Hyperfine and magnetic properties of 19e− (electron reservoir) sandwiches of Fe(I), C5R5FeC6R6 (where R=H or CH3) and (C6(CH3)6)2Fe+: Mössbauer experiments and molecular orbital calculations

The four compounds C₅H₅FeC₆H₆, C₅H₅Fe(CH₃₎₆, C₅(CH₃₎₅FeC₆(CH₃₎₆ and (C₆(CH₃₎₆₎₂Fe⁺ were studied by Mössbauer spectroscopy on powders. On the basis of semi-empirical molecular orbital calculation (Iterative Extended Hückel with self-consistence of the charge) in which parameters derived from X-ray data are used, we have modelized the thermal behaviour of the electric field gradient (EFG) tensor from 4 K to 300 K. The reduction in magnitude of electronic observables (e.g. spin orbit coupling constant and EFG magnitude) gave evidence for a dynamic Jahn-Teller effect. Spectra in a high magnetic field (6 teslas) confirmed the paramagnetic behaviour of the compounds. The sign of the EFG tensor, the magnetic hyperfine field tensor and its orientation with respect to the EFG tensor were determined.     

Benzene and thiophene formation from acetylene on sulfided Pd(111) studied by LITD/FTMS

We report the first laser-induced thermal desorption (LITD) studies of acetylene on Pd(111). LITD coupled with Fourier transform mass spectrometry (FTMS) probes the surface molecular composition. Our results show simultaneous formation of thiophene and benzene at 120 K after a 6 L dose of acetylene at 80 K on a Pd(111) surface with 0.06 ML of sulfur. Simultaneous formation implies that formation of the C₄H₄ intermediate is the slow step in the formation of both cyclic products. The relative amounts of thiophene and benzene observed with LITD/FTMS are comparable, while thermal desorption spectroscopy (TDS) yields integrated thiophene signals that are < 2% as intense as for benzene. This indicates that thiophene primarily decomposes upon heating. Low coverage (0.5 L) results confirm reports that the presence of sulfur enhances benzene production.     

Laser induced breakdown spectroscopy surface analysis correlated with the process of nanoparticle production by laser ablation in liquids

Linkage isomerism is the coexistence of iso-compositional molecules or solids differing by connectivity of the metal to a ligand. In a crystalline solid state, the rotation is possible for asymmetric ligands, e.g., for cyanide ligand. Here we report on our observation of a phase transition in anhydrous RbMn[Fe(CN)₆] (nearly stoichiometric) and on the effect of linkage isomerism ensuing our interpretation of the results of Mössbauer study in which we observe the iron spin state crossover among two phases involved into this transition. The anhydrous RbMn[Fe(CN)₆] can be prepared via prolonged thermal treatment (1 week at at 80 °C) of the as-synthesized hydrated RbMn[Fe(CN)₆]·H₂O. The latter compound famous for its charge-transfer phase transition is a precursor in our case. As the temperature is raising above 80 °C (remaining below 100 °C) we observe RbMn[Fe(CN)₆] that inherited its F-43 m symmetry from RbMn[Fe(CN)₆]·H₂O transforming to a phase of the Fm-3 m symmetry. In the latter, more than half of Fe^3?+? ions are in high-spin state. We suggest a plausible way to explain the spin-crossover that is to allow the linkage isomerism by rotation of the cyanide ligands.     

Vibration spectroscopy of benzene adsorbed on Pt(111) and Ni(111)

High resolution electron energy loss spectroscopy has been applied to study the adsorption of benzene (C₆H₆ and C₆D₆) on Pt(111) and Ni(111) single crystal surfaces between 140 and 320 K. The vibrational spectra provide evidence that benzene is chemisorbed with its ring parallel to the surface, predominantly π bonded to the platinum and nickel surface respectively. A significant frequency increase of the CH-out-of-plane bending mode, largest in the case of platinum, is observed compared to the free molecule. On both metals two phases of benzene exist simultaneously, characterized by a different frequency shift. The shifts are explained by electronic interaction between the metal d-orbitals and molecules adsorbed in on top and threefold hollow sites respectively. The vibrational spectra of the multilayer condensed phase of benzene exhibit the infrared active modes of the gasphase molecule as expected.     

Mössbauer and molecular orbital (MO) study of localized and delocalized mixed-valence organo-FeIIFeI bisandwiches

A Mössbauer study of two mixed-valence Fe^IIFe^I compounds C₅H₅Fe(C₅H₄)₂ FeC₆(CH₃)₆ (1) and ¦C₆(CH₃)₆ Fe(C₅H₄)₂ Fe C₆(CH₃)₆¦⁺ (2 ⁺) was carried out from 4.2K to room temperature. Zero-field spectra show two types of iron atoms for1 and one type for2 ⁺. Hence1 is a localized mixed-valence complex and2 ⁺ a delocalized mixed-valence complexe. High magnetic field spectra for 2⁺ give a negative sign for the EFG and show the valence electron is delocalized on the two centers. IEHT-MO calculations confirm the results and allow to explain the temperature independence of the quadrupole splitting (QS) of2 ⁺.     

Experimental and kinetic modeling study of sooting atmospheric-pressure cyclohexane flame

Experimental data and modelling results of the main products and intermediates from a fuel-rich sooting premixed cyclohexane flame were presented in this work. Model predictions well agree with experimental data both in sooting and non-sooting flames. Major and minor species are properly predicted, together with the soot yield. The initial benzene peak was demonstrated to be due to the fast dehydrogenation reactions of the cycloalkane, which gives rise to cyclohexene and cyclohexadiene both via molecular and radical pathways. Once formed cyclohexadiene quickly forms benzene whereas in the postflame zone, benzene comes from the recombination and addition reactions of small radicals, with C₃H₃ + C₃H₃ playing the most important role in these conditions. An earlier soot inception was detected in the cyclohexane flame with respect to a n-hexane flame and this feature is not reproduced by the model that foresees soot formation significant only in the second part of the flame. The model insensitivity of soot to the reactant hydrocarbon was also observed comparing the predictions of three flames of cyclohexane, 1-hexene and n-hexane with the same temperature profile. A sensitivity analysis revealed that soot primarily comes from the HACA mechanism for the three flames, acetylene being the key species in the nucleation. Experimental data on soot inception seem to indicate the importance of the early formation of benzene, that depends on the fuel structure. It is thus important to further investigate the role of benzene and aromatics in order to explain this discrepancy.     

Theoretical study of efficiency of metal cations (Mg+, Ca+, and Ag+) for effective hydrogen storage

The interaction between the metal cations and H₂ molecule has been investigated using dispersion-corrected and -uncorrected hybrid density functional and CCSD(T) methods in conjunction with the correlation consistent triple-ζ quality basis sets for the storage of the H₂ molecule. The molecular properties, potential energy surfaces, stability, binding energy and well-depth have been computed for the metal cation–dihydrogen (M⁺–H₂, M = Mg, Ca, and Ag) complexes in the gas phase. The results obtained by the dispersion-corrected hybrid density functional B2PLYP-D method agree very well with the earlier experimental and theoretical results wherever available. Different components of the interaction energy have been estimated by the symmetry adapted perturbation theory (SAPT) to get physical insight into the interaction energy. Among the three complexes, only Ag⁺–H₂ is the most stable complex and it accumulates more H₂ molecules as the interaction between the metal cation Ag⁺ and the H₂ molecule is the greatest.     

Development of the collinear laser beam line at TRIUMF

Several commercially available medicaments containing ferrous fumarate (FeC₄H₂O₄) and ferrous sulfate (FeSO₄), as a source of ferrous iron, were studied using a high velocity resolution Mössbauer spectroscopy. A comparison of the ⁵⁷Fe hyperfine parameters revealed small variations for the main components in both medicaments indicating some differences in the ferrous fumarates and ferrous sulfates. It was also found that all spectra contained additional minor components probably related to ferrous and ferric impurities or to partially modified main components.     

[2.2.2]Paracyclophane as a receptor for the cesium cation in the gas phase

By using electrospray ionisation mass spectrometry, it was proven experimentally that the cesium cation (Cs⁺) forms with [2.2.2]paracyclophane (C₂₄H₂₄) the cationic complex [Cs(C₂₄H₂₄)]⁺. Further, applying quantum chemical calculations, the most probable structure of the [Cs(C₂₄H₂₄)]⁺ complex was derived. In the resulting complex with a symmetry very close to C₃, the ‘central’ cation Cs⁺, fully located in the cavity of the parent [2.2.2]paracyclophane ligand, is bound to all three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, E(int), of the considered cation–π complex [Cs(C₂₄H₂₄)]⁺ was found to be ?73.2 kJ/mol, confirming the formation of this fascinating complex species as well. This means that [2.2.2]paracyclophane can be considered as a receptor for the Cs⁺ cation in the gas phase.     

H2O2氧化苯一步羟基化制备苯酚连续流动反应体系

采用连续流动反应器研究了苯一步羟基化制备苯酚的反应. 利用载铁活性炭为催化剂,H₂O₂为氧化剂,乙腈为溶剂,得到苯酚收率为28.1%,选择为98%,基于铁计算的转化数为3 h^-1. 对催化剂进行N₂吸附、XRD、XPS、FTIR等表征发现,负载的Fe可能与活性炭表面羧基发生作用,形成了化学环境类似于乙酸铁的铁物种,该结构是催化反应的活性中心. 计算得出,反应的阿伦尼乌斯活化能为13.4 kJ/mol,苯和H₂O₂的反应级数分别为0.2和0.7,总反应级数约为1.     

Fascinating interaction of the ammonium cation with [2.2.2]paracyclophane: experimental and theoretical study

By means of electrospray ionisation mass spectrometry, it was evidenced experimentally that the ammonium cation (NH₄⁺) reacts with the electroneutral [2.2.2]paracyclophane ligand (C₂₄H₂₄) to form the cationic complex [NH₄(C₂₄H₂₄)]⁺. Moreover, applying quantum chemical calculations, the most probable conformation of the proven [NH₄(C₂₄H₂₄)]⁺ complex was solved. In the complex [NH₄(C₂₄H₂₄)]⁺ having a symmetry very close to C₃, the ‘central’ cation NH₄⁺ is coordinated by three strong bifurcated intramolecular hydrogen bonds to the corresponding six carbon atoms from the three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, E(int), of the considered complex [NH₄(C₂₄H₂₄)]⁺ was evaluated as ?625.8 kJ/mol, confirming the formation of this fascinating complex species as well. It means that the [2.2.2]paracyclophane ligand can be considered as an effective receptor for the ammonium cation in the gas phase.     

Nanochemistry – Chemical Reactions of Iron and Benzene Within Molecular Clusters

Molecular clusters represent a nanoscale test tube where chemical reactions can be examined in a unique way for the effects of the local environment and the possibility of size-dependent reactions. Previous experiments have shown that the ionization/dissociation of iron pentacarbonyl clusters can lead to the formation of iron ions and iron cluster ions and that these species can further react with dopant molecules to yield chemically rearranged products. The present experiments characterize similar reactions with benzene molecules and clusters. Heteroclusters of the form [Fe(CO)₅]_m(C₆H₆)_nAr_p are created in an expanding supersonic jet of the component molecules. Following ionization by a 30 ps, 266 nm laser pulse, extensive dissociation, aggregation, and chemical rearrangement occurs leading to ionic products which are characterized by mass spectrometry. Cluster ions of the type Fe_m(C₆H₆)_n ⁺ are observed as products. The stability of the sandwich form of the ion, Fe(benzene)₂ ⁺, is inferred from the data. Evidence for a similar special stability for the double-decker, Fe₂(benzene)₃ ⁺, is presented.     

Low temperature formation of benzene from acetylene on a Pd(111) surface

Experiments with UPS, metastable noble gas deexcitation spectroscopy (MDS) and thermal desorption demonstrated that C₂H₂ adsorbed on Pd(111) at 140 K undergoes cyclotrimerisation to C₆H₆ after higher (? 100 L) exposures. If the surface is intermediately warmed up to 300 K, the low temperature state of adsorbed acetylene transforms irreversibly into another species which is unreactive. The surface species formed by reaction was identified by comparison with the electron spectroscopic data of C₆H₆ adsorbed from the gas phase as well as with those of free C₆H₆. The molecules are only weaky held on the surface and start to desorb already at about 150 K.     

LEED,AES and thermal desorption studies of chemisorbed Hydrogen and hydrocarbons (C2H2, C2H4, C6H6, C6H12) on the (111) and stepped [6(111) × (100)] iridium crystal surfaces; comparison with platinum

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C₂H₄, C₂H₂, or C₆H₆ exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.     

Novel semiconducting iron–quinizarin metal–organic framework for application in supercapacitors*

We present conductivity data for a newly synthesised metal–organic framework FeQ, Fe(C₁₄H₆O₄).H₂O demonstrating significant electronic transport. The electrical conductivity of the material is expected to be through the π–π interaction of the ligand-quinizarin and is measured to be 1.73?×?10^?2?Scm^?1_. Its potential role as supercapacitor electrodes is discussed.     

Effects of subsurface Na,H and C on the bonding of carbon to nickel surfaces

This paper reports the results of a theoretical study of Na, H and C subsurface atomic species in nickel and demonstrates how these interstitial atoms influence the reactivity of the Ni(111) surface and the structure of carbon species adsorbed on the surface. The benzene molecule, C₆H₆, in planar and nonplanar geometries, is used to probe bonding at the surface. Adsorption energies are calculated by ab initio configuration interaction techniques modelling the surface as an embedded cluster. Adsorption energies of planar C₆H₆ at the most stable, three-fold, adsorption site are 18 kcal/mol for the Ni(111) surface, and 10, 19 and 44 kcal/mol in the presence of the Na, H and C interstitials, respectively. The energies required for the planar to puckered distortion are 99 kcal/mol on Ni(111), 69 kcal/mol with the Na interstitial, 83 kcal/mol with H, and 134 kcal/mol with C compared to 198 kcal/mol for distortion of C₆H₆ in the gas phase. The possible relevance of these results to the nucleation of diamond on nickel are discussed. The results indicate that subsurface Na stabilizes tetrahedrally bonded carbon subunits of the diamond structure while subsurface C may make it easier for the overlayer to revert to a planar graphite structure.     

Adsorption of hydrocarbons and various gases on Ni-SiO2 catalysts studied by high field magnetic methods

The adsorption of various gases (H₂, H₂S, alcanes, alcenes and alcynes containing up to five carbons, benzene, cyclohexane, cyclopentane, and some mixtures) on Ni-SiO₂ catalysts has been studied by measuring variations of saturation magnetization. The adsorption has been performed at ?78 °C and the system heated in steps. When coverage is small intermediate adsorbed compounds thermally stable have been detected. The observed bond number is even (2, 4, 6, 8) and it corresponds to strongly dehydrogenated states for alcanes, alcenes, cyclanes and benzene, and to an associative model for acetylene and propyne. Complete cracking occurs at relatively low temperatures (50 °C to 200 °C) with formation of Ni₃C and NiH: the greater the saturation of the hydrocarbon, the smaller its carbon number, and the lower is its cracking temperature. Sulfur of H₂S is rapidly and irreversibly dissolved into the bulk of nickel. These results which are not always in good agreement with those obtained by low field techniques, strongly supports the following view: nickel atoms, on which a molecule is adsorbed cease to participate to the collective magnetism.     

Remote substituent effects on homolytic Fe‐N bond energies of p‐G‐C6H4NHFe(CO)2(η5‐C5H5) and p‐G‐C6H4(COMe)NFe(CO)2(η5‐C5H5) studied using Hartree–Fock and density functional theory methods

The nature and strength of metal–ligand bonds in organotransition–metal complexes is crucial to the understanding of organometallic reactions and catalysis. The Fe‐N homolytic bond dissociation energies [ΔH_homo(Fe‐N)′s] of two series of para‐substituted Fp anilines p‐G‐C₆H₄NHFp [1] and p‐G‐C₆H₄N(COMe)Fp [2] were studied using the Hartree–Fock (HF) and the density functional theory methods with large basis sets. In this study, Fp is (η⁵‐C₅H₅)Fe(CO)₂ and G are NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO and NMe₂. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and accurate predictions of ΔH_homo(Fe‐N)′s. B3LYP can also satisfactorily predict the α and remote substituent effects on ΔH_homo(Fe‐N)′s [ΔΔH_homo(Fe‐N)′s]. The good correlations [r = 0.96 (g, 1), 0.99(g, 2)] of ΔΔH_homo(Fe‐N)′s in series 1 and 2 with the substituent σ_p⁺ constants imply that the para‐substituent effects on ΔH_homo(Fe‐N)′s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. ΔΔH_homo(Fe‐N)′s(1,2) conform to the captodative principle. Insight from this work may help the design of more effective catalytic processes. Copyright © 2012 John Wiley & Sons, Ltd.