Electronic structures of I2− and I4− ions in γ-irradiated rigid solutions

Radical-anions of iodine, bromine, and monoiodochloride are produced in γ-irradiated amorphous solids at 77 K, and their electronic and ESR spectra measured. On limited warming of the irradiated solution dimerization by the reaction I₂^? + I₂ → I₄^? occurs to produce the same species as reported by Fornier de Violet et al. The electronic structure of the dimeric anion is discussed in comparison with the monomeric anion.     

Endor in the 3.601 GHz zero-field transition of phosphorescent quinoline C9ND7: determination of the 14N quadrupolar splitting

The ENDOR spectrum of the high-frequency ESR transition of phosphorescent quinoline in zero field is investigated. The findings are in accord with a recent analysis of Buckley et al. From the observed ENDOR frequencies the quadrupolar splitting of the ¹⁴N nucleus in the phosphorescent state is determined: ν_? = 3.04, ν₊ = 4.57 MHz. In crystals with a concentration of 0.04% quinoline in durene strong signals are observed due to ENDOR transitions involving two nitrogen atoms on pairs of quinoline molecules.     

Synthesis of Diaryl Ethers: A Long-Standing Problem Has Been Solved

A breakthrough in the synthesis of diaryl ethers has been achieved as shown in Equation (1). The coupling of phenols with aryl boronic acids in the presence of copper(II ) acetate and a base proceeds under very mild conditions (room temperature) as described simultaneously by Evans et al. and Chan et al. Examples: R¹=2-Cl, 2-I, 2-OMe, 4-tBu, 4-CH₂CH(NHCOOtBu)CO₂Me, 3,5-tBu₂; R²=2-Me, 3-OMe, 3-NO₂, 4-H, 4-F, 4-OMe.     

Comment on “Realization of Lewis Basic Sodium Anion in the NaBH3− Cluster”

We challenge the interpretation of the chemical bond in NaBH₃^? proposed by Liu et al. We argue that NaBH₃^? has an electron‐sharing Na?BH₃^? covalent bond rather than a dative bond Na^?→BH₃.     

A comprehensive kinetic mechanism for CO,CH2O,and CH3OH combustion

New experimental profiles of stable species concentrations are reported for formaldehyde oxidation in a variable pressure flow reactor at initial temperatures of 850–950 K and at constant pressures ranging from 1.5 to 6.0 atm. These data, along with other data published in the literature and a previous comprehensive chemical kinetic model for methanol oxidation, are used to hierarchically develop an updated mechanism for CO/H₂O/H₂/O₂, CH₂O, and CH₃OH oxidation. Important modifications include recent revisions for the hydrogen–oxygen submechanism (Li et al., Int J Chem Kinet 2004, 36, 565), an updated submechanism for methanol reactions, and kinetic and thermochemical parameter modifications based upon recently published information. New rate constant correlations are recommended for CO + OH = CO₂ + H ( R23 ) and HCO + M = H + CO + M ( R24 ), motivated by a new identification of the temperatures over which these rate constants most affect laminar flame speed predictions (Zhao et al., Int J Chem Kinet 2005, 37, 282). The new weighted least‐squares fit of literature experimental data for ( R23 ) yields k₂₃ = 2.23 × 10⁵T^1.89exp(583/T) cm³/mol/s and reflects significantly lower rate constant values at low and intermediate temperatures in comparison to another recently recommended correlation and theoretical predictions. The weighted least‐squares fit of literature results for ( R24 ) yields k₂₄ = 4.75 × 10¹¹T^0.66exp(?7485/T) cm³/mol/s, which predicts values within uncertainties of both prior and new (Friedrichs et al., Phys Chem Chem Phys 2002, 4, 5778; DeSain et al., Chem Phys Lett 2001, 347, 79) measurements. Use of either of the data correlations reported in Friedrichs et al. (2002) and DeSain et al. (2001) for this reaction significantly degrades laminar flame speed predictions for oxygenated fuels as well as for other hydrocarbons. The present C₁/O₂ mechanism compares favorably against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay, and flow reactor species time history data at each level of hierarchical development. Very good agreement of the model predictions with all of the experimental measurements is demonstrated. © 2007 Wiley Periodicals, Inc. 39: 109–136, 2007     

Stability and bonding of carbon(0)-iron−N2 complexes relevant to nitrogenase co-factor: EDA-NOCV analyses

The factors/structural features which are responsible for the binding, activation and reduction of N₂ to NH₃ by FeMoco of nitrogenase have not been completely understood well. Several relevant model complexes by Holland et al. and Peters et al. have been synthesized, characterized and studied by theoretical calculations. For a matter of fact, those complexes are much different than real active N₂-binding Fe-sites of FeMoco, which possesses a central C(4-) ion having an eight valence electrons as an μ₆-bridge. Here, a series of [(S₃C(0))Fe(II/I/0)-N₂]^n- complexes in different charged/spin states containing a coordinated σ- and π-donor C(0)-atom which possesses eight outer shell electrons [carbone, (Ph₃P)₂C(0); Ph₃P→C(0)←PPh₃] and three S-donor sites (i.e. ^-S-Ar), have been studied by DFT, QTAIM, and EDA-NOCV calculations. The effect of the weak field ligand on Fe-centres and the subsequent N₂-binding has been studied by EDA-NOCV analysis. The role of the oxidation state of Fe and N₂-binding in different charged and spin states of the complex have been investigated by EDA-NOCV analyses. The intrinsic interaction energies of the Fe−N₂ bond are in the range from −42/−35 to −67 kcal/mol in their corresponding ground states. The S₃C(0) donor set is argued here to be closer to the actual coordination environment of one of the six Fe-centres of nitrogenase. In comparison, the captivating model complexes reported by Holland et al. and Peter et al. possess a stronger π-acceptor C-ring (S₂C_ring donor, π-C donor) and stronger donor set like CP₃ (σ-C donor) ligands, respectively.     

Comment on “Realization of Lewis Basic Sodium Anion in the NaBH3− Cluster”

We challenge the interpretation of the chemical bond in NaBH₃⁻ proposed by Liu et al. We argue that NaBH₃⁻ has an electron-sharing Na−BH₃⁻ covalent bond rather than a dative bond Na⁻→BH₃.     

Monte Carlo simulation of RRKM unimolecular decomposition in molecular beam experiments. V. product ox angular and energy distributions from O(3P)+X2 (X = Br,I)

Statistical simulation was applied to the unimolecular decomposition of the collision complexes formed in the crossed beam experiments on O(³P) + Br₂ by Fernie et al. and O(³P) + I₂ by Durkin et al. The simulation procedure used the fundamentals of RRKM theory and included exact angular momentum conservation. The impact parameter distributions were varied to obtain the best fits. Good agreement with experimental laboratory angular distributions measured with O atoms seeded in both He and Ne was found for impact parameter distributions which were peaked at quite small values, in most cases between 2 and 3 Å. Product OX molecules were found to be rotationally excited and inverted with a mean rotational energy close to twice the value expected without angular momentum restrictions. The differences found between the calculated and the experimental angular distributions do not support any assumptions about osculating or short-lived complexes. The normal exoergicity ΔD₀ of 27 kJ/mol for the O + I₂ reaction agrees well with the experiments by Dunkin et al.     

Etude de la resonance paramagnetique electronique (RPE) de l'ion CO−3 distordu et associe a l'impurete lithium dans la calcite synthetique

Electronic spin resonance (ESR) study of distorted CO⁻₃ ions associated with lithium impurities in synthetic calcite. A paramagnetic centre was created in a single crystal of synthetic calcite by X ray irradiation at 77 K. It is characterized by an extreme thermal instability since at 100 K it disappears completely. The shapes of the ESR spectra, as well as the values of the components of the g̃ tensor and the superhyperfine interaction tensor, allowed to identify this defect as a distorted CO⁻₃ ion, associated with a Li⁺ ion located in an interstitial site in the plane of the Ca²⁺ ions.     

The Effects of M2O3 on Stabilizing Monocopper over the Surface of Cu-ZnO-M2O3 Catalysts for Mathanol Synthesis

The effects of M₃O₃ (M = Al, Sc etc.) in Cu-ZnO-M₂O₃ catalysts on methanol synthesis at low pressure were studied with ESR, XPS and TPR spectroscopy. The results of ESR showed that the generation of monovalent cationic defects was because the valence state and electronic charge on the ZnO lattice lost their balance as M³⁺ doped into ZnO. The induced effect by Sc³⁺ is stronger than that by Al³⁺. The results of XPS and TPR indicated that the amount and stabilization of Cu⁺ on the surface of reduced copper-based catalyst and its catalytic activity were affected by the monovalent cationic defects on the surface of ZnO.     

A possible relativistic contribution to the singlet-triplet separation in methylene

The work of Desclaux et al. indicates that the Fock-Dirac relativistic correction to the s²p³-sp³ energy separation in carbon is of the same order of magnitude as the difference between experimental and theoretical estimates of the singlet-triplet separation in CH₂.     

Synthesis of Dithiohemiacetals

An interesting flavour component isolated from the headspace volatiles of beef broth by Brinksman et al.¹ was tentatively identified as 1-methyl-thioethanethiol (1; R¹[dbnd]R¹¹[dbnd]CH₃).     

Ionisation of small molecules by state-selected Ne* (3P0, 3P2) metastable atoms in the 0.06 < E < 6 eV energy range

The velocity dependence and absolute values of the total ionisation cross section for the molecules H₂, N₂, O₂, NO, CO, N₂O, CO₂, and CH₄ by metastable Ne* (³P₀) and Ne* (³P₂) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*H₂, N₂, CO, and CH₄, and the systems Ne*O₂, NO, CO₂, and N₂O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (π or σ^b) of the orbital of the electron that is removed from the target molecule. For the molecules H₂, N₂, CO, and CH₄ an electron from a σ^b orbital is removed from the molecule, whereas for O₂, NO, N₂O, and CO₂ an outer π-ortibal electron is involved. For the systems Ne* (³P₀, ³P₂)H₂ we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state NaH₂ potential of Botschwina et al. The resonance width Γ(r) as a function of the internuclear distance r shows a saturation at small r (r < 2.8 Å) for both the Ne*(³P₀)H₂ and the Ne*(³P₂)H₂ interaction. This supports previous conclusions of Verheijen et al. and Kroon et al. Reliable values for the absolute value of the total ionisation cross section have been obtained by performing a careful calibration of the density—length product of the supersonic secondary beam. The results are in good agreement with the values of West et al. for experiments without state selection. The total ionisation cross sections for molecules with π-type ionisation orbitals, with their larger spatial extent, in general are larger than those for molecules with σ^b-type ionisation orbitals.     

Crossed molecular beam chemiluminescence study of the metastable mercury reaction: Hg(3P)+Br2 → HgBr(B) + Br

Chemiluminescence from HgBr(B) formed in the reaction of Hg(6³P_o with Br₂ has been observed using a N₂-seeded nozzle beam of metastable Hg³P_o) atoms. The cross section has been estimated to be 3⁺³_?2 at a collission energy of 0.33 eV. This value is smaller by more than an order of magnitude than the corresponding value for Hg(³P₂) atoms measured by Krause et al., in accordance with their inference based on less direct evidence.     

Theoretical study of the electronic transition moment for the d 3IIga 3IIu band system of the C2 molecule

Large scale ab initio SCF and CI calculations are employed to study the potential curves for the d ³II_g, a ³II_u and X¹Σ⁺_g states of the C₂ radical. The electronic transition moment R_e′e″ for the Swan bands (d ³II_ga ³II_u) is calculated in various AO and MO basis sets as a function of the internuclear CC distance. The form of the Σ|R_e′e″|² curve is in very good agreement with that obtained recently from measurements of Danylewych and Nicholls and Tatarczyk et al.; the calculated value for Σ|R_e′e″² at 2.44 bohr is found to be 5.2 au² compared to the most recent experimental values of |R_e(r_oo)|² = 3.57 au² of Tatarczyk et al.     

Supported titanium‐magnesium catalysts for ethylene polymerization: A comparative study of catalysts containing isolated and clustered titanium ions in different oxidation states

Supported titanium–magnesium catalysts (TMC) comprising isolated and clustered titanium ions in different oxidation states, which are obtained using titanium compounds of different composition (TiCl₄, TiCl₃?nDBE (DBE – dibutyl ether), [η⁶–BenzeneTiAl₂Cl₈]), were synthesized and tested in ethylene polymerization. The state of titanium ions was studied by the ESR method both for the procatalysts and after their interaction with triisobutilaluminum. For identification of ESR‐silent Ti³⁺ ions and Ti²⁺ ions, special procedures of additional catalyst treatment with pyridine, water, and chloropentafluorobenzene were used to obtain Ti³⁺ ions that are observable in ESR spectra. In distinction to numerous earlier works performed with the TiCl₄/MgCl₂ catalyst comprising after the interaction with AlR₃ the Ti³⁺ surface compounds both as isolated ions and clusters (ESR‐silent), this work considers the [η⁶–BenzeneTiAl₂Cl₈]/MgCl₂ catalyst (TMC‐3) comprising mainly the isolated Ti²⁺ ions and a new catalyst TMC‐4 obtained by treating the TMC‐3 with chloropentafluorobenzene. This catalyst comprises only the isolated Ti³⁺ ions both before and after the interaction with triisobutylaluminum. It was shown that in spite of sharp distinctions between the catalysts under consideration concerning titanium oxidation state and the ratio of isolated Ti³⁺ ions to clustered ones, all these catalysts produce polyethylenes with similar molecular weights and molecular‐weight distributions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6362–6372, 2009     

Absolute partial and total electron impact ionization cross sections for C3H8 from threshold up to 950 eV

Electron impact ionization of propane (C₃H₈) was studied using the ion beam deflection technique and a double focussing mass spectrometer in conjunction with a recently developed correction procedure that accounts for discrimination due to the initial kinetic energy of fragment ions. The relative corrected partial ionization cross sections for the production of C₃H ₈ ⁺ , C₃H ₇ ⁺ , C₃H ₆ ⁺ , C₃H ₅ ⁺ , C₃H ₄ ⁺ , C₃H ₃ ⁺ , C₃H ₂ ⁺ , C₃H⁺, C ₃ ⁺ ; C₂H ₅ ⁺ , C₂H ₄ ⁺ , C₂H ₃ ⁺ , C₂H ₂ ⁺ , C₂H⁺, C ₂ ⁺ ; CH ₃ ⁺ , CH ₂ ⁺ , CH⁺, C⁺; C₃H ₅ ²⁺ , C₃H ₄ ²⁺ , C₃H ₃ ²⁺ and C₃H ₂ ²⁺ were determined from threshold up to 950 eV. Absolute partial ionization cross sections were obtained by charge weighted summing of all the observed partial ionization cross sections and by normalizing to a recent accurate determination of the total ionization cross section at 100 eV by Djuric et al. 1991. The absolute total ionization cross section curve obtained is in excellent agreement in shape and magnitude with the low energy data (<200 ev)=" of=" djuric=" et=" al.=" 1991=" and=" the=" high=" energy=" data=" points=" of=" schram=" et=" al.=">     

Structure and reactivity of a(n) and a(n) peptide fragments investigated using isotope labeling, tandem mass spectrometry, and density functional theory calculations

Extensive ¹⁵N labeling and multiple-stage tandem mass spectrometry were used to investigate the fragmentation pathways of the model peptide FGGFL during low-energy collision-induced-dissociation (CID) in an ion-trap mass spectrometer. Of particular interest was formation of a ₄ from b ₄ and a*₄ (a ₄-NH₃) from a ₄ ions correspondingly, and apparent rearrangement and scrambling of peptide sequence during CID. It is suggested that the original FGGF_oxa b ₄ structure undergoes b-type scrambling to form GGFF_oxa. These two isomers fragment further by elimination of CO and ¹⁴NH₃ or ¹⁵NH₃ to form the corresponding a ₄and a*₄ isomers, respectively. For (¹⁵N-F)GGFL and FGG(¹⁵N-F)L the a*₄ ion population appears as two distinct peaks separated by 1 mass unit. These two peaks could be separated and fragmented individually in subsequent CID stages to provide a useful tool for exploration of potential mechanisms along the a ₄ → a*₄ pathway reported previously in the literature (Vachet et al. J. Am. Chem. Soc. 1997, 119, 5481, and Cooper et al. J. Am. Soc. Mass Spectrom. 2006, 17, 1654). These mechanisms result in formally the same a*₄ structures but differ in the position of the expelled nitrogen atom. Detailed analysis of the observed fragmentation patterns for the separated light and heavy a*₄ ion fractions of (¹⁵N-F)GGFL indicates that the mechanism proposed by Cooper et al. is consistent with the experimental findings, while the mechanism proposed by Vachet et al. cannot account for the labeling data. In addition, a new rearrangement pathway is presented for a ₄*-CO ions that effectively transfers the former C-terminal amino acid residue to the N-terminus.     

Wechselwirkungen von Hetero-π-Systemen mit Zentralmetallatomen in niedrigen Oxydationsstufen. ESR-Untersuchungen zur Struktur und Bindung an Niedrig-spin-d5-(S = 1/2)-Tris(benzil-bis-N-phenylimin)vanadium(0) und Bis(benzil-bis-N-phenylimin)cobalt(0)

Interaction of Hetero-π-Systems with Central Metal Atoms in Low Oxidation States. ESR Investigations on Structure and Bonding in Low Spin d⁵-(S = 1/2)-Tris(benzil-bis-N-phenylimine)vanadium(0) and Bis(benzil-bis-N-phenylimine)cobalt(0) ESR investigations on V(N??N)₃ and Co(N??N)₂(N??N: Benzil-bis-N-phenylimine) are reported. The ESR parameters — g, A^V — indicate trigonal coordination geometry (D₃) for V(N??N)₃ disturbed by a small lower-symmetric ligand-field component. Interpretation of these parameters in terms of a ligand-field model for a low-spin d⁵ electronic configuration shows the electronic ground state to be ²A₁ and yields a large value for the trigonal field splitting parameter K. The ESR spectrum of Co(N??N)₂ could be obtained at 4.2 K only. Its rhombic symmetry and the absence of hyperfine interactions with the nuclei ⁵⁹Co and ¹⁴N is consistent with a rhombically distorted, compressed tetrahedral symmetry (D₂) for Co(N??N)₂. The spin-Hamiltonian parameters are discussed in terms of the metal-ligand bond character and the electronic spectrum.     

Multiple Reaction Pathways in Rhodium‐Catalyzed Hydrosilylations of Ketones

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium‐catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L_nRh^I]⁺ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh–silyl intermediates [L_nRh^III(H)SiHMe₂]⁺. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh? Si bond occurs, as previously proposed by Ojima et al., or into the Si? H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si? H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This “silylene mechanism” was found to have the lowest activation barrier for the rate‐determining step, the migration of a rhodium‐bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R₂SiH₂ compared to R₃SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C? D versus M? D.