Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute Fe(III)(TPP)(NO3)

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe(III)(TPP)(eta(2)-O(2)NO) (1, TPP = meso-tetraphenyl porphyrinate(2)(-)) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO(2))(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(eta(2)-O(2)NO) maintained under a (15)NO atmosphere was found to be the labeled analogue Fe(TPP)(eta(2)-O(2)(15)NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N(2)O(3) and (15)NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(eta(1)-ONO(2))(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO(3)(*)( )()to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N(2)O(4) concerted with electron transfer to the metal center is proposed.     

Reactions of nitrogen oxides with heme models. Characterization of NO and NO2 dissociation from Fe(TPP)(NO2)(NO) by flash photolysis and rapid dilution techniques: Fe(TPP)(NO2) as an unstable intermediate

Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.     

Comparative IR study of nitric oxide reactions with sublimed layers of iron(II)- and ruthenium(II)-meso-tetraphenylporphyrinates

The interactions of nitric oxide gas with thin layers of Fe(II)(TPP) and Ru(II)(TPP), obtained by sublimation onto low-temperature substrate (77 K), has been investigated by means of IR spectroscopy (TPP = meso-tetraphenylporphyrinate). Only simple addition of NO to form Fe(TPP)(NO) is observed for the iron-porphyrin Fe(II)(TPP), while, in contrast, Ru(II)(TPP) promotes NO disproportionation to form the nitrosyl-nitrito complex Ru(TPP)(NO)(ONO) and N(2)O. Thin layers of Fe(TPP)(NO) are inert to further reaction with excess NO; however, the nitrosyl-nitro complex Fe(TPP)(NO)(NO(2)) is readily formed when traces of dioxygen are added to the NO atmosphere. When the NO(2) concentrations in the NO/NO(2) mixture are relatively high, the nitrato complex Fe(TPP)(NO(3)) is also formed. Spectral data are given indicating that moderate shifts in the nitrosyl stretching frequency of Fe(TPP)(NO) are due to crystal packing effects, rather than to the H-bonding of coordinated NO with protic contaminants suggested in an earlier publication. Removal of NO by exhaustive evacuation from layers containing Fe(TPP)(NO)(NO(2)) leads to formation of Fe(TPP)(NO) and Fe(TPP)(NO(3)).     

Nitric oxide adsorption and reduction reaction mechanism on the Rh7(+) cluster: a density functional theory study

The transition metal rhodium has been proved the effective catalyst to convert from NO(x) to N(2.) In the present work, we are mainly focused on the NO adsorption and decomposition reaction mechanism on the surface of the Rh(7)(+) cluster, and the calculated results suggest that the reaction can proceed via three steps. First, the NO can adsorb on the surface of the Rh(7)(+) cluster; second, the NO decomposes to N and O atoms; finally, the N atom reacts with the second adsorbed NO and reduces to a N(2) molecule. The N-O bond breaks to yield N and O atoms in the second step, which is the rate-limiting step of the whole catalytic cycle. This step goes over a relatively high barrier (TS(12)) of 39.6 kcal/mol and is strongly driven by a large exothermicity of 55.1 kcal/mol during the formation of stable compound 3, accompanied by the N and O atoms dispersed on the different Rh atoms of the Rh(7)(+) cluster. In addition, the last step is very complex due to the different possibilities of reaction mechanism. On the basis of the calculations, in contrast to the reaction path II that generates N(2) from two nitrogen atoms coupling, the reaction path I for the formation of intermediate N(2)O is found to be energetically more favorable. Present work would provide some valuable fundamental insights into the behavior of the nitric oxide adsorption and reduction reaction mechanism on the Rh(7)(+) cluster.     

Role of adsorbed NO in N2O decomposition over iron-containing ZSM-5 catalysts at low temperatures

Transient response and temperature-programmed desorption/reaction (TPD/TPR) methods were used to study the formation of adsorbed NO(x) from N2O and its effect during N2O decomposition to O2 and N2 over FeZSM-5 catalysts at temperatures below 653 K. The reaction proceeds via the atomic oxygen (O)(Fe) loading from N2O on extraframework active Fe(II) sites followed by its recombination/desorption as the rate-limiting step. The slow formation of surface NO(x,ads) species was observed from N2O catalyzing the N2O decomposition. This autocatalytic effect was assigned to the formation of NO(2,ads) species from NO(ads) and (O)(Fe) leading to facilitation of (O)(Fe) recombination/desorption. Mononitrosyl Fe2+(NO) and nitro (NO(2,ads)) species were found by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) in situ at 603 K when N2O was introduced into NO-containing flow passing through the catalyst. The presence of NO(x,ads) does not inhibit the surface oxygen loading from N2O at 523 K as observed by transient response. However, the reactivity of (O)(Fe) toward CO oxidation at low temperatures (<523 K) is drastically diminished. Surface NO(x) species probably block the sites necessary for CO activation, which are in the vicinity of the loaded atomic oxygen.     

Conversion of a Fleeting Open‐Shell Iron Nitride into an Iron Nitrosyl

Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [Fe^VN(cyclam‐ac)]⁺ ( 2 , cyclam‐ac^?=1,4,8,11‐tetraazacyclotetradecane‐1‐acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam‐ac)]⁺ ( 3 ). Further theoretical studies suggest that during the reaction a closed‐shell singlet O atom is transferred to 2 . Consequently, the N?O bond formation does not follow a radical coupling mechanism proposed for the N?N bond formation but is accomplished by three mutual electron‐transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2 .     

Structure and photochemistry of nitrocobalt(III) tetraphenylporphyrin with axial triphenylphosphine in toluene solutions

Thermal and photochemical reactions of nitroaquacobalt(III) tetraphenylporphyrin, (NO(2))(H(2)O)Co(III)TPP, have been investigated in toluene solutions containing triphenylphosphine, P phi(3). It is found that Pphi(3) thermally abstracts an oxygen atom from the NO(2) moiety of (NO(2))(H(2)O)Co(III)TPP with a rate constant 0.52 M(-1) s(-1), resulting in the formation of nitrosylcobalt porphyrin, (NO)CoTPP. The 355-nm laser photolysis of (NO(2))(H(2)O)Co(III)TPP at low concentrations of P phi(3) (<1.0 x 10(-4) M) gives Co(II)TPP and NO(2) as intermediates. The recombination reaction of Co(II)TPP and NO(2) initially forms the coordinately unsaturated nitritocobalt(III) tetraphenylporphyrin, (ON-O)Co(III)TPP, which reacts with P phi(3) to yield nitro(triphenylphosphine)cobalt(III) tetraphenylporphyrin, (NO(2))(P phi(3))Co(III)TPP. Subsequently, the substitution reaction of the axial P phi(3) with H(2)O leads to the regeneration of (NO(2))(H(2)O)Co(III)TPP. From the kinetic studies, the substitution reaction is concluded to occur via a coordinately unsaturated nitrocobalt(III) porphyrin, (NO(2))Co(III)TPP. At higher concentrations of P phi(3) (>4 x 10(-3) M), (NO(2))(H(2)O)Co(III)TPP reacts with P phi(3) to form (NO(2))(P phi(3))Co(III)TPP: the equilibrium constant is obtained as K = 4.3. The X-ray structure analysis of (NO(2))(P phi(3))Co(III)TPP reveals that the P-Co-NO(2) bond angle is 175.0(2) degrees and the bond length Co-NO(2) is 2.000(7) A. In toluene solutions of (NO(2))(H(2)O)Co(III)TPP containing P phi(3) (>4 x 10(-3) M), the major light-absorbing species is (NO(2))(P phi(3))Co(III)TPP, which yields (NO)CoTPP by continuous photolysis. The laser photolysis of (NO(2))(P phi(3))Co(III)TPP gives Co(II)TPP, NO(2), and P phi(3) as initial products. The NO(2) molecule is suggested to be reduced by P phi(3) to yield NO, and the reaction between NO and Co(II)TPP gives (NO)CoTPP. The quantum yield for the photodecomposition of (NO(2))(P phi(3))Co(III)TPP is determined as 0.56.     

Mechanistic studies of the oxidative N-dealkylation of a substrate tethered to carboxylate-bridged diiron(II) complexes, [Fe2(mu-O2CAr(Tol))2(O2CAr(Tol))2(N,N-Bn2en)2

Carboxylate-bridged diiron(II) centers activate dioxygen for the selective oxidation of hydrocarbon substrates in bacterial multicomponent monooxygenases. Synthetic analogues of these systems exist in which substrate fragments tethered to the diiron(II) core through attachment to an N-donor ligand are oxidized by transient species that arise following the introduction of O2 into the system. The present study describes the results of experiments designed to probe mechanistic details of these oxidative N-dealkylation reactions. A series of diiron(II) complexes with ligands N,N-(4-R-Bn)Bnen, where en is ethylenediamine, Bn is benzyl, and R-Bn is benzyl with a para-directing group R = Cl, F, CH3, t-Bu, or OCH3, were prepared. A Hammett plot of the oxygenation product distributions of these complexes, determined by gas chromatographic analysis, reveals a small positive slope of rho = +0.48. Kinetic isotope effect (KIE(intra)) values for oxygenation of [Fe2(mu-O2CAr(Tol))2(O2CAr(Tol))2(N,N-(C6H5CDH)2en)2] and [Fe2(mu-O2CAr(Tol))2(O2CAr(Tol))2(N,N-(C6H5CD2)(C6H5CH2)en)2] are 1.3(1) and 2.2(2) at 23 degrees C, respectively. The positive slope rho and low KIE(intra) values are consistent with a mechanism involving one-electron transfer from the dangling nitrogen atom in N,N-Bn2en to a transient electrophilic diiron intermediate, followed by proton transfer and rearrangement to eliminate benzaldehyde.     

In situ FT-IR and UV-vis spectroscopy of the low-temperature NO disproportionation mediated by solid state manganese(II) porphyrinates

The heterogeneous reaction between NO gas and sublimed layers of manganese(II) porphyrinato complexes Mn(Por) (Por = TPP (tetraphenylporphyrinato dianion), TMP (tetramesitylporphyrinato dianion), or TPP(d20) (perdeuterated tetraphenylporphyrinato dianion)) has been monitored by IR and optical spectroscopy over the temperature range of 77 K to room temperature. These manganese porphyrins promote NO disproportionation to NO2 species and N2O, and the reaction proceeds via several distinct stages. At 90 K, the principal species observed spectrally are the nitric oxide dimer, cis-ONNO, two manganese nitrosyls, the simple NO adduct Mn(Por)(NO), and another intermediate (1) that is apparently critical to the disproportionation mechanism. This key intermediate is formed prior to N2O evolution, and proposals regarding its likely structure are offered. When the system is warmed to 130 K, the disproportionation products, N2O and the O-coordinated nitrito complex Mn(Por)(NO)(ONO) (2), are formed. IR spectral changes show that, upon further warming to 200 K, 2 isomerizes into the N-bonded nitro linkage isomer Mn(Por)(NO)(NO2) (3). After it is warmed to room temperature, the latter species loses NO and converts to the known 5-coordinate nitrito complex Mn(Por)(ONO) (4).     

Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.     

Ir/SiO2 as a highly active catalyst for the selective reduction of NO with CO in the presence of O2 and SO2

Coexisting SO2 considerably enhanced the catalytic activity of Ir/SiO2 for NO reduction with CO in the presence of O2 because of the formation of a cis-type coordinated species of NO and CO to one iridium atom ([formula: see text]), a possible reaction intermediate leading to N2 formation.     

Iron-catalyzed intermolecular [2π+2π] cycloaddition

The bis(imino)pyridine iron dinitrogen compounds, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(μ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = (i)Pr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.     

NO在氧化铝负载的Pd催化剂上吸附的TPD-MS研究

消除汽车尾气中的氮氧化物(NOx)对保护大气环境有着重要意义.为了除去NOx,已经进行了许多卓有成效的研究,例如NOx在分子筛上的直接分解和催化还原,在贵金属三效催化剂上的还原等.     

Hydroxylation of aromatics with the help of a non-haem FeOOH: a mechanistic study under single-turnover and catalytic conditions

Ferric-hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low-spin Fe(III)(OOH) complexes characterized to date, [(L(5)(2))Fe(OOH)](2+) is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single-turnover conditions. In this report we show that [(L(5)(2))Fe(OOH)](2+) decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first-order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first-order rate constants. Combining the kinetic data obtained at different temperatures and under different single-turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5-D(3)]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by Fe(III)(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged Fe(IV)(O) and OH(·) species from the Fe(III)(OOH) complex have been obtained for the first time. After homolytic O-O cleavage, a caged pair of oxidants [Fe(IV)O+HO(·)] is generated that act in unison to hydroxylate the aromatic ring: HO(·) attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by Fe(IV)O to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re-aromatization step. Spin-trapping experiments in the presence of 5,5-dimethyl-1-pyrroline N-oxide and GC-MS analyses of the intermediate products further support the proposed mechanism.     

微量Pt和Pd对Fe_2O_3/ZrO_2催化剂结构与性能的影响

用ＸＲＤ、ＴＰＲ及固定床微反应器技术研究了微量Ｐｔ 和Ｐｄ 对Ｆｅ２Ｏ３／ＺｒＯ２ 催化剂结构及性能的影响。结果表明，活性组分Ｆｅ３ ＋ 与载体ＺｒＯ２ 之间的相互作用会阻止载体从无定形ＺｒＯ２ 到ｔ－ＺｒＯ２ 进而ｍ － ＺｒＯ２ 的转变，同时，促进Ｆｅ２Ｏ３ 在ＺｒＯ２ 表面的分散。微量Ｐｔ 或Ｐｄ 的加入对Ｆｅ２Ｏ３ 在ＺｒＯ２ 上的分散状态无明显影响，但使Ｆｅ２Ｏ３／ＺｒＯ２ 的还原性能发生明显变化，表层Ｆｅ２Ｏ３ 的还原性显著增强，导致Ｍ－ Ｆｅ２Ｏ３／ＺｒＯ２ 催化剂在ＮＯ＋ ＣＯ反应中有更高的催化活性。Ｎ２ 生成的选择性次序为Ｆｅ２Ｏ３／ＺｒＯ２ ＞ Ｐｄ － Ｆｅ２Ｏ３／ＺｒＯ２ ＞ Ｐｔ－ Ｆｅ２Ｏ３／ＺｒＯ２ ＞ Ｐｄ／ＺｒＯ２ ＞ Ｐｔ／ＺｒＯ２ 。     

The heterogeneous chemical kinetics of NO3 on atmospheric mineral dust surrogates

Uptake experiments of NO3 on mineral dust powder were carried out under continuous molecular flow conditions at 298 +/- 2 K using the thermal decomposition of N2O5 as NO3 source. In situ laser detection using resonance enhanced multiphoton ionization (REMPI) to specifically detect NO2 and NO in the presence of N2O5, NO3 and HNO3 was employed in addition to beam-sampling mass spectrometry. At [NO3] = (7.0 +/- 1.0) x 10(11) cm(-3) we found a steady state uptake coefficient gamma(ss) ranging from (3.4 +/- 1.6) x 10(-2) for natural limestone to (0.12 +/- 0.08) for Saharan Dust with gamma(ss) decreasing as [NO3] increased. NO3 adsorbed on mineral dust leads to uptake of NO2 in an Eley-Rideal mechanism that usually is not taken up in the absence of NO3. The disappearance of NO3 was in part accompanied by the formation of N2O5 and HNO3 in the presence of NO2. NO3 uptake performed on small amounts of Kaolinite and CaCO3 leads to formation of some N2O5 according to NO((3ads)) + NO(2(g)) --> N2O(5(ads)) --> N2O(5(g)). Slow formation of gas phase HNO3 on Kaolinite, CaCO3, Arizona Test Dust and natural limestone has also been observed and is clearly related to the presence of adsorbed water involved in the heterogeneous hydrolysis of N2O(5(ads)).     

Release of NO from reduced nitroprusside ion. Iron-dinitrosyl formation and NO-disproportionation reactions

The kinetics and mechanism of the thermal decomposition of the one-electron reduction product of [Fe(CN)(5)NO](2-) (nitroprusside ion, NP) have been studied by using UV-vis, IR, and EPR spectroscopy and mass-spectrometric and electrochemical techniques in the pH range of 4-10. The reduction product contains an equilibrium mixture of [Fe(CN)(4)NO](2-) and [Fe(CN)(5)NO](3-) ions. The first predominates at pH <8 and is formed by the rapid release of trans-cyanide from [Fe(CN)(5)NO](3-), which, in turn, is the main component at pH >9-10. Both nitrosyl complexes decay by first-order processes with rate constants around 10(-5) s(-1) (pH 6-10) related to the dissociation of NO. The decomposition is enhanced at pH 4 by 2 orders of magnitude with protons (and also metal ions) favoring the release of cyanides from the [Fe(CN)(4)NO](2-) ions and the ensuing rapid delivery of NO. At pH 7, an EPR-silent intermediate I(1) is detected (nu(NO), 1695 and 1740 cm(-1)) and assigned to the trans-[Fe(II)(CN)(4)(NO)(2)](2-) ion, an {Fe(NO)(2)}(8) species. At pH 6-8, I(1) induces a disproportionation process with formation of N(2)O and the regeneration of nitroprusside in a 1:2 molar ratio. At lower pHs, I(1) leads, competitively, to a second paramagnetic (S = 1/2) dinitrosyl intermediate I(2), [Fe(CN)(2)(NO)(2)](1-), a new member of a series of four-coordinate {Fe(L)(2)(NO)(2)} complexes (L = thiolates, imidazole, etc.), described as {Fe(NO)(2)}(9). Other decomposition products are hexacyanoferrate(II) or free cyanide, depending on the pH, and precipitates of the Prussian-Blue type. This study throws light on the conditions favoring rapid release of NO, to promote vasodilatory effects upon NP injection, and describes new processes related to dinitrosyl formation and NO disproportionation, which are also relevant to the diverse biological processes associated with NO and N(2)O processing.     

均匀Fe~3O~4胶体粒子形成机理II. 低温Mossbauer谱研究

在液氮温度下, 测定了均匀Fe~3O~4胶体粒子制备过程中陈化时间为1小时内的不同时间所得样品的Mossbauer谱。结果表明, γ-FeOOH为Fe~3O~4均匀胶粒形成过程的中间产物, 并提出均匀Fe~3O~4胶粒的形成机理。     

The mechanism of N2O formation via the (NO)2 dimer: a density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.     

Reactions of nitrogen oxides with the five-coordinate Fe(III)(porphyrin) nitrito intermediate Fe(Por)(ONO) in sublimed solids

Detailed experimental studies are described for reactions of several nitrogen oxides with iron porphyrin models for heme/NxOy systems. It is shown by FTIR and optical spectroscopy and by isotope labeling experiments that reaction of small increments of NO2 with sublimed thin layers of the iron(II) complex Fe(Por) (Por = meso-tetraphenylporphyrinato dianion, TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP) leads to formation of the 5-coordinate nitrito complexes Fe(Por)(eta1-ONO) (1), which are fairly stable but very slowly decompose under vacuum giving mostly the corresponding nitrosyl complexes Fe(Por)(NO). Further reaction of 1 with new NO2 increments leads to formation of the nitrato complex Fe(Por)(eta2-O2NO) (2). The interaction of NO with 1 at low temperature involves ligand addition to give the nitrito-nitrosyl complexes Fe(Por)(eta1-ONO)(NO) (3); however, these isomerize to the nitro-nitrosyl analogs Fe(Por)(eta1-NO2)(NO) (4) upon warming. Experiments with labeled nitrogen oxides argue for an intramolecular isomerization ("flipping") mechanism rather than one involving dissociation and rebinding of NO2. The Fe(III) centers in the 6-coordinate species 3 and 4 are low spin in contrast to 1, which appears to be high-spin, although DFT computations of the porphinato models Fe(P)(nitrite) suggest that the doublet nitro species and the quartet and sextet nitrito complexes are all relatively close in energy. The nitro-nitrosyl complex 4 is stable under an NO atmosphere but decomposes under intense pumping to give a mixture of the ferrous nitrosyl complex Fe(Por)(NO) and the ferric nitrito complex Fe(Por)(eta1-ONO) indicating the competitive dissociation of NO and NO2. Hence, loss of NO from 4 is accompanied with nitro --> nitrito isomerization consistent with 1 being the more stable of the 5-coordinate NO2 complexes of iron porphyrins.