CO2 Hydrogenation on Cu/Al2O3: Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst

Selective hydrogenation of CO₂ into methanol is a key sustainable technology, where Cu/Al₂O₃ prepared by surface organometallic chemistry displays high activity towards CO₂ hydrogenation compared to Cu/SiO₂, yielding CH₃OH, dimethyl ether (DME), and CO. CH₃OH formation rate increases due to the metal–oxide interface and involves formate intermediates according to advanced spectroscopy and DFT calculations. Al₂O₃ promotes the subsequent conversion of CH₃OH to DME, showing bifunctional catalysis, but also increases the rate of CO formation. The latter takes place 1) directly by activation of CO₂ at the metal–oxide interface, and 2) indirectly by the conversion of formate surface species and CH₃OH to methyl formate, which is further decomposed into CH₃OH and CO. This study shows how Al₂O₃, a Lewis acidic and non‐reducible support, can promote CO₂ hydrogenation by enabling multiple competitive reaction pathways on the oxide and metal–oxide interface.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Loss of methyl from [H2CC(OH)?CH3]+˙ ions prepared by electron impact ionization of unstable 2-hydroxypropene

Unstable 2-hydroxpropene was prepared by retro-Diels-Alder decomposition of 5-exo-methyl-5-norbornenol at 800°C/2 × 10^?6 Torr. The ionization energy of 2-hydroxypropene was measured as 8.67±0.05 eV. Formation of [C₂H₃O]⁺ and [CH₃]⁺ ions originating from different parts of the parent ion was examined by means of ¹³C and deuterium labelling. Threshold-energy [H₂C?C(OH)? CH₃]^+˙ ions decompose to CH₃CO⁺+CH₃^˙ with appearance energy AE(CH₃CO⁺) = 11.03 ± 0.03 eV. Higher energy ions also form CH₂?C?OH⁺ + CH₃ with appearance energy AE(CH₂?C?OH⁺) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C₂H₃O]⁺ ions containing the original methyl group and [CH₃]⁺ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate-determining isomerization [H₂C?C(OH)? CH₃]^+˙ →[CH₃COCH₃]^+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.     

Encapsulation and Dynamic Behavior of Methanol and Formaldehyde inside Open‐Cage C60 Derivatives

Methanol (CH₃OH) and formaldehyde (H₂CO) molecules were inserted into an open‐cage C₆₀ derivative with a large opening, under high‐pressure and high‐temperature conditions in solution. Isolation of their molecular complexes in pure form was achieved by the use of recycling HPLC with Buckyprep columns. ¹H NMR spectroscopy, single‐crystal X‐ray diffraction studies, and DFT calculations revealed the orientation of the encapsulated CH₃OH and H₂CO, both in solution and in the solid state, and the results show that the CH₃ group of the CH₃OH and the carbonyl group of the H₂CO point to the bottom of the cages. Furthermore, the dynamic behavior of the CH₃OH and H₂CO were studied at the molecular level.     

Metal‐catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society

Proton dissociation of an aqua‐Ru‐quinone complex, [Ru(trpy)(q)(OH₂)]²⁺ (trpy = 2,2′ : 6′,2″‐terpyridine, q = 3,5‐di‐t‐butylquinone) proceeded in two steps (pK_a = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(q)(OH)]⁺, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O^?.)]⁰ (sq = 3,5‐di‐t‐butylsemiquinone), owing to an intramolecular electron transfer from the resultant O^2? to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru₂(btpyan)(q)₂(OH)₂]²⁺ (btpyan = 1,8‐bis(2,2′‐terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru₂(btpyan)(q)₂(OH)₂]²⁺ generated two oxyl radical Ru‐O^?. groups, which worked as a precursor for O₂ evolution in the oxidation of water. The [Ru₂(btpyan)(q)₂(OH)₂](SbF₆)₂ modified ITO electrode effectively catalyzed four‐electron oxidation of water to evolve O₂ (TON = 33500) under electrolysis at +1.70 V in H₂O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O^?.) groups forms a (cat)Ru‐O‐O‐Ru(sq) (cat = 3,5‐di‐t‐butylcathechol) framework with a μ‐superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru₂(btpyan)(cat)(sq)(μ‐O₂^?)]⁰ assisted with an attack of two water (or OH^?) to Ru centers, which causes smooth O₂ evolution with regeneration of [Ru₂(btpyan)(q)₂(OH)₂]²⁺. Deprotonation of an Ru‐quinone‐ammonia complex also gave the corresponding Ru‐semiquinone‐aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base. Three complexes, [Ru(bpy)₂(CO)₂]²⁺, [Ru(bpy)₂(CO)(C(O)OH)]⁺, and [Ru(bpy)₂(CO)(CO₂)]⁰ exist as an equilibrium mixture in water. Treatment of [Ru(bpy)₂(CO)₂]²⁺ with BH₄^? gave [Ru(bpy)₂(CO)(C(O)H)]⁺, [Ru(bpy)₂(CO)(CH₂OH)]⁺, and [Ru(bpy)₂(CO)(OH₂)]²⁺ with generation of CH₃OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO₂ to CH₃OH in electro‐ and photochemical CO₂ reduction is proposed. A new pbn (pbn = 2‐pyridylbenzo[b]‐1,5‐naphthyridine) ligand was designed as a renewable hydride donor for the six‐electron reduction of CO₂. A series of [Ru(bpy)_3‐n(pbn)_n]²⁺ (n = 1, 2, 3) complexes undergoes photochemical two‐ (n = 1), four‐ (n = 2), and six‐electron reductions (n = 3) under irradiation of visible light in the presence of N(CH₂CH₂OH)₃. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200800039     

Syntheses,Structures and Sorption Properties of Three Isoreticular Trinuclear Indium‐Based Amide‐Functionalized Metal–Organic Frameworks

Amide‐functionalized metal–organic frameworks (AFMOFs) as a subclass of MOF materials have received great interest recently because of their intriguing structures and diverse potential applications. In this work, solvothermal reactions between indium nitrate and two mixed‐linkers afforded two new isoreticular 8‐connected trinuclear indium‐based AFMOFs of [(In₃O)(OH)(L2)₂(IN)₂]?(solv)_x ( 2‐In ) and [(In₃O)(OH)(L2)₂(AIN)₂]?(solv)_x ( NH₂‐2‐In ) (H₂L2=4,4′‐(carbonylimino)dibenzoic acid and HIN=isonicotinic acid or HAIN=3‐aminoisonicotinic acid), respectively. Moreover, by means of reticular chemistry, an extended network of [(In₃O)(OH)(L3)₂(PB)₂]?(solv)_x (3‐In) (H₂L3=4,4′‐(terephthaloylbis(azanediyl))dibenzoic acid, HPB=4‐(4‐pyridyl)benzoic acid) was also successfully realized after prolongation of the former dicarboxylate linker and HIN, resulting in a truly 8‐connected isoreticular AFMOF platform. These frameworks were structurally determined by single‐crystal X‐ray diffraction (SCXRD). Sorption studies further demonstrate that 2‐In and NH₂‐2‐In exhibit not only high surface areas and pore volumes but also relatively high carbon capture capabilities (the CO₂ uptakes reach 60.0 and 75.5 cm³ g^?1 at 298 K and 760 torr, respectively) due to the presences of amide and/or amine functional groups. The selectivity of CO₂/N₂ and CO₂/CH₄ calculated by IAST are 10.18 and 12.43, 4.20 and 4.23 for 2‐In and NH₂‐2‐In , respectively, which were additionally evaluated by mixed‐gases dynamic breakthrough experiments. In addition, high‐pressure gas sorption measurements show that both materials could take up moderate amounts of natural gas.     

Adsorption of uranyl species onto the rutile (110) surface: a periodic DFT study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO₂ rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta‐ or hexa‐coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H₂O, OH^?, and CO₃^2? ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface–[UO₂(H₂O)₃]²⁺ system were well reproduced by our calculations. With respect to adsorbates, [UO₂(L1)_x(L2)_y(L3)_z]ⁿ (L1=H₂O, L2=OH^?, L3=CO₃^2?, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H₂O to OH^? and CO₃^2? lengthens the U? O_surf and U? Ti distances. As a result, the uranyl–surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO₂ decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U?O distances that are longer than those of aquouranyl species by 0.1–0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U?O bond lengths, shorter uranyl–surface separations in most cases, and stronger sorption interactions.     

Mechanistic Origin of Antagonist Effects of Usual Anionic Bases (OH−, CO32−) as Modulated by their Countercations (Na+, Cs+, K+) in Palladium‐Catalyzed Suzuki–Miyaura Reactions

The mechanism of the reaction of trans‐ArPdBrL₂ (Ar=p‐Z‐C₆H₄, Z=CN, H; L=PPh₃) with Ar′B(OH)₂ (Ar′=p‐Z′‐C₆H₄, Z′=H, CN, MeO), which is a key step in the Suzuki–Miyaura process, has been established in N,N‐dimethylformamide (DMF) with two bases, acetate (nBu₄NOAc) or carbonate (Cs₂CO₃) and compared with that of hydroxide (nBu₄NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M⁺ (e.g., M⁺OH^?), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M⁺ (Na⁺, Cs⁺, K⁺) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L₂, partly inhibiting its transmetalation with Ar′B(OH)₂. A decreasing reactivity order is observed when M⁺ is associated with OH^?: nBu₄N⁺> K⁺> Cs⁺> Na⁺. Acetates lead to the formation of trans‐ArPd(OAc)L₂, which does not undergo transmetalation with Ar′B(OH)₂. This explains why acetates are not used as bases in Suzuki–Miyaura reactions that involve Ar′B(OH)₂. Carbonates (Cs₂CO₃) give rise to slower reactions than those performed from nBu₄NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH^?. The mechanism of the reaction with carbonates is then similar to that established for nBu₄NOH, involving ArPd(OH)L₂ in the transmetalation with Ar′B(OH)₂. Due to the low concentration of OH^? generated from CO₃^2? in water, both transmetalation and reductive elimination result slower than those performed from nBu₄NOH at equal concentrations as Cs₂CO₃. Therefore, the overall reactivity is finely tuned by the concentration of the common base OH^? and the ratio [OH^?]/[Ar′B(OH)₂]. Hence, the anionic base (pure OH^? or OH^? generated from CO₃^2?) associated with its countercation (Na⁺, Cs⁺, K⁺) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L₂, acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar′B(OH)₃^? and by complexation of ArPd(OH)L₂ by M⁺.     

Photocatalytic Dissociation of CH3OH on ZnO(0001) Surface

Photocatalysis of CH₃OH on the ZnO(0001) surface has been investigated by using temperature-programmed desorption (TPD) method with a 266 nm laser light. TPD results show that part of the CH₃OH adsorbed on ZnO(0001) surface are in molecular form, while others are dissociated. The thermal reaction products of H₂, CH₃^·, H₂O, CO, CH₂O, CO₂ and CH₃OH have been detected. Experiments with the UV laser light indicate that the irradiation can promote the dissociation of CH₃OH/CH₃O^· to form CH₂O, which can be future converted to HCOO^- during heating or illumination. The reaction between CH₃OH_Znand OH_ad can form the H₂O molecule at the Zn site. Both temperature and illumination promote the desorption of CH₃^· from CH₃O^·. The research provides a new insight into the photocatalytic reaction mechanism of CH3OH on ZnO(0001).     

Neutral counterparts of the C2H7O+ isomers

The neutral counterparts of the C₂H₇O⁺ isomers CH₃O⁺ (H)CH₃, CH₃CH₂OH₂⁺ and $ {\rm C}_2 \,{\rm H}_4 \,\, \cdot \cdot \cdot \mathop {\rm H}\limits^ + \, \cdot \cdot \cdot {\rm OH}_2 $ were studied by neutralization-reionization mass spectrometry. Protonated dimethyl ether and its —O(D)⁺ analogue were produced by protonation (deuteration) of dimethyl ether and also generated as a fragment ion from (labeled) ionized CH₃OCH₂CH(OH)CH₃ by loss of CH₃CO?. It was observed that the dissociation characteristics of the ions and the stability of their neutral counterpart depended on the internal energy of the protonated ether ions. Stable neutral CH₃?(H)CH₃ was only produced from energy-rich ions. The classical protonated ethanol ion CH₃CH₂OH₂⁺ (a) was produced at threshold by the loss of CH₃CO?. from ionized butane-2,3-diol. Mixtures of a with the non-classical ion $ {\rm C}_2 \,{\rm H}_4 \,\, \cdot \cdot \cdot \mathop {\rm H}\limits^ + \, \cdot \cdot \cdot {\rm OH}_2 $ (b) were produced by reaction of C₂H₅⁺ ions with H₂O. As for the protonated ether, only high-energy a and/or b ions yielded stable hypervalent radicals. It is suggested that the stable C₂H₇?O radicals are Rydberg states.     

Mass spectral fragmentation pattern of 3-(2′-hydroxyethyl)quinolin-2(1H)-one and its derivatives

Fragmentation patterns resulting from electron impact ionization of 3-(2′-hydroxyethyl)quinolin-2(1H)-one, three of its monosubstituted derivatives and four of its disubstituted derivatives were studied. The molecular ion of quinolinone-2-etbanol undergoes initial fragmentation with the loss of OH^·, H₂O, CO, ^·CHO, CH₂O, CH₂OH^·, CH₂?CHOH and HCNO species. The [M – CHO]⁺ ion is tentatively suggested to have been formed by the expulsion of H^· from the [M – CO]^+· ion and the [M - CHO]⁺ peak may be considered as diagnostic of a 2-quinolone-3-ethanol.     

Dynamic Changes in the Structure,Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂ electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_x species observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄ and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^I species under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁ product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.     

Direct Conversion of Methane with Carbon Dioxide Mediated by RhVO3− Cluster Anions

Direct conversion of methane with carbon dioxide to value‐added chemicals is attractive but extremely challenging because of the thermodynamic stability and kinetic inertness of both molecules. Herein, the first dinuclear cluster species, RhVO₃^?, has been designed to mediate the co‐conversion of CH₄ and CO₂ to oxygenated products, CH₃OH and CH₂O, in the temperature range of 393–600 K. The resulting cluster ions RhVO₃CO^? after CH₃OH formation can further desorb the [CO] unit to regenerate the RhVO₃^? cluster, leading to the successful establishment of a catalytic cycle for methanol production from CH₄ and CO₂ (CH₄+CO₂→CH₃OH+CO). The exceptional activity of Rh‐V dinuclear oxide cluster (RhVO₃^?) identified herein provides a new mechanism for co‐conversion of two very stable molecules CH₄ and CO₂.     

Theoretical study on the gas-phase reaction mechanism between rhodium monoxide cation and methane

The gas-phase reaction mechanism between rhodium monoxide cation and methane has been investigated on the singlet and triplet state potential energy surfaces at the CCSD(T)/6-311+G(2d,2p), SDD//B3LYP/6-311+G(2d,2p), SDD level. Over the 300–1100 K temperature range, the branching ratios of Rh⁺ + CH₃OH and RhCH₂ ⁺ + H₂O are 83.8–52.6% and 16.2–47.4%, respectively, whereas the branching ratio of CH₂ORh⁺ + H₂ is so small to be negligible. For the main products (Rh⁺ + CH₃OH and RhCH₂ ⁺ + H₂O) formation, the minimum energy reaction pathway involves singlet–triplet spin inversion, and both b-RhCH₃OH⁺ and H₂ORhCH₂ ⁺ are the energetically preferred intermediates. Alternatively, in the CH₂ORh⁺ + H₂ reaction, both b-RhCH₃OH⁺ and H₂RhOCH₂ ⁺ are the energetically favorable intermediates, and the main products are Rh⁺ + CH₃OH. In the RhCH₂ ⁺ + H₂O reaction, the main products are Rh⁺ + CH₃OH with the energetically predominant intermediate b-RhCH₃OH⁺. In the reaction of Rh⁺ + CH₃OH, both b-RhCH₃OH⁺ and H₂RhOCH₂ ⁺ are the energetically preferable intermediates, and the main products are CH₂ORh⁺ + H₂. Besides, toward methane activation, the cation RhO⁺ exhibits higher reaction efficiency than the cation Rh⁺, the neutral RhO, and its first-row congener CoO⁺, and it exhibits lower methanol branching ratio and higher water branching ratio than RhO and CoO⁺.     

In cation interactions with some organics: ab initio molecular orbital and density functional theory

Ab initio molecular orbital and density functional theory studies were undertaken to investigate the structural and energetic characteristics of complexes of In⁺ with several different organic molecules for the first time. HF, MP2, QCISD, and CCD levels of theory in ab initio MO as well as B3LYP, B3PW91 hybrid functionals in density functional theory were used. A valence TZ+P basis set with relativistic effective core potentials was used for the In atom while the 6-311++G(3d, 2p) basis set was utilized for all other atoms. Both closed-shell (H₂O, CH₄, CH₃OH, and C₆H₆) and open-shell (CH₃ and C₂H₃) molecules were considered for complexation with In⁺. In⁺ affinities of 21.5, 24.8, 28.6, 18.4, and 23.0 kcal/mol were obtained with the B3PW91 hybrid functional for H₂O, CH₃OH, C₆H₆, CH₃, and C₂H₃, respectively. The large values for the calculated affinities indicate the validity of our recent experimental detection of In⁺ ion attachment to some organic molecules.     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

PHOTOFRAGMENTATION OF HYDROXYACETONE, 1.3-DIHYDROXYACETONE, AND 1.3-DICARBOXYACETONE IN AQUEOUS SOLUTION. AN EPR STUDY

Abstract —On photoexcitation, hydroxyacetone undergoes a Norrish-type-1 fragmentation to yield CH₃CO and CH₂OH. CH₂OH is identified by its EPR spectrum. The existence of CH₃CO is inferred from the presence of diacetyl and acetaldehyde in irradiated solutions. Above pH 5, in addition to CH₂OH, the cis and trans forms of the hydroxyacetone enediol radical anion, CH₃C(O^-)=C(O***)H, are detected. 1.3-Dihydroxyacetone is photodecomposed to HOCH₂C?O and C?H₂OH. The former radical decarbonylates to yield CH₂OH and CO. At 254 nm the overall quantum yield of CO production is 0.75. Above pH 5, in addition to CH₂OH, the cis and trans forms of the 1.3-dihydroxyacetone enediol radical anion, HOCH₂C(O^-)C(O***)H, are observed. Electronically excited hydroxyacetone and 1.3-dihydroxyacetone react exclusively by C-C fragmentation, and no H-abstraction from H-donors is observed. In contrast, electronically excited 1.3-dicarboxyacetone shows H-abstraction from H-donors in competition with C-C fragmentation. In the absence of H-donors, fragmentation resulting in CH₂CO₂^- and ^-O₂CCH₂C?O occurs followed by decarbonylation of ^-O₂H₂C?O. At 254 nm the quantum yield of CO production is 0.02. In the presence of H-donors, H-abstraction, yielding HO₂CCH₂C(OH)CH₂CO₂, predominates.     

Theoretical influence of third molecule on reaction channels of weakly bound complex CO2… HF systems

In this investigation, reaction channels of weakly bound complexes CO₂…HF, CO₂…HF…NH₃, CO₂…HF…H₂O and CO₂…HF…CH₃OH systems were established at the B3LYP/6‐311++G(3df,2pd) level, using the Gaussian 98 program. The conformers of syn‐fluoroformic acid or syn‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH) were found to be more stable than the conformers of the related anti‐fluoroformic acid or anti‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH). However, the weakly bound complexes were found to be more stable than either the related syn‐ and anti‐type fluoroformic acid or the acid plus third molecule (NH₃, H₂O, or CH₃OH) conformers. They decomposed into CO₂ + HF, CO₂ + NH₄F, CO₂ + H₃OF or CO₂ + (CH₃)OH₂F combined molecular systems. The weakly bound complexes have four reaction channels, each of which includes weakly bound complexes and related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti‐fluoroformic acid type structure (T₁₃) is significantly larger than that of internal rotation (T₂₃) between the syn‐ and anti‐FCO₂H (or FCO₂H…NH₃, FCO₂H…H₂O, or FCO₂H…CH₃OH) structures. However, adding the third molecule NH₃, H₂O, or CH₃OH can significantly reduce the activation energy of T₁₃. The catalytic strengths of the third molecules are predicted to follow the order H₂O < NH₃ < CH₃OH. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A First-Principles Study of CO2 Hydrogenation on a Niobium-Terminated NbC (111) Surface

As promising materials for the reduction of greenhouse gases, transition-metal carbides, which are highly active in the hydrogenation of CO₂, are mainly considered. In this regard, the reaction mechanism of CO₂ hydrogenation to useful products on the Nb-terminated NbC (111) surface is investigated by applying density functional theory calculations. The computational results display that the formation of CH₄, CH₃OH, and CO are more favored than other compounds, where CH₄ is the dominant product. In addition, the findings from reaction energies reveal that the preferred mechanism for CO₂ hydrogenation is thorough HCOOH^*, where the largest exothermic reaction energy releases during the HCOOH^* dissociation reaction (2.004 eV). The preferred mechanism of CO₂ hydrogenation towards CH₄ production is CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₂^*→CH₃^*→CH₄^*, where CO₂^*→t,c-COOH^*→HCOOH^*→HCO^*→CH₂O^*→CH₂OH^*→CH₃OH^* and CO₂^*→t,c-COOH^*→CO^* are also found as the favored mechanisms for CH₃OH and CO productions thermodynamically, respectively. During the mentioned mechanisms, the hydrogenation of CH₂O^* to CH₂OH^* has the largest endothermic reaction energy of 1.344 eV.     

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

OH⁺ is an extraordinarily strong oxidant. Complexed forms (L? OH⁺), such as H₂OOH⁺, H₃NOH⁺, or iron–porphyrin‐OH⁺ are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L? OH⁺ towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L? OH⁺. With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas‐phase pseudo‐first‐order reaction rates of H₂OOH⁺, H₃NOH⁺, and protonated 4‐picoline‐N‐oxide towards isobutane and different halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂ have been determined by means of Fourier transform ion cyclotron resonance meaurements. Reaction rates for H₂OOH⁺ are generally fast (7.2×10^?10–3.0×10^?9 cm³ mol^?1 s^?1) and only in the cases HCF₃ and CF₄ no reactivity is observed. In contrast to this H₃NOH⁺ only reacts with tC₄H₉Cl (k_obs=9.2×10^?10), while 4‐CH₃‐C₅H₄N‐OH⁺ is completely unreactive. While H₂OOH⁺ oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH₂OH, a series of alkanes C_nH_2n+2 (n=1–4), and halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂, were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C_nH_2n+1⁺ (n=1–4) and C_nH_2nCl⁺ (n=1–4). The hypersurface for the reaction of H₂OOH⁺ with CH₃Cl and C₂H₅Cl was calculated at the B3 LYP, MP2, and G3_m* level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.