2680-01-5Relevant articles and documents
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.
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+.
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.
