Theoretical investigation of the decarbonylation of acetaldehyde by Fe+ and Cr+.

We report a comprehensive theoretical study on the decarbonylation of acetaldehyde by Fe+ and Cr+. Various intermediates, transition states, and products involved in the decarbonylation reactions are fully optimized at the B3LYP/6-311+G(2df,2pd) level of theory. The potential energy surfaces (PESs) corresponding to [M,O,C2,H4]+(M=Cr and Fe) are examined in detail using B3LYP and CCSD(T) methods, respectively. The validity of these theoretical methods is calibrated with respect to the available thermochemical data. Calculations suggest that the Cr+ mediated decarbonylation of acetaldehyde takes place in four steps on the sextet surface: encounter complexation, C-C activation, aldehyde H-shift, and nonreactive dissociation, in good accordance with the Co+ mediated decarbonylation of acetaldehyde [Zhao, Zhang, Guo, Wu, Lu, Chem. Phys. Lett. 2005, 414, 28], while for the Fe+/acetaldehyde system decarbonylation can occur on both the quartet and the sextet PESs. The quartet pathway, which experiences spin-orbit coupling between the two surfaces, is energetically more favorable; whereas along the sextet decarbonylation coordinate several high-energy barriers are revealed. The theoretical results are compared with the experimental product kinetic energy and angular distributions of decarbonylation of acetaldehyde by Fe+ and Cr+ measured using a crossed-beam technique [Sonnenfroh, Farrar, J. Am. Chem. Soc. 1986, 108, 3521].     

Product detection of the CH radical reaction with acetaldehyde

The reaction of the methylidyne radical (CH) with acetaldehyde (CH(3)CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr(3) and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH(2)), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH(3)), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD(3)CHO) provides fine details about the reaction mechanism. The CH(2) elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C-H bond of the methyl group of acetaldehyde is likely to lead to a C(3)H(5)O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH(3), which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein.     

Theoretical survey of the reaction between osmium and acetaldehyde

The mechanism of the reaction of osmium atom with acetaldehyde has been investigated with a DFT approach. All the stationary points are determined at the UB3LYP/sdd/6-311++G** level of the theory. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of a CH₃CHO-metal complex followed by C-C, aldehyde C-H, C-O, and methyl C-H activation. These reactions can lead to four different products (HOsCH₃ + CO, OsCO + CH₄, OsCOCH₃ + H, and OsO + C₂H₄). The minimum energy reaction path is found to involve the spin inversion in the initial reaction step. This potential energy curve-crossing dramatically affects reaction exothermic. The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.     

Hydrothermal C-C bond formation and disproportionation of acetaldehyde with formic acid

Reaction pathways and kinetics of C2 (carbon-two) aldehyde, acetaldehyde (CH3CHO), and formic acid HCOOH or HOCHO, are studied in neutral and acidic subcritical water at 200-250 degrees C. Acetaldehyde is found to exhibit (i) the acid-catalyzed C-C bond formation between acetaldehyde and formic acid, which generates lactic acid (CH3CH(OH)COOH), (ii) the cross-disproportionation, where formic acid reduces acetaldehyde into ethanol, and (iii) the aldol condensation. The lactic acid formation is a green C-C bond formation, proceeding without any organic solvents or metal catalysts. The new C-C bond formation takes place between formic acid and aldehydes irrespective of the presence of alpha-hydrogens. The hydrothermal cross-disproportionation produces ethanol without base catalysts and proceeds even in acidic condition, in sharp contrast to the classical base-catalyzed Cannizzaro reaction. The rate constants of the reactions (i)-(iii) and the equilibrium constant of the lactic acid formation are determined in the temperature range of 200-250 degrees C and at HCl concentrations of 0.2-0.6 M (mol/dm(3)). The reaction pathways are controlled so that the lactic acid or ethanol yield may be maximized by tuning the reactant concentrations and the temperature. A high lactic acid yield of 68% is achieved when acetaldehyde and formic acid are mixed in hot water, respectively, at 0.01 and 2.0 M in the presence of 0.6 M HCl at 225 degrees C. The ethanol yield attained 75% by the disproportionation of acetaldehyde (0.3 M) and formic acid (2.0 M) at 225 degrees C in the absence of added HCl.     

Comparison of steric and electronic requirements for C-C and C-H bond activation. Chelating vs nonchelating case

C-H bond activation was observed in a novel PCO ligand 1 (C(6)H(CH(3))(3)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)()]BF(4) (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degrees C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH(3))Rh(C(6)H(CH(3))(2)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))BF(4)), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF(4)(-) anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.     

Competition between C-C and C-H Activation in Reactions of Neutral Nickel Atom with Cycloalkanes （n = 3-7）

A theoretical investigation of the reaction mechanisms for C-H and C-C bond activation processes in the reaction of Ni with cycloalkanes C,,H2. （n = 3-7） is carried out. For the Ni ＋ CnH2, （n = 3, 4） reactions, the major and minor reaction channels involve C-C and C-H bond activations, respectively, whereas Ni atom prefers the attacking of C-H bond over the C-C bond in CnH2n （n = 5=7）. The results are in good agreement with the experimental study. In all cases, intermediates and transition states along the reaction paths of interest are characterized, It is found that both the C-H and C-C bond activation processes are proposed to proceed in a one-step manner via one transition state. The overall C-H and C-C bond activation processes are exothermic and involve low energy barriers, thus transition metal atom Ni is a good mediator for the activity of cycloalkanes CnH2n （n = 3 -7）.     

Density Functional Study on the Reaction Mechanism for the Reaction of Ni^＋ with Ethane

The mechanism of the reaction of Ni^ (^2D) with ethane in the gas-phase was studied by using density functional theory.Both the B3LYP and BLYP functionals with standard all-electron basis sets are used to give the detailed information of the potential energy surface (PES) of [Ni,C2,H6]^ . The mechanisms forming the products CH4 and H2 in the reaction of Ni^ with ethane are proposed.The reductive eliminations of CH4 and H2 are typical addition-elimination reactions.Each of the two reactions consists of two elementary steps:C-C or C-H bond activations to form inserted species followed by isomerizations to from product-like intermediate.The rate determining steps for the elimination reactions of forming CH4 and H2 are the isomerization of the inserted species rather than C-C or C-H bond activations .The elimination reaction of forming H2 was found to be thermodynamically favored compared to that of CH4.     

Photodissociation of spatially aligned acetaldehyde cations

Photofragment translational energy and angular distributions are reported for the photodissociation of acetaldehyde cations in the wavelength range 354-363 nm obtained using the DC slice ion imaging technique. Vibrationally selected parent ions were produced by 2+1 resonance-enhanced multiphoton ionization (REMPI) via the 3s<--n Rydberg transition, with photodissociation resulting from absorption of a fourth additional photon. Three product channels were observed: HCO+, CH3CO+, and CH4+. The angular distributions reveal that all product channels have a predominantly parallel recoil anisotropy although the lower beta2 parameter of CH3CO+ indicates the concomitant presence of a perpendicular component. Furthermore, the distinct angular distribution of the CH3CO+ fragments shows a large value of the higher order Legendre polynomial term, providing evidence that acetaldehyde cations are spatially aligned during the ionization process.     

C-H vs C-C bond activation of acetonitrile and benzonitrile via oxidative addition: rhodium vs nickel and Cp* vs Tp' (Tp' = hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp* = η(5)-pentamethylcyclopentadienyl)

The photochemical reaction of (C(5)Me(5))Rh(PMe(3))H(2) (1) in neat acetonitrile leads to formation of the C-H activation product, (C(5)Me(5))Rh(PMe(3))(CH(2)CN)H (2). Thermolysis of this product in acetonitrile or benzene leads to thermal rearrangement to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(CH(3))(CN) (4). Similar results were observed for the reaction of 1 with benzonitrile. The photolysis of 1 in neat benzonitrile results in C-H activation at the ortho, meta, and para positions. Thermolysis of the mixture in neat benzonitrile results in clean conversion to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(C(6)H(5))(CN) (5). DFT calculations on the acetonitrile system show the barrier to C-H activation to be 4.3 kcal mol(-1) lower than the barrier to C-C activation. A high-energy intermediate was also located and found to connect the transition states leading to C-H and C-C activation. This intermediate has an agostic hydrogen interaction with the rhodium center. Reactions of acetonitrile and benzonitrile with the fragment [Tp'Rh(CNneopentyl)] show only C-H and no C-C activation. These reactions with rhodium are compared and contrasted to related reactions with [Ni(dippe)H](2), which show only C-CN bond cleavage.     

Activation of alkenes and H2 by [Fe]-H2ase model complexes

The established ability of the Fe(II) bridging hydride species (micro-H)(micro-pdt)[Fe(CO)2(PMe3)]2+, 1-H+, to take-up and heterolytically activate dihydrogen, resulting in H/D scrambling of H2/D2 and H2/D2O mixtures (Zhao et al. Inorg. Chem. 2002, 41, 3917) has prompted a study of simultaneous alkene/H2 activation by such [Fe]H2ase model complexes. That the required photolysis produced an open site was substantiated by substitution of CO in 1-H+ by CH3CN with formation of structurally characterized [(micro-H)(micro-pdt)[Fe(CO)2(PMe3)][Fe(CO)(CH3CN)(PMe3)]]+[PF6]-. Under similar photolytic conditions, H/D exchange reactions between D2 and terminal alkenes (ethylene, propene and 1-butene), but not bulkier alkenes such as 2-butene or cyclohexene, were catalyzed by 1-H+ and the edt (SCH2CH2S) analogue, 2-H+. Substantial regioselectivity for H/D exchange at the internal vinylic hydrogen was observed. The extent to which the olefins were deuterium enriched vs deuterated was catalyst dependent. The stabilizing effect of the binuclear chelating ligands, SCH2CH2CH2S, pdt, and SCH2CH2S, edt, is required for the activity of binuclear catalysts, as the mono-dentate micro-SEt analogue decomposed to inactive products under the photolytic conditions of the catalysis. Reactions of 1 and 2 with EtOSO2CF3 yielded the S-alkylated products, [(micro-SCH2CH2CH2SEt)[Fe(CO)2(PMe3)]2]+[SO3CF3]- (1-Et+), and 2-Et+, rather than micro-C2H5 analogues to the micro-H of 1-H+. The stability and lack of reactivity toward H2 of 1-Et+ and 2-Et+, indicates they are not on the reaction path of the olefin/D2 H/D exchange process. A mechanism with olefin binding to an open site created by CO loss and formation of an Fe-(CH2CHDR) intermediate is indicated. A likely role of a binuclear chelate effect is implicated for the unique S-XXX-S cofactor in the active site of [Fe]H2ase.     

HD isotope effect of methyl internal rotation for acetaldehyde in ground state as calculated from a multicomponent molecular orbital method

We have analyzed the differences in the methyl internal rotation induced by the HD isotope effect for acetaldehyde (CH(3)CHO) and deuterated acetaldehyde (CD(3)CDO) in ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. The rotational constant of CH(3)CHO was in reasonable agreement with experimental one due to the adequate treatment of the protonic quantum effect by the MC_MO method. The C-D bond distances were about 0.007 A shorter than the C-H distances because of the effect of anharmonicity of the potential. The Mulliken population for CD(3) in CD(3)CDO is larger than that for CH(3) in CH(3)CHO because the distribution of wavefunctions for the deuterons was more localized than that for the protons. The barrier height obtained by the MC_MO method for CH(3)CHO was estimated as 401.4 cm(-1), which was in excellent agreement with the experimentally determined barrier height. We predicted the barrier height of CD(3)CDO as 392.5 cm(-1). We suggest that the internal rotation of the CD(3) group was more facile than that of the CH(3) group because the C-D bond distance was observed to be shorter than the C-H distance. Additionally the localized electrons surrounding the CD(3) group in CD(3)CDO caused the extent of hyperconjugation between the CD(3) and CDO groups to be smaller than that in the case of CH(3)CHO, which may have also contributed to the observed differences in methyl internal rotation. The differences in bond distances and electronic populations induced by the H/D isotope effect were controlled by the difference in the distribution of wavefunctions between the protons and deuterons.     

High-pressure pulse-radiolysis study of the formation and decomposition of complexes with iron-carbon sigma bonds: mechanistic comparison for different metal centers

Volumes of activation for the formation and homolysis of the transient complexes (hedta)Fe(III)-CO(2)(2-) and (hedta)Fe(III)-CH(3)(-) (HOCH(2)CH(2)N(CH(2)CO(2-))CH(2)CH(2)N(CH(2)CO(2-))(2) = hedta) were determined using high-pressure pulse-radiolysis techniques. A comparison of the results with those for analogous complexes with other central transition-metal cations (M(n+)) and ligands (L) points out that (i) the reaction of M(n)L(m) with aliphatic radicals (R(*)) proceeds via an interchange ligand substitution mechanism, i.e. M(n)L(m) + R(*) --> L(m-1)M(n+1)-R + L, (ii) the homolysis of the metal-carbon bonds naturally follows the same mechanism, and (iii) the volume of activation for the homolysis reaction depends strongly on the nature of the central cation, i.e. larger for M(n+1) = Cr(III), Co(III), Ni(III) and smaller for Fe(III). The volume of activation for the reaction (hedta)Fe(III)-CO(2)(2-) + CO(2)(*-) + 2H(+) --> Fe(II)(hedta)(H(2)O)(-) + CO + CO(2) was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.     

C-C versus C-H activation and versus agostic C-C interaction controlled by electron density at the metal center

Based on the PCN ligand 2, a remarkable degree of control over C-C versus C-H bond activation and versus formation of an agostic C-C complex was demonstrated by choice of cationic [Rh(CO)(n)(C(2)H(4))(2-n)] (n=0, 1, 2) precursors. Whereas reaction of 2 with [Rh(C(2)H(4))(2)(solv)(n)]BF(4) results in exclusive C-C bond activation to yield product 5, reaction with the dicarbonyl precursor [Rh(CO)(2)(solv)(n)]BF(4) leads to formation of the C-H activated complex 9. The latter process is promoted by intramolecular deprotonation of the C-H bond by the hemilabile amine arm of the PCN ligand. The mixed monocarbonyl monoethylene Rh species [Rh(CO)(C(2)H(4))]BF(4) reacts with the PCN ligand 2 to give an agostic complex 7. The C-C activated complex 5 is easily converted to the C-H activated one (9) by reaction with CO; the reaction proceeds by a unique sequence of 1,2-metal-to-carbon methyl shift, agostic interaction, and C-H activation processes. Similarly, the C-C agostic complex 7 is converted to the same C-H activated product 9 by treatment with CO.     

Reaction of CH3CHO with Y+: A density functional theoretical study

The reaction mechanism of the Y⁺ cation with CH₃CHO has been investigated with a DFT approach. All the stationary points are determined at the UB3LYP/ECP/6-311++G** level of the theory. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of a CH₃CHO-metal complex followed by C-C, aldehyde C-H, methyl C-H and C-O activation. These reactions can lead to four different products (Y⁺CH₄ + CO, Y⁺CO + CH₄, Y⁺COCH₂ + H₂ and Y⁺O + C₂H₄). The minimum energy reaction path is found to involve the spin inversion in the different reaction steps, this potential energy curve-crossing dramatically affects reaction exothermic. The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.     

Reactions of atomic transition-metal ions with long-chain alkanes

Understanding metal ion interactions with long-chain alkanes not only is of fundamental importance in the areas of organometallic chemistry, surface chemistry, and catalysis, but also has significant implication in mass spectrometry method development for the analysis of polyethylene. Polyethylene represents one of the most challenging classes of polymers to be analyzed by mass spectrometry. In this work, reactions of several transition-metal ions including Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Ag+ with long-chain alkanes, C28H58 and C36H74, are reported. A metal powder and the nonvolatile alkane are co-deposited onto a sample target of a laser desorption/ionization (LDI) time-of-flight mass spectrometer. The metal ions generated by LDI react with the vaporized alkane during desorption. It is found that all these metal ions can form adduct ions with the long-chain alkanes. Fe+, Co+, and Ni+ produce in-source fragment ions resulting from dehydrogenation and dealkylation of the adduct ions. The post-source decay (PSD) spectra of the metal-alkane adduct ions are recorded. It is shown that PSD of Ag+ alkane adduct ions produces bare metal ions only, suggesting weak binding between this metal ion and alkane. The PSD spectra of the Fe+, Co+, and Ni+ alkane adduct ions display extensive fragmentation. Fragment ions are also observed in the PSD spectra of Cr+, Mn+, and Cu+ alkane adduct ions. The high reactivity of Fe+, Co+, and Ni+ is consistent with that observed in small alkane systems. The unusually high reactivity of Cr+, Mn+, and Cu+ is rationalized by a reaction scheme where a long-chain alkane first forms a complex with a metal ion via ion/induced dipole interactions. If sufficient internal energy is gained during the complex formation, metal ions can be inserted into C-H and C-C bonds of the alkane, followed by fragmentation. The thermal energy of the neutral alkane is believed to be the main source of the internal energy acquired in the complex. Finally, the implication of this work on mass spectrometry method development for polyethylene analysis is discussed.     

Bimetallic-induced tail-to-tail dimerization and C-H activation of methyl acrylate

An organometallic complex resulting from tail-to-tail dimerization and C-H activation of methyl acrylate (MA), [Mo(CO2Cp(eta 3-(MeO2C)CH[symbol: see text]CH[symbol: see text]CHCH2(CO2Me)] 2, has been fully characterized from the reaction of the heterobimetallic complex [Cp*Ni=Mo(mu-CO)(CO)2Cp] with MA and an exclusively eta 3-allyl bonding mode of the coupled ligand was established for the first time by X-ray diffraction; formation of 2 is accompanied by that of the mu 3-alkylidyne-capped cluster [NiMo2(mu 3-CCH2CO2Me)(CO)4Cp*Cp2] 3 which results from a double C-H activation of the CH2 group of MA; none of these reactions occur with the corresponding homodinuclear complexes.     

Shock‐tube and modeling study of acetaldehyde pyrolysis and oxidation

Pyrolysis and oxidation of acetaldehyde were studied behind reflected shock waves in the temperature range 1000–1700 K at total pressures between 1.2 and 2.8 atm. The study was carried out using the following methods, (1) time‐resolved IR‐laser absorption at 3.39 μm for acetaldehyde decay and CH‐compound formation rates, (2) time‐resolved UV absorption at 200 nm for CH₂CO and C₂H₄ product formation rates, (3) time‐resolved UV absorption at 216 nm for CH₃ formation rates, (4) time‐resolved UV absorption at 306.7 nm for OH radical formation rate, (5) time‐resolved IR emission at 4.24 μm for the CO₂ formation rate, (6) time‐resolved IR emission at 4.68 μm for the CO and CH₂CO formation rate, and (7) a single‐pulse technique for product yields. From a computer‐simulation study, a 178‐reaction mechanism that could satisfactorily model all of our data was constructed using new reactions, CH₃CHO (+M) → CH₄ + CO (+M), CH₃CHO (+M) → CH₂CO + H₂(+M), H + CH₃CHO → CH₂CHO + H₂, CH₃ + CH₃CHO → CH₂CHO + CH₄, O₂ + CH₃CHO → CH₂CHO + HO₂, O + CH₃CHO → CH₂CHO + OH, OH + CH₃CHO → CH₂CHO + H₂O, HO₂ + CH₃CHO → CH₂CHO + H₂O₂, having assumed or evaluated rate constants. The submechanisms of methane, ethylene, ethane, formaldehyde, and ketene were found to play an important role in acetaldehyde oxidation. © 2007 Wiley Periodicals, Inc. 40: 73–102, 2008     

Direct dynamics study on the reaction of acetaldehyde with ozone

The hydrogen abstraction reaction of ozone with acetaldehyde has been studied theoretically over the temperature range 250-2500 K. Two different reactive sites of acetaldehyde molecule, CH(3) and CHO groups have been investigated, and results confirm that the CHO group is a highly reactive site. In this study, the geometries and harmonic vibrational frequencies of all stationary points are calculated at the MPW1K, BHandHLYP, and MPWB1K levels of theory. The minimum energy paths (MEPs) were obtained at the MPW1K/6-31+G(d,p) level of theory. To refine the energies along the MEPs of each channel, single-point energy calculations were performed by a higher-level energy calculation method (denoted as HL). The rate constants were evaluated based on the MEPs from the HL method in the temperature range 250-2500 K by using the conventional transition state theory (TST), the canonical variational transition state theory (CVT), the microcanonical variational transition state theory (muVT), the CVT coupled with small-curvature tunneling (SCT) correction (CVT/SCT), and the muVT coupled with Eckart tunneling correction (muVT/Eckart). The fitted three-parameter Arrhenius expressions of the calculated CVT/SCT and muVT/Eckart rate constants of the H abstraction from CHO group are k CVT/SCT(T) = 4.92 x 10(-27).T 3.77.e(-7867.0/T) and k muVT/Eckart(T) = 2.10 x 10(-27).T(3.90).e(-7706.2/T), respectively. The fitted three-parameter Arrhenius expressions of the calculated CVT/SCT and muVT/Eckart rate constants of the H abstraction from CH3 group are k(CVT/SCT)(T) = 1.27 x 10(-27).T(3.94).e(-14554.1/T) and k muVT/Eckart(T) = 1.62 x 10(-26).T(3.66).e(-15459.8/T), respectively.     

CO2 reforming of CH4 on Ni(111): a density functional theory calculation

CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.     

CH2CHOH2+ + PN: a proton-transfer triple play

Quantum chemical calculations are used to explore the proton-transfer reactivity of O-protonated vinyl alcohol, CH2CHOH2+, with phosphorus nitride, PN. This reaction is relevant to the chemical evolution of interstellar clouds, since O-protonated vinyl alcohol has been postulated (and tentatively identified) as a product of the association reaction between interstellar H3O+ and C2H2, while PN is the most widespread and abundant phosphorus-containing molecule seen in astrophysical environments. Furthermore, the reaction exhibits an unusual mechanistic feature, namely, an extended "proton-transport catalysis" mechanism, which we characterize here as a "proton-transfer triple play". The reaction proceeds initially by proton transfer from CH2CHOH2+ to PN, then from PNH+ to CH2CHOH, and finally from CH3CHOH+ to PN, where the emphasized atom indicates the resultant site of protonation/deprotonation. Thus, the ultimate overall bimolecular proton-transfer reaction is expected to occur as CH2CHOH2+ + PN --> CH3CHO + PNH+; that is, the apparent favored product channel exhibits not only proton transfer but also keto/enol tautomerization. The triple-play mechanism can be rationalized in terms of the proton affinities of vinyl alcohol, acetaldehyde, and phosphorus nitride, which here are satisfactorily reproduced by high-level ab initio calculations. Other neutrals with a proton affinity appropriate for the possible triple-play mechanism converting CH2CHOH2+ to CH3CHO are also identified, with a view to encouraging experimental investigation of this mechanism.