Efficient Room‐Temperature Activation of Methane by TaN+ under C−N Coupling

The thermal reaction of diatomic tantalum nitride cation [TaN]⁺ with methane has been explored using FT‐ICR mass spectrometry complemented by high‐level quantum chemical calculation; based on this combined experimental/computational approach, mechanistic aspects of this novel, highly efficient C?N coupling process have been uncovered. In distinct contrast to [TaN]⁺, its lighter congeners [VN]⁺ and [NbN]⁺ are inert towards methane under ambient conditions, and the origins of the remarkably variable efficiencies of the three metal nitrides are uncovered by CCSD(T) calculations.     

Efficient Room‐Temperature Methane Activation by the Closed‐Shell,Metal‐Free Cluster [OSiOH]+: A Novel Mechanistic Variant

The closed‐shell cluster ion [OSiOH]⁺ is generated in the gas phase and its reactivity towards the thermal activation of CH₄ has been examined using Fourier transform‐ion cyclotron resonance (FT‐ICR) mass spectrometry in conjunction with state‐of‐the‐art quantum chemical calculations. Quite unexpectedly at room temperature, [OSiOH]⁺ efficiently mediates C?H bond activation, giving rise to [SiOH]⁺ and [SiOCH₃]⁺ with the concomitant formation of methanol and water, respectively. Mechanistic aspects for this unprecedented reactivity pattern are presented, and the properties of the [OSiOH]⁺/CH₄ couple are compared with those of the closed‐shell systems [OCOH]⁺/CH₄ and [MgOH]⁺/CH₄; the last two couples exhibit an entirely different reactivity scenario.     

Gas‐Phase Reactions of Cationic Vanadium‐Phosphorus Oxide Clusters with C2Hx (x=4, 6): A DFT‐Based Analysis of Reactivity Patterns

The reactivities of the adamantane‐like heteronuclear vanadium‐phosphorus oxygen cluster ions [V_xP_4?xO₁₀]^.+ (x=0, 2–4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas‐phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT‐based calculations; homolytic C? H bond activation constitutes the initial step, and for all systems the P? O^. unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen‐atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e^? reduction of the cationic clusters.     

Metal‐Free,Room‐Temperature Oxygen‐Atom Transfer in the N2O/CO Redox Couple as Catalyzed by [Si2Ox].+ (x=2–5)

The thermal reduction of N₂O by CO mediated by the metal‐free cluster cations [Si₂O_x]^.+ (x =2–5) has been examined in the gas phase using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry in conjunction with quantum chemical calculations. Three successive oxidation/reduction steps occur starting from [Si₂O₂]^.+ and N₂O to form eventually [Si₂O₅]^.+; the latter as well as the intermediate oxide cluster ions react sequentially with CO molecules to regenerate [Si₂O₂]^.+. Thus, full catalytic cycles occur at ambient conditions in the gas phase. Mechanistic aspects of these sequential redox processes have been addressed to reveal the electronic origins of these unparalleled reactions.     

An Original L‐shape,Tunable N‐Heterocyclic Carbene Platform for Efficient Gold(I) Catalysis

The synthesis and characterization of original NHC ligands based on an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands for efficient gold‐catalyzed C?N, C?O, and C?C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5^Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic Au^I active species by ligand 1^Me .     

Gas‐Phase Kinetics of Chlorosilylene Reactions II. ClSiH + C2H4: Absolute Rate Measurements and Quantum Chemical and RRKM Calculations for the Prototype π Addition Reaction

Time‐resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1‐chloro‐1‐silacyclopent‐3‐ene, are carried out to obtain rate constants for its bimolecular reaction with ethene, C₂H₄, in the gas‐phase. The reaction is studied over the pressure range 0.13–13.3 kPa (with added SF₆) at five temperatures in the range 296–562 K. The second order rate constants, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equation: log(k^∞/cm³ molecule^?1 s^?1)=(?10.55±0.10) + (3.86±0.70) kJ mol^?1/RT ln10. The Arrhenius parameters correspond to a loose transition state and the rate constant at room temperature is 43 % of that for SiH₂ + C₂H₄, showing that the deactivating effect of Cl‐for‐H substitution in the silylene is not large. Quantum chemical calculations of the potential energy surface for this reaction at the G3MP2//B3LYP level show that, as well as 1‐chlorosilirane, ethylchlorosilylene is a viable product. The calculations reveal how the added effect of the Cl atom on the divalent state stabilisation of ClSiH influences the course of this reaction. RRKM calculations of the reaction pressure dependence suggest that ethylchlorosilylene should be the main product. The results are compared and contrasted with those of SiH₂ and SiCl₂ with C₂H₄.     

The Unique Gas‐Phase Chemistry of the [AuO]+/CH4 Couple: Selective Oxygen‐Atom Transfer to,Rather than Hydrogen‐Atom Abstraction from,Methane

The thermal reaction of [AuO]⁺ with methane has been explored using FT‐ICR mass spectrometry complemented by high‐level quantum chemical calculations. In contrast to the previously studied congener [CuO]⁺, and to [AgO]⁺, [AuO]⁺ reacts with CH₄ exclusively via oxygen‐atom transfer to form CH₃OH, and a novel mechanistic scenario for this selective oxidation process has been revealed. Also, the origin of the inertness of the [AgO]⁺/CH₄ couple has been addressed computationally.     

Quantum Chemical Study of the Enzymatic Repair of T(6‐4)C/C(6‐4)T UV‐Photolesions by DNA Photolyases

Several strategies have evolved to repair one of the abundant UV radiation‐induced damages caused to DNA, namely the mutagenic pyrimidine (6‐4) pyrimidone photolesions. DNA (6‐4)‐photolyases are enzymes repairing these lesions by a photoinitiated electron transfer. An important aspect of a possible repair mechanism is its generality and transferability to different (6‐4) lesions. Therefore, previously suggested mechanisms for the repair of the T(6‐4)T lesion are here transferred to the T(6‐4)C and C(6‐4)T lesions and investigated theoretically using quantum chemical methods. Despite the different functional groups of the pyrimidine bases involved, a general valid molecular mechanism was identified, in which the initial step is an electron transfer coupled to a proton transfer from the protonated HIS365 to the N3^′ nitrogen of the 3^′ pyrimidine, followed by an intramolecular OH/NH₂ transfer in one concerted step, which does not require an oxetane/azetidine or isolated water/ammonia intermediate.     

On the Origin of the Remarkably Variable Reactivities of [AlCeOx]+ (x=2–4) towards Methane as a Function of Oxygen Content

The thermal gas‐phase reactions of the closed‐shell heteronuclear metal–oxide clusters [AlCeO_x]⁺ (x =2–4) with methane have been explored by FT‐ICR mass spectrometry and high‐level quantum‐chemical calculation. Whereas [AlCeO₂]⁺ and [AlCeO₄]⁺ are inert towards methane under ambient conditions, [AlCeO₃]⁺ spontaneously abstracts one hydrogen atom from methane. Mechanistic aspects have been addressed to reveal the reasons for the rather distinct reactivities of the [AlCeO_x]⁺/CH₄ couples, and the electronic origins of the unprecedented single hydrogen‐atom abstraction from methane by closed‐shell [AlCeO₃]⁺ are discussed.     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

The Origin of the Efficient,Thermal Chemisorption of Methane by the Heteronuclear Metal‐Oxide Cluster [Al2TaO5]+

The thermal gas‐phase reactions of the closed‐shell metal‐oxide cluster [Al₂TaO₅]⁺ with methane have been explored by using FT‐ICR mass spectrometry complemented by high‐level quantum chemical calculations. Mechanistic aspects have been addressed to reveal the origins of the efficient addition process which results in activating the C?H bond of methane. The [Al₂TaO₅]⁺/CH₄ couple has been compared with several other systems reported previously, and the electronic origins of their rather distinct performances are discussed.     

Density Functional Theory Investigation of the Alkyl–Alkyl Negishi Cross‐Coupling Reaction Catalyzed by N‐Heterocyclic Carbene (NHC)–Pd Complexes

Why bigger is better : A “steric wall” created by the N‐(2,6‐diisopropylphenyl) substituent on the bulky NHC ligand IPr (1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) guides the reactants to and from the Pd center through weak, fleeting (IPr)H–Pd interactions that help the oxidative addition intermediate escape “the anti‐trap”. The alternative “side” approach leads to transmetalation (the rate‐limiting step) for which a novel Pd–Zn interaction was identified.

     

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co‐adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂O₂(CO)_n]^? (n=2–6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂O₂(CO)_n]^? (n=2–6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co‐adsorbed on Au₂^? with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂O₂(CO)]^? core ion is observed around =2032–2042 cm^?1, which is about 200 cm^?1 higher than that in [Au₂(CO)₂]^?. This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co‐adsorbed O₂ and CO on Au₂^? may have relevance to the elementary reaction steps on real gold catalysts.