Direct Hydrogen Transfer Research Articles

Mechanism of Generation of Volatile Hydrides of Trace Elements by Aqueous Tetrahydroborate(III). Mass Spectrometric Studies on Reaction Products and Intermediates

The mechanism of generation of volatile metal/metalloid hydrides by derivatization with borane complexes is presented. This reaction has been employed for ultratrace element analysis since 1972 and has been the source of much controversy in regard to the reaction mechanism. Here we investigated this derivatization by using As(III), Sb(III), Bi(III), MeAsO(OH)2, and Me2AsO(OH) as model analytes and NaBH4, NaBD4, tert-BuNH2BH3, and Me2NHBH3 as borane reagents. The identification of reaction products and intermediates observed under various reaction conditions was performed by gas chromatography/mass spectrometry and electrospray ionization mass spectrometry. An alternative reaction model, based on the formation of analyte-borane complex (ABC) intermediates, is able to reconcile all the experimental evidence reported in the literature. In this study, we provide definitive evidence of the ABC hydride generation mechanism, which shows that the generation of volatile hydrides occurs via formation of ABC intermediates between hydroboron species and the analyte substrate followed by the direct transfer of hydrogen from boron to the analyte atom, and fast hydrolysis leading to the final product.

Mechanistic Borderline between One-Step Hydrogen Transfer and Sequential Transfers of Electron and Proton in Reactions of NADH Analogues with Triplet Excited States of Tetrazines and Ru(bpy)32+*

Efficient energy transfer from Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine, denotes the excited state) to 3,6-disubstituted tetrazines [R(2)Tz: R = Ph (Ph(2)Tz), 2-chlorophenyl [(ClPh)(2)Tz], 2-pyridyl (Py(2)Tz)] occurs to yield the triplet excited states of tetrazines ((3)R(2)Tz(*)), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)(3)(2+). The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using (3)R(2)Tz(*) by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to (3)R(2)Tz(*) proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of (3)R(2)Tz(*) and the protonation reactivity of R(2)Tz(*)(-). In the case of (3)Ph(2)Tz(*), which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)(2)Tz; Py(2)Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH(2)) to (3)Ph(2)Tz(*) without formation of the radical cation (AcrH(2)(*)(+)). The rate constant of the direct hydrogen transfer from AcrH(2) to (3)Ph(2)Tz(*) is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to (3)Ph(2)Tz(*), exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPr(i)) and 1-benzyl-1,4-dihydronicotinamide (BNAH) to (3)R(2)Tz(*) occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements.

Addition of Difluorocarbene to 4‘,5‘-Unsaturated Nucleosides: Synthesis and Deoxygenation Reactions of Difluorospirocyclopropane Nucleosides1

Synthetic routes to 4'-(2,2-difluorospirocyclopropane) analogues of adenosine, cytidine, and uridine are described. Treatment of 2',3'-O-isopropylidene-4',5'-unsaturated compounds derived from adenosine and uridine with difluorocarbene (generated from PhHgCF3 and NaI) gave diastereomeric mixtures of the 2,2-difluorospirocyclopropane adducts. Stereoselectivity resulting from hindrance by the isopropylidene group favored addition at the beta face. Removal of base and sugar protecting groups gave new difluorospirocyclopropane nucleoside analogues. The protected uridine analogue was converted into its cytidine counterpart via a 4-(1,2,4-triazol-1-yl) intermediate. Stannyl radical-mediated deoxygenation of the 3'-O-TBS-2'-thionocarbamate derivatives gave the 2'-deoxy products of direct hydrogen transfer. In contrast, identical treatment of the 2'-O-TBS-3'-thionocarbamate isomers resulted in opening of the vicinal difluorocyclopropane ring upon generation of a C3' radical followed by homoallylic hydrogen transfer to give 4'-(1,1-difluoroethyl)-3',4'-unsaturated nucleoside derivatives. Structural aspects and biological effect considerations are discussed.

Mechanistic aspects of transition metal-catalyzed hydrogen transfer reactions

In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.

The mechanism of formation of volatile hydrides by tetrahydroborate(III) derivatization: A mass spectrometric study performed with deuterium labeled reagents

In order to clarify the mechanism of hydride formation, the isotopic composition of arsine, stibine, bismuthine, germane, stannane and hydrogen selenide formed by derivatization with either NaBD 4 (TDB) or NaBH 4 (THB) with inorganic As(III), Sb(III), Bi(III), Ge(IV), Sn(IV) and Se(IV) in aqueous reaction media and under various reaction conditions was determined. Batch hydride generation and gas chromatography–mass spectrometry (GC–MS) were employed. The analyte, present in 0.5–5 ml of acid solution (0.1–10 M in HCl or HNO 3 or HClO 4) was derivatized with 1 ml of 0.25–0.5 M TDB / THB in 0.1 M NaOH solution. For TDB derivatization in H 2O reaction media, almost pure BiD 3 and SbD 3 were always obtained for Bi(III) and Sb(III). Nearly pure AsD 3 could be obtained only under some reaction conditions. In general, for As(III), the isotopic composition of the arsines depends strongly on reaction conditions and included all possible AsH n D 3− n from almost pure AsD 3 to almost pure AsH 3. For Ge(IV) and Sn(IV), the isotopic composition of generated GeH n D 4− n and SnH n D 4− n depends on reaction conditions, but pure GeD 4 and SnD 4 could never be obtained. Pure H 2Se was obtained in all cases, independent of reaction conditions. The occurrence of side reactions involving D–H exchange in TBD during its hydrolysis and before the derivatization step, as well as on recently formed hydrides following derivatization was investigated. D–H exchange in TDB during acid hydrolysis appears to occur to a limited extent. Amongst the hydrides, H 2Se undergoes H–D exchange whereas germane and stannane do not exchange at all. Arsine undergoes D–H exchange at elevated acidities (pH < 0) whereas stibine and bismuthine do not exchange significantly during the generation and stripping steps. A reaction model for hydride generation is proposed accounting for primary reactions giving rise to hydride formation through reaction intermediates, as well as side reactions involving D–H exchange and decomposition of reactive hydroboron species, reaction intermediates and final products. Hydrides are formed by direct hydrogen transfer from boron to the analyte atom, most likely through concerted mechanisms taking place via reaction intermediates.

The Mechanism of Aluminum-Catalyzed Meerwein−Schmidt−Ponndorf−Verley Reduction of Carbonyls to Alcohols

The mechanistic details of the Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of ketones to the corresponding alcohols were investigated both experimentally and computationally. Density functional theory (DFT) was used to assess the energetics of several proposed pathways (direct hydrogen transfer, hydridic, and radical). Our results demonstrate that a direct hydrogen transfer mechanism involving a concerted six-membered ring transition state is the most favorable pathway for all calculated systems starting from a small model system and concluding with the experimentally investigated BINOLate/Al/(i)PrOH/MePhC=O system. Experimental values for the activation parameters of acetophenone reduction using the BINOLate/Al/(i)PrOH system (DeltaG# = 21.8 kcal/mol, DeltaH# = 18.5 kcal/mol, DeltaS# = -11.7 au) were determined on the basis of kinetic investigation of the reaction and are in good agreement with the computational findings for this system. Calculated and experimental kinetic isotope effects support the concerted mechanism.

Iridium(I) versus Ruthenium(II). A Computational Study of the Transition Metal Catalyzed Transfer Hydrogenation of Ketones

We present a density functional theory based computational study comparing simplified models for the ruthenium(II)- and iridium(I)-catalyzed transfer hydrogenation of ketones. For the ruthenium compound our results confirm earlier findings that the hydrogenation involves a ruthenium hydride and occurs via a concerted hydrogen transfer mechanism with no direct ruthenium−ketone binding along the reaction path. In contrast, for the iridium compound our calculations suggest that the reaction proceeds via direct hydrogen transfer between simultaneously coordinated ketone and alcohol. We find that for both metal complexes the formation of a very stable metal−alkoxide complex plays an important role. For the ruthenium-catalyzed reaction it constitutes a resting state that does not take an active part in the transfer hydrogenation, while for the iridium-catalyzed reaction it is an important intermediate along the reaction path.

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Quenching of n,pi*-excited states in the gas phase: variations in absolute reactivity and selectivity.

The quenching of the n,pi*-excited azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by 19 heteroatom-containing electron and hydrogen donors, that is, amines, sulfides, ethers, and alcohols, was investigated in the gas phase. Deuterium isotope effects were measured for 9 selectively deuterated derivatives. The data support the involvement of an excited charge-transfer complex, that is, an exciplex, for tertiary amines and sulfides, and a competitive direct hydrogen transfer from the C-H bonds of ethers or from the N-H or O-H bonds of secondary and primary amines or alcohols. The recently observed "inverted" solvent effect for the fluorescence quenching of azoalkanes by amines and sulfides in solution is supported by the observed rate constants in the gas phase, which are substantially larger than those in solution. A more pronounced inverted solvent effect for the weaker electron-donating sulfides and a presumably faster exciplex deactivation result in a switch-over in absolute reactivity relative to tertiary amines in the gas phase. Most importantly, the kinetic data demonstrate that the reactivity of the strongly dipolar O-H and N-H bonds in photoinduced hydrogen abstraction reactions shows a larger decrease upon solvation than that of the less polar C-H bonds. The azoalkane data are compared with previous studies on quenching of n,pi*-triplet-excited ketones in the gas phase.

Energetics of Radical Transfer in DNA Photolyase

Charge separation and radical transfer in DNA photolyase from Escherichia coli is investigated by computing electrostatic free energies from a solution of the Poisson-Boltzmann equation. For the initial charge separation 450 meV are available. According to recent experiments [Aubert et al. Nature 2000, 405, 586-590] the flavin receives an electron from the proximal tryptophan W382, which consequently forms a cationic radical WH(*)(+)382. The radical state is subsequently transferred along the triad W382-W359-W306 of conserved tryptophans. The radical transfer to the intermediate tryptophan W359 is nearly isoenergetic (58 meV uphill); the radical transfer from the intermediate W359 to the distal W306 is 200 meV downhill in energy, funneling and stabilizing the radical state at W306. The resulting cationic radical WH(*)(+)306 is further stabilized by deprotonation, yielding the neutral radical W(*)306, which is 214 meV below WH(*)(+)306. The time scale of the charge recombination process yielding back the resting enzyme with FADH(*) is governed by reprotonation of W306, with a calculated lifetime of 1.2 ms that correlates well with the measured lifetime of 17 ms. In photolyase from Anacystis nidulans the radical state is partially transferred to a tyrosine [Aubert et al. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 5423-5427]. In photolyase from Escherichia coli, there is a tyrosine (Y464) close to the distal tryptophan W306 that could play this role. We show that this tyrosine cannot be involved in radical transfer, because the electron transfer from tyrosine to W306 is much too endergonic (750 meV) and a direct hydrogen transfer is likely too slow. Coupling of specific charge states of the tryptophan triad with protonation patterns of titratable residues of photolyase is small.

Tyr275 and Lys279 stabilize NADPH within the catalytic site of NADPH:protochlorophyllide oxidoreductase and are involved in the formation of the enzyme photoactive state.

Fluorescence spectroscopic and kinetic analysis of photochemical activity, cofactor and substrate binding, and enzyme denaturation studies were performed with highly purified, recombinant pea NADPH:protochlorophyllide oxidoreductase (POR) heterologously expressed in Escherichia coli. The results obtained with an individual stereoisomer of the substrate [C8-ethyl-C13(2)-(R)-protochlorophyllide] demonstrate that the enzyme photoactive state possesses a characteristic fluorescence maximum at 646 nm that is due to the presence of specific charged amino acids in the enzyme catalytic site. The photoactive state is converted directly into an intermediate having fluorescence at 685 nm in a reaction involving direct hydrogen transfer from the cofactor (NADPH). Site-directed mutagenesis of the highly conserved Tyr275 (Y275F) and Lys279 (K279I and K279R) residues in the enzyme catalytic pocket demonstrated that the presence of these two amino acids in the wild-type POR considerably increases the probability of photoactive state formation following cofactor and substrate binding by the enzyme. At the same time, the presence of these two amino acids destabilizes POR and increases the rate of enzyme denaturation. Neither Tyr275 nor Lys279 plays a crucial role in the binding of the substrate or cofactor by the enzyme. In addition, the presence of Tyr275 is absolutely necessary for the second step of the protochlorophyllide reduction reaction, "dark" conversion of the 685 nm fluorescence intermediate and the formation of the final product, chlorophyllide. We propose that Tyr275 and Lys279 participate in the proper coordination of NADPH and PChlide in the enzyme catalytic site and thereby control the efficiency of the formation of the POR photoactive state.

Submicron Multiphoton Free-Form Fabrication of Proteins and Polymers: Studies of Reaction Efficiencies and Applications in Sustained Release

Nonlinear multiphoton photo-cross-linking and photopolymerization of proteins and polymers in solution have been used to direct the three-dimensional assembly of micron scale objects. Two aspects of fabricated proteinatious matrixes are examined in this paper: the efficiency of protein photopolymerization and the application of fabricated matrixes as sustained release devices. The efficiency of photoactivated cross-linking of the proteins bovine serum albumin and fibrinogen, using rose bengal, have been determined and found to vary with photosensitizer concentration. This concentration dependence suggests that the mechanism for protein cross-linking is a direct hydrogen transfer between an amino acid residue of the protein and the dye molecule itself. A comparison of the surface structure of single and multiple protein oligomers is undertaken and shown to vary significantly depending on fabrication materials. Alkaline phosphatase bioactivity, upon entrapment in a protein structure, is maintained. The properties of fabricated protein matrixes as sustained release devices is also examined. The rates of diffusion of fluorescently labeled dextrans (10 and 40 kDa) from an optically fabricated BSA matrix vary with molecular weight and are linear with cross-link density. The half-life of release of 10 kDa dextran-TMR from a BSA micron scale structure is less than or equal to 6 min while 40 kDa dextran-TMR half-life of release is 25 min. Finally, rhodamine 610, a typical drug size molecule, was entrapped in an acrylamide structure, and its release is found to be diffusion-limited with half-lives of 10−31 min, depending on cross-link density.

Solid–solid palladium-catalysed water reduction with zinc: mechanisms of hydrogen generation and direct hydrogen transfer reactions

Facile generation of hydrogen gas from water takes place under moderate conditions in the presence of zinc powder and catalytic palladium on carbon; 82% conversion of zinc is obtained. An unusually large kinetic isotope effect is observed using D2O (kH/kD=14), which may reflect the cleavage of both O–H bonds in the rate-determining step. Experiments using D2O–H2O mixtures evidence that water molecules adsorbed on the catalyst surface undergo H–D exchange reactions (with molecules from the solvent bulk) that are approximately 100 times faster than the hydrogen generation reaction. The primary factors in this system appear to be palladium–hydrogen and zinc–oxygen interactions. Conversely, in the presence of an organic hydrogen acceptor, such as benzaldehyde, a different course is realised, consisting of direct hydrogen transfer from ‘‘zinc-activated ’’ water to the substrate, without the participation of Pd–H intermediates. Quantitative hydrogenation of benzaldehyde to benzyl alcohol, and of aromatic nitro compounds to the corresponding amines, is obtained. Another application of the above system is the specific deutero-dehalogenation of aromatic halides. Possible mechanisms and the implications of a chemical reaction involving two macroscopic solid particles are discussed.

A density functional study on the formation of stereoerrors in the stereoselective propene polymerization with zirconocene catalysts

Abstract Recent labeling experiments have detected stereoerrors in the isotactic polymerization of propene with zirconocenium catalysts, which are characterized by a deuterium transfer from the polymer chain to a methyl group. Using Cp 2 Zr- iso -butyl + as a model system, two alternative mechanisms leading to the observed products are investigated using fully nonlocal levels of density functional theory (DFT). The direct hydrogen transfer starting from a γ-agostic intermediate requires an overall activation relative to the β-agostic resting state in excess of 200 kJ mol −1 and can be excluded. The sequential mechanism involves a (i) β-elimination to yield an initial iso -butene π-complex; (ii) a subsequent internal rotation of this π-complex; and (iii) reinsertion of the olefin into the ZrH bond via two consecutive formations of β-agostic interactions, with maximum relative energies along this path between 35 and 54 kJ mol −1 (depending on functionals and basis sets employed). The subsequent steps are the reverse of the first three steps and can lead to an inverted iso -butyl complex with migrated deuterium. Although certain barrier heights are somewhat sensitive to the density functional employed (BP86 vs. B3LYP), the sequential mechanism is much more favorable energetically. Loss of olefin from the intermediate π-complex is endothermic by 104–114 kJ mol −1 and should therefore not occur.

Ab initio MO and TST calculations for the rate constant of the HNO+NO2→HONO+NO reaction

Potential-energy surfaces for various channels of the HNO+NO2 reaction have been studied at the G2M(RCC,MP2) level. The calculations show that direct hydrogen abstraction leading to the NO+cis-HONO products should be the most significant reaction mechanism. Based on TST calculations of the rate constant, this channel is predicted to have an activation energy of 6–7 kcal/mol and an A factor of ca. 10−11 cm3 molecule−1 s−1 at ambient temperature. Direct H-abstraction giving NO+trans-HONO has a high barrier on PES and the formation of trans-HONO would rather occur by the addition/1,3-H shift mechanism via the HN(O)NO2 intermediate or by the secondary isomerization of cis-HONO. The formation of NO+HNO2 can take place by direct hydrogen transfer with the barrier of ca. 3 kcal/mol higher than that for the NO+cis-HONO channel. The formation of HNO2 by oxygen abstraction is predicted to be the least significant reaction channel. The rate constant calculated in the temperature range 300–5000 K for the lowest energy path producing NO+cis-HONO gave rise to © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 729–736, 1998

Ab initio MO and TST calculations for the rate constant of the HNO+NO2?HONO+NO reaction

Potential-energy surfaces for various channels of the HNO+NO2 reaction have been studied at the G2M(RCC,MP2) level. The calculations show that direct hydrogen abstraction leading to the NO+cis-HONO products should be the most significant reaction mechanism. Based on TST calculations of the rate constant, this channel is predicted to have an activation energy of 6–7 kcal/mol and an A factor of ca. 10−11 cm3 molecule−1 s−1 at ambient temperature. Direct H-abstraction giving NO+trans-HONO has a high barrier on PES and the formation of trans-HONO would rather occur by the addition/1,3-H shift mechanism via the HN(O)NO2 intermediate or by the secondary isomerization of cis-HONO. The formation of NO+HNO2 can take place by direct hydrogen transfer with the barrier of ca. 3 kcal/mol higher than that for the NO+cis-HONO channel. The formation of HNO2 by oxygen abstraction is predicted to be the least significant reaction channel. The rate constant calculated in the temperature range 300–5000 K for the lowest energy path producing NO+cis-HONO gave rise to © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 729–736, 1998

Coal hydroliquefaction using highly dispersed catalyst precursors

In order to discuss the hydrogen transfer process in coal liquefaction with a catalyst in the presence of a donor solvent, hydroliquefaction of Yallourn, Wyoming, Illinois No. 6, and Mi-ike coals and cracking of benzyl phenyl ether (BPE) were carried out in tetralin or tetralin/naphthalene mixed solvent under a hydrogen atmosphere with highly dispersed catalyst precursors such as Fe(CO) 5S, Mo(CO) 6S, and Ru 3(CO) 12. In the absence of the catalyst, more than 70% of hydrogen was transferred from tetralin, as determined by the formation of naphthalene. In the presence of Mo(CO) 6S and Ru 3(CO) 12, however, the amount of hydrogen transferred from tetralin decreased to 15–40% of the total hydrogen and that from gas phase increased to 60–85% of the hydrogen required to stabilize coal fragment radicals even with an excess amount of tetralin. When the reaction was carried out in the tetralin/naphthalene mixed solvent, little hydrogenation of naphthalene occurred even with the active catalyst. This strongly supports the assertion that a decrease in the amount of naphthalene in the catalyzed liquefaction of coal in tetralin with a catalyst can be ascribed to the direct hydrogen transfer from molecular hydrogen to coal fragment radicals. In the presence of coal or benzyl phenyl ether, little or no hydrogenation of naphthalene occurred.

Anthracene singlet quenching by indoles in media of different polarities: mechanism and chemical photoreaction

The quenching of excited singlet anthracene by indole derivatives in solvents of different polarity has been studied using static and time-resolved techniques. The quenching process is dependent on the nature of the indole derivative and on the polar properties of the medium. In non-polar solvents photoreaction takes place with unit efficiency in compounds which have a hydrogen atom bound to the nitrogen. The efficiency decreases in polar solvents and is considerably suppressed in aqueous solvent mixtures. The results here obtained indicate that charge transfer is the primary step for the photoreaction in polar solvents. For compounds with the indolic N—H group, direct hydrogen transfer is the dominant mechanism in non-polar media. The importance of radical ion formation in these anthracene–indole systems is investigated as a function of the indole structure and solvent polarity.

A novel hydrogen rearrangement of bifunctionalo-xylylene derivatives in chemical ionization mass spectrometry

A novel hydrogen rearrangement reaction was found in benzyl ions that have a (benzyl) group (Z=O or N) at their ortho position. From deuterium labeling experiments, mass analyzed ion kinetic energy spectra with collisional activation, and examination of structure-peak intensity correlation, a mechanism is proposed which involves a direct transfer of hydrogen (hydride) to the structurally remote but spatially close cationic center. The possibility of an alternative, less likely mechanism including an ion/neutral complex and/or a proton-bridged complex has also been discussed.

Reactions of carboxylic acids on the Pd(111)-(2 × 2)O surface: multiple roles of surface oxygen atoms

The reactions of formic acid, acetic acid, and propanoic acid were examined on the oxygen-dosed Pd(111) surface. Adsorption of these carboxylic acids on Pd(111)-(2 × 2)O at 170 K produced adsorbed carboxylate species via direct transfer of the acid hydrogen to the oxygen adlayer. In addition, the presence of adsorbed oxygen suppressed the formation of the hydrogen-bonded catemers observed for carboxylic acids on the clean surface. The stability of carboxylate species on Pd(111) was found to depend upon the substituent on the carboxyl group and upon the presence of adsorbed oxygen. Formate species were the least stable on Pd(111) and decomposed primarily through a dehydrogenation pathway at 280 K, evolving CO 2 and releasing hydrogen atoms to the surface. The temperature required for formate decomposition was 20 K higher on the oxygen-dosed surface than on the clean surface, suggesting that the presence of oxygen adatoms stabilized the formate adlayer. Acetate decarboxylation occurred at 305 and 420 K on the oxygen-dosed surface. The lower temperature acetate decomposition pathway was also observed for acetate species adsorbed on the clean Pd(111) surface, and resulted in the desorption of primarily CO 2. In contrast, the decomposition channel at 420 K was associated with acetate species stabilized by the presence of adsorbed oxygen. Decomposition of oxygen-stabilized acetates released CO 2, CH 1COOH, and H 2. Propanoate species produced from propanoic acid on the Pd(111)-(2 × 2)O surface decomposed at 380 K. 25 K higher than on the clean surface. The multiple roles of oxygen adatoms in carboxylate formation and stabilization on Pd(111) may be generalized both to the reactions of other oxygenates and to the reactivity of other group-VIII and -IB metals.