Heterogeneous recombination of O + CO on solid CO2 at 77 K

Heterogeneous recombination of O + CO → CO₂ over a solid CO₂ surface at 77 K was investigated. A modified discharge flow setup was used to generate low O atom concentrations by the reaction N + NO → N₂ + O(³P). The O atom concentrations were measured upstream and downstream of the solid CO₂ substrate using resonance fluorescence by monitoring the unresolved 130.3 nm triplet transition ³S₁ ? ³P_2,1,0 at the two fixed points. CO₂ formed was determined by measuring the β activity from C¹⁴O₂ produced from CO containing C¹⁴O as a reactant gas. The CO₂ formation was found to be first order in CO and independent of O atom concentration over the entire range of 4.3 × 10¹² to 1.9 × 10¹⁴ cm^?3 and 1.2 × 10¹¹ to 5.6 × 10¹² cm^?3 for CO and O respectively. The first order recombination coefficient, λ_CO was found to be 1.4 (±.38) × 10^?5.     

Negative ion chemical ionization mass spectrometry of some isocyanates

Summary Negative ion mass spectra for 3 aliphatic and 4 aromatic isocyanates have been obtained by low pressure chemical ionization, using CH₄, CO₂ and N₂O as reagent gases. All compounds furnished intense anions at m/z 42. With CH₄, quasi-molecular anions were observed at m/z M+1 for aliphatic and m/z M+1 and M–1 for aromatic isocyanates. With N₂O, anionic substitution products at m/z M+15 and M+30 were observed, and with CO₂ and N₂O, peaks at m/z M–12 could be detected for all aromatic isocyanates. Studies with ¹³CO₂ and C¹⁸O₂ as reagent gases showed that the anions at m/z M–12 and M+15 correspond to [M–CO+O]^– and [M–H+O]^–, respectively.

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Negativionen-Massenspektrometrie mit chemischer Ionisierung von einigen Isocyanaten

Zusammenfassung Die Negativionen-Massenspektren von 3 aliphatischen und 4 aromatischen Isocyanaten wurden mittels chemischer Ionisation bei tiefem Quellendruck aufgenommen, und zwar mit den Reagensgasen CH₄, CO₄ und N₂O. Alle Verbindungen lieferten intensive Anionen mit m/z 42. Mit CH₄ erhielten wir die quasi-molekularen Anionen M+1 für aliphatische sowie M+1 und M–1 für aromatische Isocyanate. Das Reagens N₂O ergab die anionischen Substitutionsprodukte M+15 und M+30. Sowohl CO₂ als auch N₂O führten mit aromatischen Isocyanaten zur Bildung von M–12 Anionen. Versuche mit ¹³CO₂ und mit C¹⁸O₂ als Reagensgase zeigten, daß die Anionen M–12 und M+15 den Ionen [M–CO+O]^– und [M–H+O]^– entsprechen.

     

A chemically pumped CO2 laser from the CS2O2 reaction

A chemical laser has been constructed in which simultaneous output is obtained from both CO and CO₂ laser species at 5 and 10 μm, respectively. Excitation of CO₂ is via energy transfer from vibrationally excited CO (CO²) produced in the photolytically initiated reaction: O + CS → CO² + S. Gain measurements at the CO₂ laser frequencies are also reported.     

Theoretical study of the mechanism for spin‐forbidden quenching process O(1D) + CO2(1Σ) → O(3P) + CO2(1Σ)

The mechanism of the spin‐forbidden quenching process O(¹D) + CO₂(¹Σ) → O(³P) + CO₂(¹Σ) was investigated by ab initio quantum chemistry methods. The calculations showed the singlet potential surface [O(¹D)+CO₂] is attractive where a strongly bound intermediate complex CO₃ is formed in the potential basin without a transition state, whereas the complex CO₃ that is formed on the triplet surface [O(³P)+CO₃] must overcome a barrier. The complex channel was documented by searching minimum energy intersection points in the region of the bound complex CO₃ and calculating spin‐orbit coupling at the point. A direct channel was proposed by a study of cross point of singlet and triplet PESs with different collision angles and calculations of spin‐orbit coupling at those cross points in a nonbound region of the [O(¹D)+CO₃] system. The mechanism of the energy transfer is discussed on the basis of the theoretical results. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

The radiolysis of mixtures of carbon dioxide and hydrogen

The self-radiolysis of CO₂ in excess tritium (³H₂) has been studied at pressures of 0.1 to 1.0 atm, temperatures of ?80° to +100°C, and in the presence of added H₂O, He, or Ar. The primary products of decomposition are CO and ³H₂O. Secondary products are C³H₄, C₂³H₄, and a white polymer. The rates of disappearance of CO₂ and formation of products and G-;values were measured. The disappearance of CO₂ initially obeys first-order kinetics, then slows down with time at a rate depending upon the initial pressure of ³H₂. The initial rates are proportional to pressures of CO₂ and ³H₂. They are independent of temperature, decreased by addition of H₂O vapor, and increased by addition of He or Ar. The proposed mechanism of decomposition of CO₂ and formation of products involves ionization of CO₂ followed by dissociative recombination forming CO and O. Then the O reacts with a hydrogen-containing species forming OH and H₂O, and a back reaction forms CO₂ from CO and OH.     

Collision-induced intramolecular vibration—vibration energy transfer in CO2: CO2(0001) + H2/D2

Intramolecular vibration—vibration energy transfer cross sections have been calculated for CO₂(00⁰1) + H₂/D₂ → CO₂(11¹0) + H₂/D₂, → CO₂(10⁰0) + H₂/D₂, and → CO₂(20⁰0) + H₂/D₂ based on the mechanism that the energy mismatch is transferred to the translational motion. For CO₂ + H₂, the calculated cross section for CO₂(00⁰1) + H₂ → CO₂(10⁰0) + H₂ is in good agreement with experimental data. Cross sections for the processes (00⁰1 → 11¹O) and (00⁰1 → 20⁰0) are found to be too small compared with experimental data. For CO₂ + D₂, (00⁰1 → 10⁰0) is also the most important process and appears to represent experimental data at room temperature. The sum of three cross sections of CO₂ + H₂ is always greater than that of CO₂ + D₂ over the temperature range of 100–2500 K.     

Vibrational relaxation of CO2(v3) by O atoms

The vibrational relaxation time for CO₂(v₃) + O(³P) has been measured by laser fluorescence. The observed value, βCO₂.O = 0.21 ± 0.04 μsec, is an order of magnitude lower than the relaxation time for self-collisions.     

Collision-induced dissociations of deprotonated peptides,dipeptides and tripeptides with hydrogen and alkyl α groups. An aid to structure determination

The characteristic collision-induced dissociations of [M ? H]^? ions of dipeptides and tripeptides involve proton transfer to the carboxylate centre as a prelude to fragmentation. Dipeptides show the process NH₂CH(R¹)CONHCH(R²)CO₂^? → NH₂C(R¹)CONHCH(R²)CO₂H → ^?NHCH(R²)CO₂H + NH₂C(R¹)?C?O (R = H or alkyl) while tripeptides show the analogous processes NH₂CH(R¹)CONHCH(R²)CONHCH(R³)CO₂^? → NH₂CH(R¹)CONHC(R²)CONHCH(R³)CO₂^? → NHCH(R³)CO₂H + NH₂CH(R¹)CONHC(R²)?C?O and NH₂CH(R¹)CONHCH(R²)? CONHCH(R³)CO₂^? → NH₂C(R¹)CONHCH(R²)CONHCH(R³)CO₂H → ^?NHCH(R²)CONHCH(R³)CO₂H + NH₂C(R¹)?C?O. These fragmentations provide ready identification of the peptide.     

Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2(H2O)n].+, n=0–8

We investigate activation of carbon dioxide by singly charged hydrated magnesium cations Mg ^.+(H₂O)_n, through infrared multiple photon dissociation (IRMPD) spectroscopy combined with quantum chemical calculations. The spectra of [MgCO₂(H₂O)_n]^.+ in the 1250–4000 cm^?1 region show a sharp transition from n=2 to n=3 for the position of the CO₂ antisymmetric stretching mode. This is evidence for the activation of CO₂ via charge transfer from Mg ^.+ to CO₂ for n≥3, while smaller clusters feature linear CO₂ coordinated end‐on to the metal center. Starting with n=5, we see a further conformational change, with CO₂^.? coordination to Mg²⁺ gradually shifting from bidentate to monodentate, consistent with preferential hexa‐coordination of Mg²⁺. Our results reveal in detail how hydration promotes CO₂ activation by charge transfer at metal centers.     

Theoretical study of carbonic anhydrase-catalyzed hydration of co2: A brief review

Quantum mechanical calculations have been used to study the reaction mechanism of human carbonic anhydrase-catalyzed hydration of CO₂. This reaction is responsible for fast metabolism of CO₂ in the human body. For each of the reaction steps, possible catalytic effects of active site residues are examined. The pertinent results are as follows. (1) For CO₂ binding, the experimentally observed 2.5 cm^?1 frequency shift of the asymmetic stretching frequency between measurements taken in the aqueous solution and in the enzyme is reproduced in our theoretical calculations. Our results suggest that CO₂ binds to the zinc ion within the hydrophobic pocket. (2) No energy barrier is found for the nucleophilic attack from Zn²⁺?bound OH^? to C of CO₂ to form Zn²⁺?bound HCO₃^?. (3) For the internal proton transfer within zinc-bound HCO₃^?, the barrier of 35.6 kcal/mol for the direct internal proton transfer is reduced to 3.5 and 1.4 kcal/mol, respectively, when one or two water molecules are included for proton relay. (4) Displacement of Zn²⁺?bound HCO₃^? by H₂O is facilitated by the presence of the negatively charged Glu 106-Thr 199 chain and by the association and the subsequent ionization of a fifth water ligand. (5) For the intramolecular proton transfer between Zn²⁺-bound H₂O and His 64, the Zn²⁺ ion lowers the pK_a of Zn²⁺?bound water and repels the proton. His 64, or a similar proton receptor with a larger proton affinity than H₂O, functions as proton receiver; and the active site water molecules visualized by x-ray crystallography are important for the proton relay function. In summary, it is demonstrated that in order to achieve effective catalysis, a sequence of precisely coordinated catalytic events among all participating catalytic elements in the enzyme's active site is essential.     

Infrared and mass spectral studies of proton irradiated H2O + CO2 ice: Evidence for carbonic acid

The effects of proton irradiation on mixed H₂O + CO₂ (1:1) ices at 20 K were investigated by infrared and mass spectroscopy. Infrared bands due to several radical (HCO, CO₃) and molecular (CO) product species were identified. In addition, several new broad and complex i.r. features were observed. On slow warming, the broad features evolved into a 215–250 K residual film whose absorptions have been tentatively assigned to carbonic acid. This identification agrees with the spectral data for irradiated H₂O + ¹³CO₂ ice and the results of an approximate normal coordinate analysis.     

Three Lanthanide Metal‐Organic Frameworks Based on an Ether‐Decorated Polycarboxylic Acid Linker: Luminescence Modulation,CO2 Capture and Conversion Properties

Three new isostructural 3D lanthanide metal–organic frameworks (Ln‐MOFs), {H[LnL(H₂O)]?2 H₂O}_n ( 1‐Ln ) (Ln=Eu³⁺, Gd³⁺ and Tb³⁺), based on infinite lanthanide‐carboxylate chains were constructed by employing an ether‐separated 5,5′‐oxydiisophthalic acid (H₄L) ligand under solvothermal reaction. 1‐Eu and 1‐Tb exhibit strong red and green emission, respectively, through the antenna effect, as demonstrated through a combination of calculation and experimental results. Moreover, a series of dichromatic doped 1‐Eu_xTb_y MOFs were fabricated by introducing different concentrations of Eu³⁺ and Tb³⁺ ions, and they display an unusual variation of luminescent colors from green, yellow, orange to red. 1‐Eu with channels decorated by ether O atoms and the open metal sites displays good performance for CO₂ capture and conversion between CO₂ and epoxides into cyclic carbonates.     

Photodissociation reaction CO−3

Photodissociation reaction CO^?₃ + hv → CO₂ + O^? has been observed at seven photon energies between 2.35 and 2.71 eV using a drift tube mass spectrometer and an argon ion laser. The total cross sections for the destruction of CO^?₃ due to photons of these energies have been measured, and it is concluded that essentially all of this destruction is due to photodissociation. The photodestruction of CO^?₃.H₂O has also been observed at four photon energies.     

Direct determination of the rate constant for the reaction CO + N2O → CO2 + N2

The rate constant for the bimolecular reaction CO + N₂O → CO₂ + N₂ was determined by comparison of calculated infrared emission profiles of CO₂ with those observed in shock-tube experiments in the temperature range 1350–2100 K for CO-N₂O-He-Ar mixtures. The rate constant was found to be k₁ = 3.2 × 10¹¹exp(−85 kJ/RT) cm³ mol⁻¹ s⁻¹.     

The reaction of photochemically generated CF3O radicals with CO

CF₃O₂CF₃ was photolyzed at 254 nm in the presence of CO in 760 torr N₂ or air at 296 K in a static reactor. In N₂, the products CF₃OC(O)C(O)OCF₃ and CF₃OC(O)O₂C(O)OCF₃ were detected by FTIR spectroscopy. In air, the only observed products were CF₂O and CO₂ and a chain process, initiated by CF₃O, was invoked for the conversion of CO to CO₂. From both product studies, a mechanism for the CF₃O initiated oxidation of CO was derived, involving the addition reaction CF₃O₂ + CO → CF₃OC(O). The rate constant for the reaction CF₃O + CO at 296 K at a total pressure of 760 torr air was determined to be k(CF₃O + CO) = (5.0 ± 0.9) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹. © 1997 John Wiley & Sons, Inc.     

Infrared spectroscopic study of the CO2 adsorption effect on the surface acidity of zinc oxide

Taking account the bands ν_a(O¹²CO) and ν_a(O¹³CO) (in natural abundance) due to linear species, the study by FT-infrared spectroscopy of the CO₂ adsorption on ZnO has shown particular sites which can be described as Zn²⁺ ions with two vacancies, with a reactive oxygen ion in an adjacent position. Using the usual probe molecules (CO, acetonitrile, ammonia, pyridine), we have compared the Lewis acidity of ZnO and ZnO which has been pretreated with CO₂. This preadsorption causes an increase of the Lewis acidity. Moreover we have found that it prevents the dissociative adsorption of acetonitrile, ammonia, pyridine, propene and butenes. The model site described above well accounts for these results. It is concluded that the increase of the acidity due to CO₂ preadsorption is related to the formation of bidentate carbonates.     

Kinetics of CN reactions with N2O and CO2

The rate constants for the reaction of CN with N₂O and CO₂ have been measured by the laser dissociation/laser-induced fluorescence (two-laser pump-probe) technique at temperatures between 300 and 740 K. The rate of CN + N₂O was measurable above 500 K, with a least-squares averaged rate constant, k = 10^−11.8±0.4 exp(−3560 ± 181/T) cm³/s. The rate of CN + CO₂, however, was not measurable even at the highest temperature reached in the present work, 743 K, with [CO₂] ⩽ 1.9 × 10¹⁸ molecules/cm³. In order to rationalize the observed kinetics, quantum mechanical calculations based on the BAC-MP4 method were performed. The results of these calculations reveal that the CN + N₂O reaction takes place via a stable adduct NCNNO with a small barrier of 1.1 kcal/mol. The adduct, which is more stable than the reactants by 13 kcal/mol, decomposes into the NCN + NO products with an activation energy of 20.0 kcal/mol. This latter process is thus the rate-controlling step in the CN + N₂O reaction. The CN + CO₂ reaction, on the other hand, occurs with a large barrier of 27.4 kcal/mol, producing an unstable adduct NCOCO which fragments into NCO + CO with a small barrier of 4.5 kcal/mol. The large overall activation energy for this process explains the negligibly low reactivity of the CN radical toward CO₂ below 1000 K. Least-squares analyses of the computed rate constants for these two CN reactions, which fit well with experimental data, give rise to for the temperature range 300–3000 K.     

Relative rates of reaction of hydroxyl radicals with O(3P) atoms and CO molecules

The rates of decay of O(³P) atoms in H₂/CO/N₂ mixtures in a discharge flow system have been measured, using O + CO chemiluminescence. The mechanism is: O + H₂ → OH + H (1), O + OH → O₂ + H (2), CO + OH → CO₂ + H (3). At 425 K, k₂/k₃ = 260 ± 20; literature values of k₃ combine to yield k₂ = (2.65 ± 0.52) × 10¹⁰ dm³ mol^?1 s^?1.     

Dual Metal Active Sites in an Ir1/FeOx Single-Atom Catalyst: A Redox Mechanism for the Water-Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water-gas shift (WGS) reaction on Ir₁/FeO_x single-atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first-neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second-neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺−O⋅⋅⋅Ir²⁺−O_vac to Fe²⁺−O_vac⋅⋅⋅Ir³⁺−O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Dual Metal Active Sites in an Ir1/FeOx Single‐Atom Catalyst: A Redox Mechanism for the Water‐Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water‐gas shift (WGS) reaction on Ir₁/FeO_x single‐atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first‐neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second‐neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺?O???Ir²⁺?O_vac to Fe²⁺?O_vac???Ir³⁺?O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.