Rearrangement as a probe for radical formation: bromomethylcyclopropane on oxygen-covered Mo(110)

Levinson, Joshua A.; Kretzschmar, Ilona; Sheehy, Michael; Deiner, L. Jay; Friend, Cynthia M.

doi:10.1016/s0039-6028(01)00998-0

Cited by 6 publications

(38 citation statements)

References 38 publications

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“…Since the onset of the β 1 -H 2 O peak at 430 K coincides with the maximum of the 410 K product peak, we propose that oxygen in the alkoxide is deposited on the surface prior to water formation. [11][12][13] Similarly, the second hightemperature water peak (β 2 ) at 525 K also lags slightly behind the maximum of the 520 K product.…”

Section: Resultsmentioning

confidence: 93%

“…These results provide evidence that 2-methylcyclopropylmethoxide is present and that the 900 cm -1 peak is due to some combination of C-O and C-C stretch vibrations. [11][12][13] Notably, the 900 cm -1 peak shifts to higher energy as a function of coverage. The ν(C-O) modes in alkoxides are known to shift to higher energy as a function of coverage due to dipole-dipole coupling, [30][31][32] reinforcing our assignment.…”

Section: Resultsmentioning

confidence: 98%

“…Definitive assignment of the products is hampered by the close resemblance of the fragmentation patterns for the C 5 hydrocarbons involved (Table 3). In analogy to 3-buten-1-ol, 12 the products evolved from the 2-methyl-4-butenoxide intermediate are 3-methyl-1-butene and 2-methyl-1,3-butadiene. Comparison with mass spectra from the NIST database 27 (Table 3) yields an estimate of 35:65 for the 3-methyl-1-butene/2-methyl-1,3-butadiene ratio in the 520 K peak of 2-methyl-4-buten-1-ol reaction.…”

Section: Resultsmentioning

confidence: 99%

“…The foundation of our conclusions in this study is provided by earlier extensive studies of alkoxide species on O-covered Mo(110). [11][12][13]19,29,[37][38][39][40] Our previous studies clearly demonstrated that (1) homolytic C-O bond dissociation in alkoxides bound to O-covered Mo(110) yields transient hydrocarbon radicals; 19,37 (2) gas-phase hydrocarbon radicals readily add to oxygen on Mo(110) at low temperature in the microsopic reverse of radical elimination from alkoxides; 38 (3) the temperature at which radical elimination from the alkoxides occurs correlates with the homolytic C-O bond strengths in the alkoxides; 11,29,39,40 (4) surface-mediated hydrogenation and dehydrogenation compete with radical elimination, leading to formation of gaseous olefins and alkanes, respectively; 29,39,40 (5) rearrangement of cyclopropylmethyl radical formed in the vicinity of the surface occurs at ∼400 K, indicating that the lifetime of this radical is on the order of ∼1 ns; [11][12][13] and (6) rearrangement of cyclopropyl rings is not induced by the surface. 11,41 An important aspect of our studies of radical-rearrangement processes is that once the radical rearranges, it can react with oxygen and remain stable on the surface to higher temperatures.…”

Section: Discussionmentioning

confidence: 99%

“…In an ongoing effort to develop tools for the detection of radicals and the determination of the radical lifetimes in the vicinity of surfaces, we have studied various radical-clock precursors and their reactivity on the oxygenmodified Mo(110) surface. Three different precursors (hydroxymethylcyclopropane, 11 bromomethylcyclopropane, 12 and cyclopropyl-methanethiol 13 ) form a cyclopropylmethoxide intermediate on the O/Mo(110) surface. Adsorbed cyclopropylmethoxide subsequently undergoes homolytic C-O bond cleavage at ∼350 K, yielding cyclopropylmethyl radical, 2 (X,Y ) H) in the vicinity of the surface.…”

Section: Introductionmentioning

confidence: 99%

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Kinetic Control of Surface Reactions: Regioselectivity in the Reaction of 2-Methylcyclopropylmethanol on Mo(110)−(6×1)−O

Kretzschmar

Friend

2002

J. Phys. Chem. B

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The reactions of a diastereomeric mixture of 2-methylcyclopropylmethanol are investigated on oxygen-modified Mo(110) as a means of probing the influence of lifetime and stability of hydrocarbon radicals formed in a surface reaction on selectivity. 2-Methylcyclopropylmethanol forms 2-methylcyclopropylmethoxide at ∼250 K. The C−O bond of 2-methylcyclopropylmethoxide subsequently breaks near 350 K, yielding the (2-methylcyclopropyl)methyl radical. The (2-methylcyclopropyl)methyl radical can undergo fast rearrangement to two different linear C5 homoallyl radical species. The two different ring-opening pathways are due to the asymmetric methyl substitution of the cyclopropyl unit. Only one pathway results in a surface-bound product: adsorbed 4-penten-2-oxide is formed via rearrangement to 4-penten-2-yl radical and subsequent trapping on surface oxygen. The 4-penten-2-oxide is identified by comparison of infrared and temperature-programmed reaction data for the reaction product to analogous data for the reference compound. Evolution of C5 hydrocarbons competes with trapping at 350 K. Careful analysis of temperature-programmed reaction data indicates that 3-methyl-1-butene, 2-methyl-1,3-butadiene, and 1,2-dimethylcyclopropane contribute to the 410 K product. However, the close resemblance of the fragmentation patterns for all C5 hydrocarbons renders quantitative analysis impossible. At elevated temperatures the C−O bond of 4-penten-2-oxide breaks and C5 hydrocarbon evolution sets in. 1-Pentene and 1,3-pentadiene, as well as propene, are formed in a ratio of 20:70:10 at ∼520 K. Above 900 K CO is formed from recombination of adsorbed carbon and oxygen. Our results are discussed in the context of relative reaction rates for different pathways available to hydrocarbon radicals and their relation to product selectivity.

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Section: Resultsmentioning

confidence: 93%

Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Kinetic Control of Surface Reactions: Regioselectivity in the Reaction of 2-Methylcyclopropylmethanol on Mo(110)−(6×1)−O

Kretzschmar

Friend

2002

J. Phys. Chem. B

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Insight into the Partial Oxidation of Propene: The Reactions of 2-Propen-1-ol on Clean and O-Covered Mo(110)

et al. 2003

J. Am. Chem. Soc.

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The reactions of 2-propen-1-ol (allyl alcohol) were studied on clean and O-covered Mo(110) to understand the effect of resonance stabilization and the presence of surface oxygen on reaction selectivity. Propene is the only gaseous hydrocarbon product evolved from allyl alcohol reaction on O-covered Mo(110). Water and dihydrogen are also produced, along with a small amount of adsorbed carbon. We estimated, using X-ray photoelectron spectroscopy, that approximately 70% of the 0.11 ML of 2-propen-1-ol that reacts forms propene. In contrast, the dominant reaction pathway on the clean surface is nonselective decomposition to adsorbed carbon and hydrogen, leading to a 23% selectivity for propene formation. On both clean and O-covered Mo(110), X-ray photoelectron spectroscopy and infrared spectroscopy identify allyloxy as the reaction intermediate yielding propene. These results are discussed in the context of propene oxidation and periodic trends in reactivity.

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Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces: Evidence for Transient Cation Formation near O-Covered Mo(110)

et al. 2007

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Rearrangement reactions of C4-alkoxides on O-covered Mo(110) have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclobutoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropylmethoxide during high-temperature annealing. In light of compelling new data, previous results are reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopropylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D2-cyclopropylmethanol was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 x 6)-O. The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered radicals are not known to rearrange in this manner. However, in none of the cases discussed could contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are discussed in the context of heterogeneous hydrocarbon oxidation.

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Rearrangement as a probe for radical formation: bromomethylcyclopropane on oxygen-covered Mo(110)

Cited by 6 publications

References 38 publications

Kinetic Control of Surface Reactions: Regioselectivity in the Reaction of 2-Methylcyclopropylmethanol on Mo(110)−(6×1)−O

Kinetic Control of Surface Reactions: Regioselectivity in the Reaction of 2-Methylcyclopropylmethanol on Mo(110)−(6×1)−O

Insight into the Partial Oxidation of Propene: The Reactions of 2-Propen-1-ol on Clean and O-Covered Mo(110)

Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces: Evidence for Transient Cation Formation near O-Covered Mo(110)

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