Mechanistic analysis of reductive nitrosylation on water-soluble cobalt(III)-porphyrins

The reactions of NO and/or NO2- with three water-soluble cobalt porphyrins [Co(III)(P)(H2O)2]n, where P = TPPS, TCPP, and TMPyP, were studied in detail. At pH < 3, the reaction with NO proceeds through a single reaction step. From the kinetic data and activation parameters, the [Co(III)(P)(NO)(H2O)]n complex is proposed to be the primary product of the reaction with NO. This complex reacts further with a second NO molecule through an inner-sphere electron-transfer reaction to generate the final product, [Co(III)(P)(NO-)](n-1). At pH > 3, although a single reaction step is also observed, a systematic study as a function of the NO and NO2- concentrations revealed that two reaction steps are operative. In the first, NO2- and NO compete to substitute coordinated water in [Co(III)(P)(H2O)2]n to yield [Co(III)(P)(NO)(H2O)]n and [Co(III)(P)(NO2-)(H2O)](n-1) as the primary reaction products. Only the nitrite complex could be detected and no final product formation was observed during the reaction. It is proposed that [Co(III)(P)(NO)(H2O)]n rapidly reacts with NO2- to form the nitrite complex, which in the second reaction step reacts with another NO molecule to generate the final product through an inner-sphere electron-transfer reaction. The reported results are relevant for the interaction of vitamin B(12a) with NO and NO2-.     

Co—MgO催化剂上NO—程序升温表面反应和NO—CH4反应

ＮＯ，程序升温表面反应（ＴＰＳＲ），ＮＯ－ＣＨ４反应，Ｃｏ－ＭｇＯ     

TEA法测定NO2的反应机理研究

应用FABMS、NMR、IR、HPLC和紫外吸收光谱研究了室温下三乙醇胺与较高浓度NO₂(2.48%)的反应情况,证明反应产物为三乙醇胺硝酸盐和三乙醇胺亚硝酸盐,提出了可能的反应机理,为TEA法监测空气中NO₂的浓度提供了依据.     

Theoretical study on reaction mechanism of the cyanogen radical with nitrogen dioxide

The complex singlet potential energy surface for the reaction of CN with NO2, including 9 minimum isomers and 10 transition states, is explored computationally using a coupled cluster method and a density functional method. The most favorable association of CN with NO2 was found to be a barrierless carbon-to-nitrogen approach process forming an energy-rich adduct a (NCNO2) followed by C-N bond rupture along with C-O bond formation to give b1 (trans-NCONO), which can easily convert to b2 (cis-NCONO). Our results show that the product P1 (NCO + NO) is the major product, while the product P2 (CNO + NO) is a minor product. The other products may be of significance only at high temperatures. Product P1 (NCO + NO) can be obtained through path 1 P1: R --> a --> b1 (b2) --> P1 (NCO + NO), whereas the product P2 (CNO + NO) can be formed through path P2: R --> a --> b1 --> b2 --> c1 (c2) --> P2 (CNO + NO). Because the intermediates and transition states involved in the above two channels are all lower than the reactants in energy, the CN + NO2 reaction is expected to be rapid, as is confirmed by experiment. Therefore, it may be suggested as an efficient NO2-reduction strategy. These calculations indicate that the title reaction proceeds mostly through singlet pathways and less go through triplet pathways. The present results can lead us to understand deeply the mechanism of the title reaction and can be helpful for understanding NO2-combustion chemistry.     

Theoretical mechanistic study on the radical-molecule reaction of CH2OH with NO2

The complex singlet potential energy surface for the reaction of CH2OH with NO2, including 14 minimum isomers and 28 transition states, is explored theoretically at the B3LYP/6-311G(d,p) and Gaussian-3 (single-point) levels. The initial association between CH2OH and NO2 is found to be the carbon-to-nitrogen approach forming an adduct HOCH2NO2 (1) with no barrier, followed by C-N bond rupture along with a concerted H-shift leading to product P1 (CH2O + trans-HONO), which is the most abundant. Much less competitively, 1 can undergo the C-O bond formation along with C-N bond rupture to isomer HOCH2ONO (2), which will take subsequent cis-trans conversion and dissociation to P2 (HOCHO + HNO), P3 (CH2O + HNO2), and P4 (CH2O + cis-HONO) with comparable yields. The obtained species CH2O in primary product P1 is in good agreement with kinetic detection in experiment. Because the intermediate and transition state involved in the most favorable pathway all lie blow the reactants, the CH2OH + NO2 reaction is expected to be rapid, as is confirmed by experiment. These calculations indicate that the title reaction proceeds mostly through singlet pathways; less go through triplet pathways. In addition, a mechanistic comparison is made with the reactions CH3 + NO2 and CH3O + NO2. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for understanding NO2-combustion chemistry.     

Nitrite impurities are responsible for the reaction observed between vitamin B12 and nitric oxide in acidic aqueous solution

The reaction between aquacobalamin, Cbl(H2O), and NO was studied at low pH. As previously reported, the final product of the reaction is the same as that obtained in the reaction of NO and reduced Cbl(H2O), viz. Cbl(NO-). Nevertheless, this reductive nitrosylation is preceded by a faster reaction (accompanied by small absorbance changes) that depends on the HNO2 concentration but not on the NO concentration. Kinetic and UV-vis spectroscopic data show that Cbl(NO2-) is generated during this reaction. Spectroscopic data show that the dimethylbenzimidazole group trans to the NO2- ligand is protonated and partially dechelated at pH 1, by which a reaction with NO is induced. DFT calculations were performed to compare the ability of NO and NO2- to bind to cobalamin and their influence on the stability of the dimethylbenzimidazole group. The reductive nitrosylation reaction shows a quadratic dependence on the HNO2 concentration and an inverse dependence on the NO concentration. It also strongly depends on pH and is no longer observed at pH > 4. On the basis of earlier work performed on a series of Co(III) porphyrins, a mechanism is proposed that can quantitatively account for the HNO2 and NO dependencies. The reductive nitrosylation reaction is practically dominated by a back reaction, i.e., the reaction between Cbl(NO-) and HNO2, which accounts for the strange NO and HNO2 concentration dependencies observed.     

A study of the reaction of N+ with O2: experimental quantification of NO+(a 3Sigma+) production (298-500 K) and computational study of the overall reaction pathways

The product branching ratios for NO+(X 1Sigma+) and NO+(a 3Sigma+) produced from the reaction of N+ with O2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO+(a) at both temperatures, despite the fact that the reaction to form NO+(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N+ + O2 reaction show that the reaction from N+(3P) + O2(3Sigma(g)) reactants starts with an efficient early stage charge transfer to the N(2D) + O2+(X 2Pi) channel, which gives rise to the O2+(X 2Pi) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO+ and O+ products. Pathways to produce NO+(a 3Sigma+) are found to be less favorable than pathways leading to the major product NO+(X 1Sigma+). Production of N(2D) has implications for the concentration of NO in the mesosphere.     

Theoretical study on reaction mechanism of the ketenylidene radical with nitrogen dioxide

The complex doublet potential-energy surface for the reaction of CCO with NO2, including 8 minimum isomers and 17 transition states, is explored theoretically using the coupled cluster and density functional theory. The association of CCO with NO2 was found to be a barrierless process forming an energy-rich adduct a (OCCNO2) followed by oxygen shift to give b (O2CCNO). Our results show that the product P1 (CO2 + CNO) is the major product with absolute yield, while the product P4 (2CO + NO) is the minor product with less abundance. The other products may be undetectable. The product P1 (CO2 + CNO) can be obtained through R --> a --> b --> P1 (CO2 + CNO), whereas the product P4 (2CO + NO) can be obtained through two channels R --> a--> b --> c --> (d, g) --> P2 (OCNO + CO) --> P4 (2CO + NO) and R --> a --> b --> f --> P3 (c-OCC-O + NO) --> P4 (2CO + NO). Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the CCO + NO2 reaction is expected to be rapid, which is consistent with the experimental measurement in quality. The present study may be helpful for further experimental investigation of the title reaction.     

Mechanistic aspects of HC-SCR over HZSM-5: hydrocarbon activation and role of carbon-nitrogen intermediates

This study focuses on the mechanism of lean NO(2) reduction by hydrocarbons (propane, propene, and isobutane) over HZSM-5. In-situ FTIR measurements indicate a close correlation between formation of isocyanate species, consumption of water (formed in the reaction), and formation of amine species. The results in this investigation confirm our previously suggested reaction mechanism, which involves reaction of NO(+) species and hydrocarbon-derived species over Br?nsted acid sites, forming isocyanate species. These species are hydrolyzed by water, forming amine species and, finally, N(2). Experiments with (18)O(2) show an enhanced oxidation of propane by oxygen, in the presence of NO(2). This effect can possibly be explained by a type of reaction mechanism where gas-phase and/or loosely bound NO(2) react with the adsorbed hydrocarbon-derived species (i.e., carbenium ion adsorbates and/or alkenes), which then more easily react with oxygen.     

The catalytic chemistry of HCN + NO2 over Na- and Ba-Y,FAU: an in situ FTIR and TPD/TPR study

The adsorption of HCN and the reaction of HCN with NO(2) over Na-, and Ba-Y,FAU zeolite catalysts were investigated using in situ FTIR and TPD/TPR spectroscopies. Both catalysts adsorb HCN molecularly at room temperature, and the strength of adsorption is higher over Ba-Y than Na-Y. Over Na-Y, the reaction between HCN and NO(2) is slow at 473 K. On Ba-Y, HCN reacts readily with NO(2) at 473K, forming N(2), CO, CO(2), HNCO, NO, N(2)O, and C(2)N(2). The results of this investigation suggest that initial step in the HCN + NO(2) reaction over these catalysts is the hydrogen abstraction from HCN, and the formation of ionic CN- and NC- species. The formation of N(2) can proceed directly from these ionic species upon their interaction with NO+. Alternatively, these cyanide species can be oxidized to isocyanates which then can be further transformed to N(2), N(2)O and CO(x) in their subsequent reaction with NO(x).     

Theoretical investigation of the mechanisms of reactions of H2CN and H2SiN with NO

The mechanisms of the reaction of H2XN (X = C, Si) with NO were studied at the level CCSD(T)/aug-cc-PVTZ//B3LYP/6-31++G(d,p). The results indicate that there are two most favorable reaction pathways in the reaction H2CN + NO that have similar energy barriers; these two pathways lead to the formation of HCN + HNO (P1) and H2CO + N2 (P3), with the calculated barriers 11.1 and 10.2 kcal/mol, respectively, with respect to the reactants (H2CN + NO). In the reaction H2SiN + NO the difference of the barriers in these two analogous pathways becomes large, and the preferable pathway shifts to the production of H2SiO + N2 (P3s), which has no barrier with respect to the reactants (H2SiN + NO). A direct reduction of NO into a stable and nontoxic nitrogen molecule with no energy input becomes possible. As a consequence, H2SiN might be an effective reagent to convert the reactive and toxic NO into a benign gas N2 in several NO-producing combustion systems. We offer a possible explanation of the differences between H2CN and H2SiN toward NO as well as the calculated potential energies for these reactions.     

NO(2)(+) nitration mechanism of aromatic compounds: electrophilic vs charge-transfer process

The nitration of methylnaphthalenes with NO(2)BF(4) and NOBF(4) was examined in order to shed light on the controversial aromatic nitration mechanism, electrophilic vs charge-transfer process. The NO(2)(+) nitration of 1,8-dimethylnaphthalene showed a drastic regioselectivity change depending on the reaction temperature, where ortho-regioselectivity at -78 degrees C and para-regioselectivity at 0 degrees C were considered to reflect the electrophilic and the direct or alternative charge-transfer process, respectively, because the NO(+) nitration through the same reaction intermediates as in the NO(2)(+) nitration via a charge-transfer process resulted in para-regioselectivity regardless of the reaction temperature. The NO(2)(+) nitration of redox potential methylnaphthalenes higher than 1,8-dimethylnaphthalene gave a similar ortho-regioselectivity enhancement to 1,8-dimethylnaphthalene at lower temperature, thus reflecting the electrophilic process. On the other hand, the NO(2)(+) nitration of redox potential methylnaphthalenes lower than 1,8-dimethylnaphthalene showed para-regioselectivity similar to the NO(+) nitration, indicating the direct or alternative charge-transfer process. In the presence of strong acids where the direct charge-transfer process will be suppressed by protonation, the ortho-regioselectivity enhancement was observed in the NO(2)(+) nitration of 1,8-dimethylnaphthalene, suggesting that the direct charge-transfer process could be the main process to show para-regioselectivity. These experimental results imply that the NO(2)(+) nitration proceeds via not only electrophilic but also direct charge-transfer processes, which has been considered to be unlikely because of the high energy demanding process of a bond coordination change between NO(2)(+) and NO(2). Theoretical studies at the MP2/6-31G(d) level predicted ortho- and para-regioselectivity for the NO(2)(+) nitration via electrophilic and charge-transfer processes, respectively, and the preference of the direct charge-transfer process over the alternative one, which support the experimental conclusion     

Reaction of a superoxochromium(III) ion with nitrogen monoxide: kinetics and mechanism.

The kinetics of the rapid reaction between Cr(aq)OO(2+) and NO were determined by laser flash photolysis of Cr(aq)NO(2+) in O(2)-saturated acidic aqueous solutions, k = 7 x 10(8) M(-1) s(-1) at 25 degrees C. The reaction produces an intermediate, believed to be NO(2), which was scavenged with ([14]aneN(4))Ni(2+). With limiting NO, the Cr(aq)OO(2+)/NO reaction has a 1:1 stoichiometry and produces both free NO(3)(-) and a chromium nitrato complex, Cr(aq)ONO(2)(2+). In the presence of excess NO, the stoichiometry changes to [NO]/[Cr(aq)OO(2+)] = 3:1, and the reaction produces close to 3 mol of nitrite/mol of Cr(aq)OO(2+). An intermediate, identified as a nitritochromium(III) ion, Cr(aq)ONO(2+), is a precursor to a portion of free NO(2)(-). In the proposed mechanism, the initially produced peroxynitrito complex, Cr(aq)OONO(2+), undergoes O-O bond homolysis followed by some known and some novel chemistry of Cr(aq)O(2+) and NO(2). The reaction between Cr(aq)O(2+) and NO generates Cr(aq)ONO(2+), k > 10(4) M(-1) s(-1). Cr(aq)OO(2+) reacts with NO(2) with k = 2.3 x 10(8) M(-1) s(-1).     

Ag-ZSM-5催化剂上CH4选择催化还原NOx的研究

摘要研究Ag-ZSM-5催化剂上CH₄选择性催化还原NO_x的反应性能,采用TPD和TPSR技术研究NO和O₂共吸附于Ag-ZSM-5催化剂表面形成的吸附物种及其和CH₄之间的反应。结果表明,Ag-ZSM-5催化剂上CH₄选择性还原NO_x活性和选择性较高。NO和O₂共吸附在Ag-ZSM-5催化剂上形成的NO₃(s)吸附物种能被CH₄还原生成N₂.在NO₃(s)和O₂共存的体系中,CH₄能优先并选择性还原NO₃(s)生成N₂.     

Thermodynamic and kinetic studies on the binding of nitric oxide to a new enzyme mimic of cytochrome p450

A new model for the P450 enzyme carrying a SO(3)(-) ligand coordinated to iron(III) (complex 2) reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of the same order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450(cam) (E.S-P450(cam)). Large and negative activation entropy and activation volume values for the binding of NO to complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin change from S = (5)/(2) to S = 0, as proposed for the reaction between NO and E.S-P450(cam). In contrast, the dissociation of NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction for the E.S-P450(cam)/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5 in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. This behavior can be explained in terms of a mechanism in which methanol must be displaced during Fe-NO bond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450 (2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions of cytochrome P450(cam) (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III) model complex.     

Selective reduction of nitric oxide with methane on In/H-ZSM-5-based catalysts

There have been many studies on the catalysis of selective catalytic reduction of nitric oxide with hydrocarbons. It was shown in our previous works that Ir/In/H-ZSM-5 has high catalytic activity and selectivity for this reaction by use of methane as a reductant. The reaction of CH4 -SCR proceeds consecutively as NO oxidation to NO2 and NO2 reduction with CH4 . These two reactions take place bifunctionally on different kinds of catalytic sites: NO oxidation on Ir and NO 2 reduction on InO+ sites. The studies of NOx chemisorption and kinetics of NOx reduction with CH4 lead us to conclude that the bifunctional catalysis is remarkably facilitated by the coexistence of these sites in the identical pores of zeolite, which may be called "intrapore catalysis". In this review, the design of highly active and selective catalysts for this reaction system will be discussed on the basis of bifunctionality.     

Theoretical study on the reaction mechanism of the methyl radical with nitrogen oxides

The radical-molecule reaction mechanism of CH3 with NOx (x = 1, 2) has been explored theoretically at the B3LYP/6-311Gd,p and MC-QCISD (single-point) levels of theory. For the singlet potential energy surface (PES) of the CH3 + NO2 reaction, it is found that the carbon to middle nitrogen attack between CH3 and NO2 can form energy-rich adduct a (H3CNO2) with no barrier followed by isomerization to b1 (CH3ONO-trans), which can easily convert to b2 (CH3ONO-cis). Subsequently, starting from b (b1, b2), the most feasible pathway is the direct N-O bond cleavage of b (b1, b2) leading to P1 (CH3O + NO) or the 1,3-H-shift and N-O bond rupture of b1 to form P2 (CH2O + HNO), both of which may have comparable contribution to the reaction CH3 + NO2. Much less competitively, b2 can take a concerted H-shift and N-O bond cleavage to form product P3 (CH2O + HON). Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the CH3 + NO2 reaction is expected to be rapid, as is consistent with the experimental measurement in quality. For the singlet PES of the CH3 + NO reaction, the major product is found to be P1 (HCN + H2O), whereas the minor products are P2 (HNCO + H2) and P3 (HNC +H2O). The CH3 + NO reaction is predicted to be only of significance at high temperatures because the transition states involved in the most feasible pathways lie almost above the reactants. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. The present study may be helpful for further experimental investigation of the title reactions.     

Sn0.5Ti0.5O2催化剂上SO2、NO和CO反应的机理

Sn0.5Ti0.5O2催化剂对NO＋CO反应活性不高， 350 ℃时NO的转化率只有50％,但反应气中含有SO2时， NO的转化率接近100％,说明SO2对Sn0.5Ti0.5O2催化剂上的NO＋CO反应具有促进作用. XPS表征发现,SO2＋CO、SO2＋NO＋CO反应后催化剂表面有微量硫存在，而反应前没有检测到硫的存在.结合反应性能测定、瞬变应答实验、XRD、TPD研究等，发现催化剂上的表面硫参与了NO的催化还原反应，是NO＋CO反应更重要的活性中心.据此，提出了SO2＋NO＋CO反应的氧化还原反应机理.     

Theoretical study on the mechanism of the CH2F + NO2 reaction

Despite the importance of the Fluoromethyl radicals in combustion chemistry, very little experimental information on their reactions toward stable molecules is available in the literature. Motivated by recent laboratory characterization about the reaction kinetics of Chloromethyl radicals with NO2, we carried out a detailed potential energy survey on the CH2F + NO2 reaction at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels as an attempt toward understanding the CH2F + NO2 reaction mechanism. It is shown that the CH2F radical can react with NO2 to barrierlessly generate adduct a (H2FCNO2), followed by isomerization to b1 (H2FCONO-trans) which can easily interconvert to b2 (H2FCONO-cis). Subsequently, Starting from b (b1, b2), the most feasible pathway is the C--F and N--O1 bonds cleavage along with N--F bond formation of b (b1, b2) leading to P1 (CH2O + FNO), or the direct N--O1 weak-bond fission of b (b1, b2) to give P2 (CH2FO + NO), or the 1,3-H-shift associated with N--O1 bond rupture of b1 to form P3 (CHFO + HNO), all of which may have comparable contribution to the reaction CH2F + NO2. Much less competitively, b2 either take the 1,4-H-shift and O1--N bond cleavage to form product P4 (CHFO + HON) or undergo a concerted H-shift to isomer c2 (HFCONOH), followed by dissociation to P4. Because the rate-determining transition state (TSab1) in the most competitive channels is only 0.3 kcal/mol higher than the reactants in energy, the CH2F + NO2 reaction is expected to be rapid, and may thus be expected to significantly contribute to elimination of nitrogen dioxide pollutants. The similarities and discrepancies among the CH2X + NO2 (X = H, F, and Cl) reactions are discussed in terms of the electronegativity of halogen atom. The present article may assist in future experimental identification of the product distributions for the title reaction, and may be helpful for understanding the halogenated methyl chemistry.     

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with nitric oxide

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with NO. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas at five temperatures in the range 299-592 K. The second-order rate constants at 10 Torr fitted the Arrhenius equation log(k/cm3 molecule(-1) s(-1)) = (-11.66 +/- 0.01) + (6.20 +/- 0.10 kJ mol(-1))/RT ln 10 The rate constants showed a variation with pressure of a factor of ca. 2 over the available range, almost independent of temperature. The data could not be fitted by RRKM calculations to a simple third body assisted association reaction alone. However, a mechanistic model with an additional (pressure independent) side channel gave a reasonable fit to the data. Ab initio calculations at the G3 level supported a mechanism in which the initial adduct, bent H2SiNO, can ring close to form cyclo-H2SiNO, which is partially collisionally stabilized. In addition, bent H2SiNO can undergo a low barrier isomerization reaction leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are NH2 + SiO. The rate controlling barrier for this latter pathway is only 16 kJ mol(-1) below the energy of SiH2 + NO. This is consistent with the kinetic findings. A particular outcome of this work is that, despite the pressure dependence and the effects of the secondary barrier (in the side reaction), the initial encounter of SiH2 with NO occurs at the collision rate. Thus, silylene can be as reactive with odd electron molecules as with many even electron species. Some comparisons are drawn with the reactions of CH2 + NO and SiCl2 + NO.