those for the d4 species ca. 10percent less than for the d0), in accord with values estimated from the potential well depth.The data are moderately sensitive to P(E,E').Assuming this for downward transitions to be a function of E-E' alone, we found that this function falls off more steeply than exponential, as found previously for chloroethane.Average bromoethane/M downward energy transfer values ((ΔEdown)) are 250 (M = Ne), 600 (M = CO2), 850 (M = C2H4), and 1200 (M = benzene) cm-1, the variation of (ΔEdown) with temperature being less than experimental uncertainty over the small experimental temperature range.
La photolyse du cyclohexene gazeux a 184,9 nm
Collin, Guy J.,Deslauriers, Helene
, p. 1970 - 1972 (2007/10/02)
We have studied the 184.9 nm photolysis of gaseous cyclohexene either in the absence or in the presence of radical scavengers such as O2, NO, H2S, or HI at pressures between 1 and 70 Torr.Propane and sulfur hexafluoride has also been used as stabilizing agent.In all cases, ethylene and 1,3-butadiene have rather high quantum yields (Φ>/=0.5).The isotopic analysis of the ethylene formed in the photolysis of cyclohexene-3,3,6,6-d4 shows the high importance of the perhydrogenated species.These results, together with others taken from the literature, favor a one-step fragmentation mechanism or a double-step mechanism involving an intermediate which has a lifetime shorter than that of the photoexcited molecule.
Hydrogenation of Ethylene on Metal Electrodes. Part 5. Reduction of Light Ethylene on Pt in Deuteroperchloric Acid Solution and the Dual-pathway Mechanism
Fujikawa, Keikichi,Kita, Hideaki,Sato, Shinri
, p. 3055 - 3072 (2007/10/02)
Electroreduction of light ethylene on a platinum electrode was conducted in a heavy-water solution of deuteroperchloric acid.Deuterium-atom distributions in the product, ethane, support the previous conclusion that ethylene diffusion is rate-controlling at potentials less positive than ca. 100 mV, whereas the surface reaction is rate-controlling at more positive potentials where the Tafel line holds.The D-atom distribution in the latter potential region reveals double maxima at - and -ethanes.This distribution is explained by the dual-pathway mechanism which assumes two reaction rates for the step C2H4(a) + H(a) C2H5(a).The difference in the reaction rate will be attributed to the difference in the adsorption state of C2H4(a) but not of H(a), since only the weakly adsorbed hydrogen atoms are active in the hydrogenation.Reduction of light ethylene with D2 on platinum in deuteroperchloric acid solution gives the same results.A computer simulation based on the above mechanism can reproduce quantitatively not only the present distributions but also others given in the literature, even those observed for the gas-phase heterogeneous reduction.
