pH- and wavelength-dependent photodecarboxylation of ketoprofen

The pH- and wavelength-dependent pathways for the photodecarboxylation of ketoprofen (KP) were mapped by CASSCF/CASPT2 computations. The decarboxylation of the basic form (KP(-)) was found to start from a long-distance charge transfer (CT) excited state when populated by photoexcitation at 330 nm. A short-distance CT excited state populated with photoexcitation at λ < 260 nm appears to be responsible for the decarboxylation of the acidic form (KP). The H(2)O molecules function as a bridge to assist proton transfer in the reactions examined here.     

气态丙烯酸光致脱羧反应AM1法研究

半经验的自洽场分子轨道法(AM1)被用来研究激发单态(~1ππ~*)和三态(~3ππ~*)丙烯酸的脱羧反应. 计算结果支持Robert等人提出的光解机理. 与实验结合. 进一步推测, 丙烯酸光致脱羧反应的第一步, 是沿单态途径进行, 第二步沿三态途径进行. 单态和三态反应途径中的反应物、过渡态、中间体和产物都用能量梯度技术进行了优化. 对于过渡态和中间体, 还作了振动分析, 确证它们分别是一级鞍点和能量极小值点.     

Influence of H2 on the gas-phase decomposition of formic acid: a theoretical study

Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H2. With competitive energy, H2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)2, conformers required for decarboxylation, are less populated due to interactions with H2.     

Evidence for the formation of a mo-h intermediate in the catalytic cycle of formate dehydrogenase

DFT/BP86/TZVP and DFT/B3LYP/TZVP have been used to investigate systematically the reaction pathways associated with the H-transfer step, which is the rate-determining step of the reaction HCOO(-) ? CO(2) + H(+) + 2e(-), as catalyzed by metalloenzyme formate dehydrogenase (FDH). Actually, the energetics associated with the transfer from formate to all H (proton or hydride) acceptors that are present within the FDH active site have been sampled. This study points to a viable intimate mechanism in which the metal center mediates H transfer from formate to the final acceptor, i.e. a selenocysteine residue. The Mo-based reaction pathway, consisting of a β-H elimination to metal with concerted decarboxylation, turned out to be favored over previously proposed routes in which proton transfer occurs directly from HCOO(-) to selenocysteine. The proposed reaction pathway is reminiscent of the key step of metal-based catalysis of the water-gas shift reaction.     

Hydrothermal reactions of formaldehyde and formic acid: free-energy analysis of equilibrium

The chemical equilibria concerning formaldehyde and formic acid are computationally investigated in water over a wide range of thermodynamic conditions. The free energy is evaluated in the method of energy representation for the solvent effect on the decomposition processes of these two compounds. The solvation is found to suppress the production of nonpolar species from a polar. In the two competitive decomposition reactions of formic acid, the solvent strongly inhibits the decarboxylation (HCOOH-->CO2+H2) and its effect is relatively weak for the decarbonylation (HCOOH-->CO+H2O). The equilibrium weights for the two decomposition pathways of formic acid are determined by the equilibrium constant of the water-gas-shift reaction (CO+H2O-->CO2+H2), which is an essential and useful process in fuel technology. The reaction control by the solvent is then examined for the water-gas-shift reaction. Through the comparison of the equilibrium constants in the absence and presence of solvent, even the favorable side of the reaction is shown to be tuned by the solvent density and temperature. The reaction equilibrium is further treated for aldehyde disproportionation reactions involving formaldehyde and formic acid. The disproportionation reactions are found to be subject to relatively weak solvent effects and to be dominated by the electronic contribution.     

Theoretical study of the gas-phase Fe(CO)5 catalyzed water gas shift reaction: a new mechanism proposed

A novel mechanism for the gas-phase Fe(CO)(5) and base catalyzed water gas shift reaction has been examined. The reaction pathway described here is predicted at the B3LYP/6-31++G(d,p) level to be energetically competitive with the classic mechanism. The reaction path explored here involves the energetically barrierless formation of (CO)(4)FeCOOH(-) (the catalyst of the system) decarboxylation induced by the addition of CO to give (CO)(4)FeCHO, and evolution of H(2) upon addition of H(2)O to the (CO)(4)FeCHO intermediate. The energetic barriers predicted for the last two steps are 21.2 and 42.0 kcal/mol, respectively, using the B3LYP method.     

Theoretical study of formamide decomposition pathways

The chemical transformations of formamide (NH(2)CHO), a molecule of prebiotic interest as a precursor for biomolecules, are investigated using methods of electronic structure computations and Rice-Rampserger-Kassel-Marcus (RRKM) theory. Specifically, quantum chemical calculations applying the coupled-cluster theory CCSD(T), whose energies are extrapolated to the complete basis set limit (CBS), are carried out to construct the [CH(3)NO] potential energy surface. RRKM theory is then used to systematically examine decomposition channels leading to the formation of small molecules including CO, NH(3), H(2)O, HCN, HNC, H(2), HNCO, and HOCN. The energy barriers for the decarboxylation, dehydrogenation, and dehydration processes are found to be in the range of 73-78 kcal/mol. H(2) loss is predicted to be a one-step process although a two-step process is competitive. CO elimination is found to prefer a two-step pathway involving the carbene isomer NH(2)CHO (aminohydroxymethylene) as an intermediate. This CO-elimination channel is also favored over the one-step H(2) loss, in agreement with experiment. The H(2)O loss is a multistep process passing through a formimic acid conformer, which subsequently undergoes a rate-limiting dehydration. The dehydration appears to be particularly favored in the low-temperature regime. The new feature identifies aminohydroxymethylene as a transient but crucial intermediate in the decarboxylation of formamide.     

Quantum chemical study of the mechanism of reaction between NH (X 3sigma-) and H2, H2O, and CO2 under combustion conditions

Reactions of ground-state NH (3sigma-) radicals with H2, H2O, and CO2 have been investigated quantum chemically, whereby the stationary points of the appropriate reaction potential energy surfaces, that is, reactants, products, intermediates, and transition states, have been identified at the G3//B3LYP level of theory. Reaction between NH and H2 takes place via a simple abstraction transition state, and the rate coefficient for this reaction as derived from the quantum chemical calculations, k(NH + H2) = (1.1 x 10(14)) exp(-20.9 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K, is found to be in good agreement with experiment. For reaction between triplet NH and H2O, no stable intermediates were located on the triplet reaction surface although several stable species were found on the singlet surface. No intersystem crossing seam between triplet NH + H2O and singlet HNO + H2 (the products of lowest energy) was found; hence there is no evidence to support the existence of a low-energy pathway to these products. A rate coefficient of k(NH + H2O) = (6.1 x 10(13)) exp(-32.8 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K for the reaction NH (3sigma-) + H2O --> NH2 (2B) + OH (2pi) was derived from the quantum chemical results. The reverse rate coefficient, calculated via the equilibrium constant, is in agreement with values used in modeling the thermal de-NO(x) process. For the reaction between triplet NH and CO2, several stable intermediates on both triplet and singlet reaction surfaces were located. Although a pathway from triplet NH + CO2 to singlet HNO + CO involving intersystem crossing in an HN-CO2 adduct was discovered, no pathway of sufficiently low activation energy was discovered to compare with that found in an earlier experiment [Rohrig, M.; Wagner, H. G. Proc. Combust. Inst. 1994, 25, 993.].     

Kinetic and equilibrium study on formic acid decomposition in relation to the water-gas-shift reaction

Kinetics and equilibrium are studied on the hydrothermal decarbonylation and decarboxylation of formic acid, the intermediate of the water-gas-shift (WGS) reaction, in hot water at temperatures of 170-330 degrees C, to understand and control the hydrothermal WGS reaction. (1)H and (13)C NMR spectroscopy is applied to analyze as a function of time the quenched reaction mixtures in both the liquid and gas phases. Only the decarbonylation is catalyzed by HCl, and the reaction is first-order with respect to both [H(+)] and [HCOOH]. Consequently, the reaction without HCl is first and a half (1.5) order due to the unsuppressed ionization of formic acid. The HCl-accelerated decarbonylation path can thus be separated in time from the decarboxylation. The rate and equilibrium constants for the decarbonylation are determined separately by using the Henry constant (gas solubility data) for carbon monoxide in hot water. The rate constant for the decarbonylation is 1.5 x 10(-5), 2.0 x 10(-4), 3.7 x 10(-3), and 6.3 x 10(-2) mol(-1) kg s(-1), respectively, at 170, 200, 240, and 280 degrees C on the liquid branch of the saturation curve. The Arrhenius plot of the decarbonylation is linear and gives the activation energy as 146 +/- 3 kJ mol(-1). The equilibrium constant K(CO) = [CO]/[HCOOH] is 0.15, 0.33, 0.80, and 4.2, respectively, at 170, 200, 240, and 280 degrees C. The van't Hoff plot results in the enthalpy change of DeltaH = 58 +/- 6 kJ mol(-1). The decarboxylation rate is also measured at 240-330 degrees C in both acidic and basic conditions. The rate is weakly dependent on the solution pH and is of the order of 10(-4) mol kg(-1) s(-1) at 330 degrees C. Furthermore, the equilibrium constant K(CO2) = [CO(2)][H(2)]/[HCOOH] is estimated to be 1.0 x10(2) mol kg(-1) at 330 degrees C.     

Photodecarboxylation and photohydration of pyrimidine derivatives

Thymine-1-acetic acid is shown to undergo photodecarboxylation while uracil-1-acetic acid is subject to photohydration and photodecarboxylation. In contrast, no photodecarboxylation was observed for thymine-1-propionic acid and uracil-1-propionic acid. With the esters and amides of these acids, only photohydration occurred. In no instance was lactonization or azalactone formation detected in these photoreactions. Irradiation of these compounds in the dry solid state resulted in little change; however, in frozen aqueous solutions cyclobutyl dimers were isolated without decarboxylation. Detailed mechanistic study revealed that photohydration and photodecarboxylation of these pyrimidines probably are simultaneous processes which may occur from the vibrationally excited levels of the ground state.     

Near infrared photochemistry of pyruvic acid in aqueous solution

Recent experimental and theoretical results have suggested that organic acids such as pyruvic acid, can be photolyzed in the ground electronic state by the excitation of the OH stretch vibrational overtone. These overtones absorb in the near-infrared and visible regions of the spectrum where the solar photons are plentiful and could provide a reaction pathway for the organic acids and alcohols that are abundant in the earth's atmosphere. In this paper the overtone initiated photochemistry of aqueous pyruvic acid is investigated by monitoring the evolution of carbon dioxide. In these experiments CO(2) is being produced by excitation in the near-infrared, between 850 nm and ～1150 nm (11,765-8696 cm(-1)), where the second OH vibrational overtone (Δν = 3) of pyruvic acid is expected to absorb. These findings show not only that the overtone initiated photochemical decarboxylation reaction occurs but also that in the aqueous phase it occurs at a lower energy than was predicted for the overtone initiated reaction of pyruvic acid in the gas phase (13,380 cm(-1)). A quantum yield of (3.5 ± 1.0) × 10(-4) is estimated, suggesting that although this process does occur, it does so with a very low efficiency.     

Photoionization-induced water migration in the hydrated trans-formanilide cluster cation revealed by gas-phase spectroscopy and ab initio molecular dynamics simulation

Photoionization-induced water migration in the trans-formanilide-water 1:1 cluster, FA-(H(2)O)(1), has been investigated by using IR-dip spectroscopy, quantum chemical calculations, and ab initio molecular dynamics simulations. In the S(0) state, FA-(H(2)O)(1) has two structural isomers, FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1), where a water molecule is hydrogen-bonded (H-bonded) to the NH group and the CO group, respectively. In addition, the S(1)-S(0) origin transition of FA(CO)-(H(2)O)(2), where a water dimer is H-bonded to the CO group, was observed only in the [FA-(H(2)O)(1)](+) mass channel, indicating that one of the water molecules evaporates completely in the D(0) state. These results are consistent with a previous report [Robertson, E. G. Chem. Phys. Lett., 2000, 325, 299]. In the D(0) state, however, [FA-(H(2)O)(1)](+) produced by photoionization via the S(1)-S(0) origin transitions of FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1) shows essentially the same IR spectra. Compared with the theoretical calculations, [FA-(H(2)O)(1)](+) can be assigned to [FA(NH)-(H(2)O)(1)](+). This means that the water molecule in [FA-(H(2)O)(1)](+) migrates from the CO group to the NH group when [FA-(H(2)O)(1)](+) is produced by photoionization of FA(CO)-(H(2)O)(1). [FA-(H(2)O)(1)](+) produced by photoionization of FA(CO)-(H(2)O)(2) also shows the IR spectrum corresponding to [FA(NH)-(H(2)O)(1)](+). In this case, the water migration from the CO group to the NH group occurs with the evaporation of a water molecule. Ab initio molecular dynamics simulations revealed the water migration pathway in [FA-(H(2)O)(1)](+). The calculations of classical electrostatic interactions show that charge-dipole interaction between FA(+) and H(2)O induces an initial structural change in [FA-(H(2)O)(1)](+). An exchange repulsion between the lone pairs of the CO group and H(2)O in [FA-(H(2)O)(1)](+) also affects the initial direction of the water migration. These two factors play important roles in determining the initial water migration pathway.     

Generation and interrogation of a pure nuclear spin state by parahydrogen-enhanced NMR spectroscopy: a defined initial state for quantum computation

We describe a number of studies used to establish that parahydrogen can be used to prepare a two-spin system in a pure state, which is suitable for implementing NMR quantum computation. States are generated by pulsed and continuous-wave (CW) UV laser initiation of a chemical reaction between Ru(CO)(3)(L(2)) [where L(2) = dppe = 1,2-bis(diphenylphosphino)ethane or L(2) = dpae = 1,2-bis(diphenylarsino)ethane] with pure parahydrogen (generated at 18 K). This process forms Ru(CO)(2)(dppe)(H)(2) and Ru(CO)(2)(dpae)(H)(2) on a sub-microsecond time-scale. With the pulsed laser, the spin state of the hydride nuclei in Ru(CO)(2)(dppe)(H)(2) has a purity of 89.8 +/- 2.6% (from 12 measurements). To achieve comparable results by cooling would require a temperature of 6.6 mK, which is unmanageable in the liquid state, or an impractical magnetic field of 0.44 MT at room temperature. In the case of CW initiation, reduced state purities are observed due to natural signal relaxation even when a spin-lock is used to prevent dephasing. When Ru(CO)(3)(dpae) and pulsed laser excitation are utilized, the corresponding dihydride product spin state purity was determined as 106 +/- 4% of the theoretical maximum. In other words, the state prepared using Ru(CO)(3)(dpae) as the precursor is indistinguishable from a pure state.     

The loss of carbon dioxide from activated perbenzoate anions in the gas phase: unimolecular rearrangement via epoxidation of the benzene ring

The unimolecular reactivities of a range of perbenzoate anions (X-C6H5CO3-), including the perbenzoate anion itself (X = H), nitroperbenzoates (X = para-, meta-, ortho-NO2), and methoxyperbenzoates (X = para-, meta-OCH3) were investigated in the gas phase by electrospray ionization tandem mass spectrometry. The collision-induced dissociation mass spectra of these compounds reveal product ions consistent with a major loss of carbon dioxide requiring unimolecular rearrangement of the perbenzoate anion prior to fragmentation. Isotopic labeling of the perbenzoate anion supports rearrangement via an initial nucleophilic aromatic substitution at the ortho carbon of the benzene ring, while data from substituted perbenzoates indicate that nucleophilic attack at the ipso carbon can be induced in the presence of electron-withdrawing moieties at the ortho and para positions. Electronic structure calculations carried out at the B3LYP/6-311++G(d,p) level of theory reveal two competing reaction pathways for decarboxylation of perbenzoate anions via initial nucleophilic substitution at the ortho and ipso positions, respectively. Somewhat surprisingly, however, the computational data indicate that the reaction proceeds in both instances via epoxidation of the benzene ring with decarboxylation resulting--at least initially--in the formation of oxepin or benzene oxide anions rather than the energetically favored phenoxide anion. As such, this novel rearrangement of perbenzoate anions provides an intriguing new pathway for epoxidation of the usually inert benzene ring.     

Formation and reactivity of Ir(III) hydroxycarbonyl complexes

Kinetic studies show that the reaction of [TpIr(CO)2] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO2H)(CO)H] (2) is second order (k = 1.65 x 10(-4) dm(3) mol(-1) s(-1), 25 degrees C, MeCN) with activation parameters DeltaH++= 46+/-2 kJ mol(-1) and DeltaS++ = -162+/-5 J K(-1) mol(-1). A kinetic isotope effect of k(H2O)/k(D2O) = 1.40 at 20 degrees C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp*Ir(CO)2] (1*, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp*Ir(CO2H)(CO)H] (2*). A proposed mechanism consistent with the relative reactivity of 1 and 1* involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2* shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O...O) 2.65 A). Complex 2* is thermally stable but (like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp*Ir(CO)2H]+ (3*) and decarboxylation with OH- to give [TpIr(CO)H2] (4*). Complex 2 undergoes thermal decarboxylation above ca. 50 degrees C to give [TpIr(CO)H2] (4) in a first-order process with activation parameters DeltaH++ = 115+/-4 kJ mol(-1) and DeltaS++ = 60+/-10 J K(-1) mol(-1).     

Cu_2O(111)催化水煤气变换反应机理的理论研究

采用密度泛函理论结合周期平板模型方法系统地研究了水煤气变换反应在Cu_2O(111)表面上的反应机理,包括氧化还原机理、羧基机理和甲酸根机理.结果表明,在Cu_2O(111)表面,羧基机理和甲酸根机理均可行,且甲酸根机理更为有利,其最佳反应途径为H_2O~*→H~*+OH~*;CO(g)+H~*+OH~*→trans-HCOOH~*(1)→cis-HCOOH~*→CO_2~*+H_2(g).其中trans-HCOOH~*(1)→cis-HCOOH~*为其决速步,该基元反应的能垒仅为59 kJ·mol~(-1).羧基机理的最优反应路径同样是以H_2O的解离反应开始,随后CO(g)+OH~*→cis-COOH~*→trans-COOH~*→CO_2(g)+H~*,最后产生的两个吸附的H原子先迁移再结合生成H_2,整个反应的控速步骤为H原子的迁移,迁移能垒为96 kJ·mol~(-1).氧化还原机理则由于OH解离需要越过一个很高的能垒(254 vs.187 kJ·mol~(-1))而不可行.     

I(+1) transfer from diiodomalonic acid to malonic acid and a complete inhibition of the CO and CO2 evolution in the Briggs-Rauscher reaction by resorcinol

A recent report on an intense CO 2 and CO evolution in the Briggs-Rauscher (BR) reaction revealed that iodination of malonic acid (MA) is not the only important organic reaction in the classical BR oscillator. To disclose the source of the gas evolution, iodomalonic (IMA) and diiodomalonic (I2MA) acids were prepared by iodinating MA with nascent iodine in a semibatch reactor. The nascent iodine was generated by an iodide inflow into the reactor, which contained a mixture of MA and acidic iodate. Some CO2 and a minor CO production was observed during these iodinations. It was found that in an aqueous acidic medium the produced I2MA is not stable but decomposes slowly to diiodoacetic acid and CO2. The first-order rate constant of the I 2MA decarboxylation at 20 degrees C was found to be k1 = 9 x 10(-5) s(-1), which is rather close to the rate constant of the analogous decarboxylation of dibromomalonic acid under similar conditions (7 x 10(-5)s(-1)). From the rate of the CO2 evolution, the I2MA concentration can be calculated in a MA-IMA-I2MA mixture as only I2MA decarboxylates spontaneously but MA and IMA are stable. Following CO2 evolution rates, it was proven that I2MA can react with MA in the reversible reaction I2MA + MA <--> 2 IMA. The equilibrium constant of this reaction was calculated as K = 380 together with the rate constants of the forward k 2 = 6.2 x 10 (-2) M (-1)s(-1) and backward k-2 = 1.6 x 10(-4) M(-1)s(-1) reactions. The probable mechanism of the reaction is I(+1) transfer from I2MA to MA. The presence of I(+1) in a I2MA solution is demonstrated by its reduction with ascorbic acid. To estimate the fraction of CO2 coming from the decarboxylation of I2MA in an oscillatory BR reaction, the oscillations were inhibited by resorcinol. Unexpectedly, all CO2 and CO evolution was interrupted for more than one hour after injecting a small amount of resorcinol (10(-5) M initial concentration in the reactor). Finally, some implications of the newly found I(+1) transfer reactions and the surprisingly effective inhibition by resorcinol regarding the mechanism of the oscillatory BR reaction are discussed. The latter is explained by the ability of resorcinol to scavenge free radicals including iodine atoms without producing iodide ions.     

Backbone cleavages and sequential loss of carbon monoxide and ammonia from protonated AGG: A combined tandem mass spectrometry,isotope labeling,and theoretical study

The fragmentation characteristics of protonated alanylglycylglycine, [AGG + H](+), were investigated by tandem mass spectrometry in MALDI-TOF/TOF, ion trap, and hybrid sector instruments. b(2) is the most abundant fragment ion in MALDI-TOF/TOF, ion trap, and hybrid sector metastable ion (MI) experiments, while y(2) is slightly more abundant than b(2) in collision activated dissociation (CAD) performed in the sector instrument. The A-G amide bond is cleaved on the a(1)-y(2) pathway resulting in a proton-bound dimer of GG and MeCH=NH. Depending on the fragmentation conditions employed, this dimer can then (1) be detected as [AGG + H - CO](+), (2) dissociate to produce y(2) ions, [GG + H](+), (3) dissociate to produce a(1) ions, [MeCH=NH + H](+), or (4) rearrange to expel NH(3) forming a [AGG + H - CO - NH(3)](+) ion. The activation method and the experimental timescale employed largely dictate which of, and to what extent, these processes occur. These effects are qualitatively rationalized with the help of quantum chemical and RRKM calculations. Two mechanisms for formation of the [AGG + H - CO - NH(3)](+) ion were evaluated through nitrogen-15 labeling experiments and quantum chemical calculations. A mechanism involving intermolecular nucleophilic attack and association of the GG and imine fragments followed by ammonia loss was found to be more energetically favorable than expulsion of ammonia in an S(N)2-type reaction.     

氮掺杂石墨烯负载单原子Zr催化CO2加氢的密度泛函理论研究

基于密度泛函理论计算,研究了H2和CO2在氮掺杂石墨烯负载单原子Zr催化剂(Zr Nx-Gr)上的吸附和CO2催化加氢反应. H2和CO2在Zr N3-Gr上单独吸附的吸附能分别为-0.49和-2.17 e V,在H2和CO2共吸附状态下,吸附能为-2.24 e V,均高于在Zr N4-Gr表面的吸附能,表明Zr N3-Gr表面更利于CO2加氢反应的发生.在Zr N3-Gr表面, CO2在共吸附后保持了其单独吸附时的特性,削弱了H2分子的吸附. CO2在Zr Nx-Gr表面催化加氢反应起始于H2和CO2的共吸附构型,沿反式HCOOH路径形成甲酸盐(HCOO*)中间体,然后HCOO*基团吸附H原子形成反式甲酸,在Zr N3-Gr和Zr N4-Gr表面该路径的反应能垒分别为1.85和2.48 e V.另一路径为产生CO与H2O的反应,在Zr N3-Gr和Zr N4-Gr表面的反应能垒分别为1.86和1.73 e V,表明Zr N3-Gr更利于CO2加氢生成甲酸反应的发生,而Zr N4-Gr表面更利于CO的产生.     

Energetics and kinetics of the reaction of HOCO with hydrogen atoms

The potential energy surface for the reaction of HOCO radicals with hydrogen atoms has been explored using the CCSD(T)/aug-cc-pVQZ ab initio method. Results show that the reaction occurs via a formic acid (HOC(O)H) intermediate, and produces two types of products: H(2)O+CO and H(2)+CO(2). Reaction enthalpies (0 K) are obtained as -102.0 kcalmol for the H(2)+CO(2) products, and -92.7 kcalmol for H(2)O+CO. Along the reaction pathways, there exists a nearly late transition state for each product channel. However, the transition states locate noticeably below the reactant asymptote. Direct ab initio dynamics calculations are also carried out for studying the kinetics of the H+HOCO reaction. At room temperature, the rate coefficient is predicted to be 1.07x10(-10)cm(3) molec(-1) s(-1) with a negligible activation energy E(a)=0.06 kcalmol, and the branching ratios are estimated to be 0.87 for H(2)+CO(2), and 0.13 for H(2)O+CO. In contrast, the product branching ratios have a strong T dependence. The branching ratio for H(2)O+CO could increase to 0.72 at T=1000 K.