Photo- and radiation-produced H-atoms in acid low temperature glasses containing Fe2+ ions

Frozen solutions of H₂SO₄ and acid 7 M NaClO₄ containing Fe²⁺ ions have been exposed to X rays and UV light and examined by EPR methods. It is concluded that the H atoms formed in UV-irradiated acid perchlorate matrices do not have mobile electrons as precursors. Thus no trapped electrons are found after UV irradiation even though these matrices provide efficient traps for electrons. The effect of the electron scavenger NO^?₃ is identical in UV- and X-irradiated matrices and largest in the H₂SO₄ matrix. Thus it seems that NO^?₃ acts on a similar H-atom precursor in the two cases, and that this precursor may be an excited CTTS state, rather than a mobile electron. The absence of allyl alcohol scavening effect on H atoms produced by UV light in the perchlorate matrix indicates that this H-atom scavenger does not interact with the CTTS state, and that the H atom is trapped in the vicinity of the Fe²⁺ ion where it was formed. The presence of small amounts of Fe²⁺ ions (< 10^?3 M) in the matrices causes a marked decrease in the spin—lattice relaxation time of the trapped hydrogen atoms as well as a decrease in the intensities of the satellite lines relative to the main hydrogen lines.     

Zerovalent Rhodium and Iridium Silatranes Featuring Two‐Center,Three‐Electron Polar σ Bonds

Species with 2‐center, 3‐electron (2c/3e^?) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^? σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^? bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z² parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e^? σ “half‐bonds”.     

Zerovalent Rhodium and Iridium Silatranes Featuring Two‐Center,Three‐Electron Polar σ Bonds

Species with 2‐center, 3‐electron (2c/3e^?) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^? σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^? bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z² parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e^? σ “half‐bonds”.     

A Study of picosecond dehalogenation of chlorobenzene anions in liquids by positronium inhibition measurements

Radiation chemistry and results of Ps yields indicate that the following processes occur in the positron spur in solution of halogen-substituted hydrocarbons, RX_n: e⁺ + e^? → Ps, e^? + RX _n → (RX_n)^? → RX_n?1 + X^?, e⁺ + (RX_n)^? → Ps + RX_n, e⁺ + X^? → [X^?, e⁺]. Hence the trapped electron can form Ps only if (RX _n)^? is stable or has a lifetime that is longer than o comparable to the Ps formation time. Previous studies have shown that some of the strongly chlorinated benzenes (n = 4.5 give reasonable inhibition in benzene but not in linear hydrocarbons. The reason is very probably that the dechlorination time is much shorter in benzene than in saturated hydrocarbons because Cl^? is more strongly solvated in benzene than in non-aromatic hydrocarbons. To test those ideas further we have begun detailed studies of solutions of the possible “intermediate” inhibitors, viz. 1,2,3,5- and 1,2,4,5-C₆H₂Cl₄, in mixtures of C₆H₆/C₆H₁₄ different methyl-substituted benzene aniline, anisole, dioxane and ethylbenzene. The results are discussed and interpreted in terms of the spur model. The Ps inhibition efficiency of the two isomeric forms of tetrachlorobenzene studied, appears most probably to depend on intramolecular electron transfer with subsequent dehalogenation of the molecular anion on a picosecond timescale. The divergence in inhibitor efficiency obtained for the chlorobenzenes when dissolved in aromatic solvents compared to the same solutes when dissolved in a saturated alkane appears most probably to be caused by complex formation between the initially formed chlorobenzene anion and benzene molecules, which permits a rapid relaxation of the molecular anion with subsequent bond stretching and expulsion of the chloride anion.     

Temperature dependence of trapped electrons in crystalline D2O ice from 77 to 6 k

The temperature dependence of the abundance of trapped electrons which absorb in the visible (e^?_vis) and infrared (e^?_IR) in crystalline D₂O ice has been studied by pulse radiolysis between 77 and 6 k. The yield (G) and decay of e^?_vis show little dependence on temperature or doping with NH₄F. At 6 K G(e^?_vis) is 0.54 and the electron decays by half within 5 μs. These observations are consistent with e^?_vis being mainly located in spurs. The yield and decay of e^?_IR, on the other hand, show a more marked dependence on temperature. In the pure crystal G(e^?_IR) increases more than tenfold from ≈ 0.1 at 77 K to 1.3 at 6 K and its decay rate is greatly decreased at the lower temperature. Doping with NH₄F increases G(e^?_IR) to 0.85 to 77 K and to 1.8 at 6 K and some decay is observed at 6 K but not at 77 K. These results are interpreted on the basis that geminate recombination between electrons and holes is very fast at 77 K but becomes sufficiently slow for the electrons to be observed at 6 K. It is also inferred that the hole is more mobile than e^?_IR. The mechanisms causing the decays of e^?_vis and e^?_IR are discussed.     

On the relaxation of trapped electrons in glassy hydrocarbons irradiated at 4 K

In contrast to previous reports e^?_t in 3MHx produced at 4 K shows trap relaxation when warmed to 77 K. The relaxation in 3 MHx and 3MP occurs without loss of e^?_t, and results in an increase in ?_max as well as a shift of λ_max.     

Photobleaching of radiation induced trapped electrons in neutral and alkaline glasses. Hydrogen atom formation

Photomobilization of trapped electrons in 7 M NaClO₄- and 9 M NaOH-glass gives rise to trapped hydrogen atoms. This is probably due to the reaction e^?_m + H₂O → H + OH^?. Experiments with an electron scavenger indicate that electrons are not precursors to radiolytically produced hydrogen atoms. It seems that the mobile electrons produced during photobleaching are not slowed down to thermal energy before they react to produce hydrogen atoms, since the yield of the latter species is strongly dependent on the wavelength of the bleaching light.     

Theoretical modeling of electrochemical interactions in bimetallic molybdenum nitrosyl complexes incorporating saturated bridges

Hybrid B3LYP and non-hybrid OLYP DFT formalism has been applied to neutral and reduced forms of bimetallic hydrotris(3-methylpyrazol-1-yl)borato (Tp^3-Me) molybdenum nitrosyl complexes incorporating ethane-1,2-diolate bridges. Direct evidence for localization of an extra electron in mixed-valence compounds {16e:17e}⁻ is based on the analysis of electron density, energetic stabilization of asymmetric structures with an electron trapped on one Mo and the splitting of both calculated and experimental ν_NO stretching frequencies. Differences in the first and second electron affinities calculated in PCM solvent model have been successfully related to cyclic voltammetry measurements. Electronic interactions through saturated ethanediolato bridges are evidenced by the extent of spin density delocalization towards the second Mo center.     

Absorption spectrum of electrons trapped in single crystals of sucrose.: A pulse-radiolysis study in the temperature range 6–270 K

Pulse radiolysis of single crystals of sucrose at 6 K yields an absorption spectrum with λ_maxat 450–475 nm which is attributed to the trapped electron. The yields and decay of this species are reported for the temperature range 6–270 K. It is concluded that there is only one form of e_t^? in these crystals and that it is deeply trapped.     

Photogeneration of superoxide and decarboxylated peptide radicals by carboquone, mitomycin C and streptonigrin. An electron spin resonance and spin trapping study

Visible light (405–615 nm) excitation of carboquone, mitomycin C, and streptonigrin dissolved in dimethylsulfoxide in the presence of oxygen generates superoxide anion radicals (O₂^?). The quantum yields for these reactions range from 4.2 times 10^?2 (carboquone, λ= 615 ± 10 nm) to 7.3 times 10^?6 (streptonigrin, λ=545 ± 10 nm). O₂^? radicals were spin trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance (ESR). The efficiency of DMPO to spin trap O₂^? in dimethylsulfoxide was determined and indicated that 91% of the O₂^? present in dimethylsulfoxide is trapped by DMPO. The oxidation of the photoexcited drug molecules occurs via a direct electron transfer to dissolved oxygen in solution. Ultraviolet irradiation (λ= 313 ± 10 nm) of the aminoquinone drug solutions (80% H₂O, 20% dimethylsulfoxide) in the presence of peptides results in the decarboxylation of the peptides. In this case the photoexcited drugs are reduced, abstracting an electron from the C-terminal carboxyl group of the peptides. The reaction is specific to the C-terminal amino acid of the peptide. The decarboxylated peptide radicals were spin trapped with 2-methyl-2-nitrosopropane (MNP) and identified by ESR.     

Yields of trapped electrons in pulse irradiated aqueous LiCl Glasses at temperatures down to 6K

Pulse radiolysis of deuterated aqueous LiCl glasses at temperatures in the range 6 K to 70 K show that the yield G(e^?_IR) of infrared absorbing electrons (e^?_IR) increases sharply as the temperature is lowered when [LiCl] ? 10 M. Under these conditions the yield of visible absorbing electrons (e^?_vis) decreases, but to a lesser extent. When [LiCl] ? 8 M, G(e^?_IR) and G(e^?_vis) are both much less dependent on temperature. These data suggest that at very low temperatures e^?_IR are not trapped exclusively in a purely aqueous environment.     

A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Carbon-free D3d [E3ME3]2− (E=P, As; M=Ni, Pd, Pt): The smallest inorganic sandwich complexes with aromatic η3-P 3 − and η3-As 3 − ligands

A density functional theory and wave function theory investigation on the possibility of carbon-free phosphametallocenes [P₃MP₃]^2? and arsenametallocenes [As₃MAs₃]^2? (M=Ni, Pd, Pt) is presented in this work. Staggered singlet D_3d [E₃ME₃]^2? (E=P, As)-the smallest inorganic metallocenes possible to construct-proved to be the global minima of the heptaatomic systems and may be targeted in future experiments. Cyclo-P ₃ ^? and cyclo-As ₃ ^? turned out to possess similar aromaticity to cyclo-P ₅ ^? and cyclo-As ₅ ^? and may serve as effective ligands to sandwich a wide range of transition metals. The first vertical electron detachment energies of C_s [E₃ME₃]Li^? monoanions with a staggered [E₃ME₃]^2? sandwich core were predicted to be between 2.7 and 2.9 eV; the extent of stabilization by Li⁺ suggests that such materials be viable targets for experimental characterization.     

Thomson scattering on argon surfatron plasmas at intermediate pressures: Axial profiles of the electron temperature and electron density

The axial profiles of the electron density n_e and electron temperature T_e of argon surfatron plasmas in the pressure range of 6–20 mbar and microwave power between 32 and 82 W have been determined using Thomson Scattering of laser irradiation at 532 nm. For the electron density and temperature we found values in the ranges 5 × 10¹⁸ < n_e < 8 × 10¹⁹ m^− 3 and 1.1 < T_e < 2.0 eV. Due to several improvements of the setup we could reduce the errors of n_e and T_e down to 8% and 3%, respectively. It is found that n_e decreases in the direction of the wave propagation with a slope that is nearly constant. The slope depends on the pressure but not on the power. Just as predicted by theories we see that increasing the power leads to longer plasma columns. However, the plasmas are shorter than what is predicted by theories based on the assumption that for the plasma-wave interaction electron–atom collisions are of minor importance (the so-called collisionless regime). The plasma vanishes long before the critical value of the electron density is reached. In contrast to what is predicted by the positive column model it is found that T_e does not stay constant along the column, but monotonically increases with the distance from the microwave launcher. Increases of more than 50% over 30 cm were found.     

Phenomenology of EPR Trapped Electron Spectra Produced by Photoionization in Alkalines at 4.2°k and 77°k

Trapped electrons, e_c^?, are produced by photoionization of K₄Fc(CN)₆ in 10M NaOH/H₂O and 10M NaOD/D₂O glasses at 4.2°K. The linewidth, ΔH_ms, of e_c^? changes reversibly between these two temperatures in contrast to the observations for organic matrics. Three proposed mechanisms including dipole relaxation of pendent molecules surrounding the electron cavity, contraction-expansion model of cavity, and electron retrapping process are each discussed. It appears that no one physical model satisfactorily accounts for the present observations. The electron retrapping process, however, might be a better condidate for interpreting the experimental results in compatible with the optical absorption data.     

Infrared photodetachment measurements near thresholds of C−

The relative cross section has been measured in the threshold regions of the following transitions: C^?(⁴S) + hv → C³P_0,1,2) + e^? and C^?(²D) + hv → C(¹D) + e^?. The electron affinity of carbon is determined EA(C) = 1.2629 ± = 0.0003 eV, and the binding energy of the metastable C^?(²D) state with respect to the C(³P₀) ground state is 0.033 ± 0.001 eV.     

Optical Absorption Spectra for the Solvated Electron Observed in the Pulse Radiolysis of cis- and trans- Isomers of 3- and 4-Methylcyclohexanols

The absorption band maximum of solvated electrons, λ_max(e^?_s), in 3- or 4-methylcyclohexanols is observed at longer wavelengths (818–837 nm), if the OH group is axial, and at shorter wavelengths (721–723 nm), if it is equatorial. It is surmised that the size of cavity for the solvated electron is larger in the former case and smaller in the latter.     

Langmuir probe study of the charged particle characteristics in an analytical radio frequency-glow discharge. Roles of discharge conditions and sample conductivity

The application of a tuned Langmuir probe to the measurement of the charged particle characteristics of electron number density, ion number density, electron energy distribution function, average electron energy and electron temperature, in an analytical radio frequency (r.f.)-glow discharge is described. Studies focus on the roles of discharge operating conditions and plasma sampling position for conductive (copper) and nonconductive (Macor) samples. Based on the data obtained here, apparent differences in plasma characteristics between conductive and nonconductive samples can be reasonably explained. For example, the sputtering of conductive samples results in plasmas with obviously higher electron and ion number densities than the sputtering of nonconductive samples (e.g. n_i = 1.8 × 10¹⁰ cm⁻³ and n_e = 1.5 × 10⁹ cm⁻³ for copper, and n_i = 8 × 10⁹ cm⁻³ and n_e = 5 × 10⁸ cm⁻³ for Macor under the conditions of argon pressure = 4 Torr, r.f. power = 30 W and sampling distance = 4.5 mm). Conversely, nonconductive samples yield electrons with higher energies (average electron energies of 15 and 7.5 eV and temperatures of 6.5 and 3.5 eV respectively for the Macor and copper samples). Lower d.c. bias potentials for the case of sputtering nonconductive samples yield reduced sputtering rates and charged particle densities, though the electrons in the latter case have higher energies and thus improved excitation capabilities. The differences between r.f.- and d.c.-glow discharge optical emission spectra are also discussed relative to reported electron energy characteristics. Studies such as these will lay the ground-work for extensive evaluation of inter-matrix type standardization for r.f.-glow discharge atomic emission spectrometry.     

Shallow and deep trap states of solvated electrons in methanol and their formation,electronic excitation,and relaxation dynamics

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (e_met⁻) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of e_met⁻ are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (e_aq⁻). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Condensed-phase first-principles molecular dynamics simulations elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons.     

Ionic bonding of lanthanides,as influenced by d‐ and f‐atomic orbitals,by core–shells and by relativity

Lanthanide trihalide molecules LnX₃ (X = F, Cl, Br, I) were quantum chemically investigated, in particular detail for Ln = Lu (lutetium). We applied density functional theory (DFT) at the nonrelativistic and scalar and SO‐coupled relativistic levels, and also the ab initio coupled cluster approach. The chemically active electron shells of the lanthanide atoms comprise the 5d and 6s (and 6p) valence atomic orbitals (AO) and also the filled inner 4f semivalence and outer 5p semicore shells. Four different frozen‐core approximations for Lu were compared: the (1s²–4d¹⁰) [Pd] medium core, the [Pd+5s²5p⁶ = Xe] and [Pd+4f¹⁴] large cores, and the [Pd+4f¹⁴+5s²5p⁶] very large core. The errors of Lu? X bonding are more serious on freezing the 5p⁶ shell than the 4f¹⁴ shell, more serious upon core‐freezing than on the effective‐core‐potential approximation. The Ln? X distances correlate linearly with the AO radii of the ionic outer shells, Ln³⁺‐5p⁶ and X^?‐np⁶, characteristic for dominantly ionic Ln³⁺‐X^? binding. The heavier halogen atoms also bind covalently with the Ln‐5d shell. Scalar relativistic effects contract and destabilize the Lu? X bonds, spin orbit coupling hardly affects the geometries but the bond energies, owing to SO effects in the free atoms. The relativistic changes of bond energy BE, bond length R_e, bond force k, and bond stretching frequency v_s do not follow the simple rules of Badger and Gordy (R_e～BE～k～v_s). The so‐called degeneracy‐driven covalence, meaning strong mixing of accidentally near‐degenerate, nearly nonoverlapping AOs without BE contribution is critically discussed. © 2015 Wiley Periodicals, Inc.