Electrocatalytic Conversion of Products of Chemisorption of Simple Iodoaromatic Molecules on Smooth Platinum

The electrochemical behavior of chemisorption products of iodoaromatic compounds (C₆H₅I, C₆H₅IO) and iodide anions on a platinum electrode is studied. An assumption is made of a possible cleavage of the C–I bond upon C₆H₅IO adsorption and the formation of a product with properties similar to those of adsorbed iodine atoms.     

State Selectivity and Dynamics in Dissociative Electron Attachment to CF3I Revealed through Velocity Slice Imaging

In light of its substantially more environmentally friendly nature, CF₃I is currently being considered as a replacement for the highly potent global‐warming gas CF₄, which is used extensively in plasma processing. In this context, we have studied the electron‐driven dissociation of CF₃I to form CF₃^? and I, and we compare this process to the corresponding photolysis channel. By using the velocity slice imaging (VSI) technique we can visualize the complete dynamics of this process and show that electron‐driven dissociation proceeds from the same initial parent state as the corresponding photolysis process. However, in contrast to photolysis, which leads nearly exclusively to the ²P_1/2 excited state of iodine, electron‐induced dissociation leads predominantly to the ²P_3/2 ground state. We believe that the changed spin state of the negative ion allows an adiabatic dissociation through a conical intersection, whereas this path is efficiently repressed by a required spin flip in the photolysis process.     

The Rules Governing the Formation of Silver Azide Photolysis Products

Irradiation of silver azide at λ = 365 nm (I > 1 × 10¹⁵ quantum cm^?2 s^?1) in a vacuum (1 × 10^?5 Pa) leads to an increase in the rate of photolysis and photoinduced current and the appearance of a new long-wave region of spectral sensitivity. The photolysis products, silver metal and gaseous nitrogen, are formed in a stoichiometric ratio on the surface of silver azide. The rate constants for silver azide photolysis were determined. Measurements of contact potential difference, current—voltage characteristics, photoelectromotive force, and photocurrent showed that AgN₃(A₁)—Ag (photolysis product) microheterogeneous systems were formed in silver azide photolysis. The limiting stage of silver azide photolysis is the diffusion of interstitial silver cations to the (T_nAg_m)⁰ neutral center.     

Absolute yields I(2P1/2 as a function of wavelength in the photolysis of n-C3F7I

The absolute branching ratio I(²P_1/2)/I(²P_3/2) in the photolysis of n-C₃F₇I is examined optoacoustically as a functionof excitation wavelength. The yield of I(²P_1/2) is much less than the normally assumed value of unity. Comparison in terms of the sub-bands present is made with CH₃I, CD₃I and CF₃I.     

The use of ferrocene in organometallic synthesis: A two-step preparation of cyclopentadienyliron acetonitrile and phosphine cations via photolysis of cyclopentadienyliron tricarbonyl or arene cations

UV photolysis of [CpFe^II(CO)₃]⁺ PF6₆^? (I) or [CpFe^II(η⁶-toluene)]⁺ PF₆^?? (II) in CH₃CN in the presence of 1 mole of a ligand L gives the new air sensitive, red complexes [CpFe^II(NCCH₃)₂L]⁺PF₆^? (III, L = PPh₃; IV; L = CO; VIII, L = cyclohexene; IX, L = dimethylthiophene) and the known air stable complex [CpFe^II(PMe₃)₂(NCMe)]⁺ PF₆^? (V). The last product is also obtained by photolysis in the presence of 2 or 3 moles of PMe₃. In the presence of dppe, the known complex [CpFe^II (dppe)(NCCH₃)]⁺ (XI) is obtained. Complex III reacts with CO under mild conditions to give the known complex [CpFe(NCCH₃)(PPh₃)CO]⁺ PF₆^? (X). UV photolysis of I in CH₃CN in the presence of 1-phenyl-3,4-dimethylphosphole (P) gives [CpFe^IIP₃]⁺ PF₆^? (XII); UV photolysis of II in CH₂Cl₂ in the presence of 3 moles of PMe₃ or I mole of tripod (CH₃C(CH₂Ph₂)₃) provides an easy synthesis of the known complexes [CpFe^II(PMe₃)₃]⁺ PF₆^? (VII) or [CpFe^IIη³-tripod]⁺ PF₆^t- (XIII). Since I and II are easily accessible from ferrocene, these photolytic syntheses provide access to a wide range of piano-stool cyclopentadienyliron(II) cations in a 2-step process from ferrocene.     

Rate constants for the gas-phase reaction of OH with amines: tert-Butyl amine, 2,2,2-Trifluoroethyl amine,and 1,4-Diazabicyclo[2.2.2] octane

The title compounds were selected to test the current predictions of OH reaction rate constants. The reactions are studied employing flash photolysis/resonance fluorescence in 130 mbar He at room temperature and up to 420 K. Experimental difficulties were encountered due to the polar and hygroscopic properties of the compounds. The rate constants obtained at 298 K, 12, 0.9, and 22·10⁻¹²cm³/s for tert-Butyl amine (TBA), 2,2,2-Trifluoroethyl amine (TFEA), and 1,4-Diazabicyclo[2.2.2]octane (DABCO®), respectively, are much lower (up to a factor > 20) than calculated with the incremental system devised by Atkinson, indicating that electronic and steric effects should not be neglected. Activation energies are slightly negative for TBA and DABCO and some +450 K for TFEA. The reactivity of TFEA was also determined (as 1.2·10⁻¹²cm³/s) relative to a set of hydrocarbons employing continuous photolysis of H₂O₂ in a glass chamber. © 1996 John Wiley & Sons, Inc.     

Photoinduced intramolecular electron transfer reactions within donor-acceptor linked compounds in solution and within donor-acceptor ion-pairs in crystal

Intramolecular electron transfer (ET) processes within donor-acceptor linked compounds in solution and donor-acceptor ion-pairs in crystal have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electron-transfer to either ruthenium(III) ion, rhodium(III) ion or a cobalt(III) ion, followed by back ET to regenerate the original reactant. An Arrhenius plot of the ET rate gave a straight line with an intercept (frequency factor) and a slope (activation energy) for the photoinduced ET and the back ET. Mixed-valence isomer states produced via photoinitiated ET rapidly decayed via back ET. A common and large frequency factor observed for Ru(II)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. For the back ET in the Ru(II)-Co(III) compounds, the frequency factors are reduced because of negative entropy change. ET within donor-acceptor ion-pair of Ru(bpy)₂³ and Co(CN)₃⁶ in crystal took place very rapidly compared with in water.     

Photolysis of thallium azide

Preliminary treatment of TlN₃(A) with light (λ = 365 nm, I > 1 × 10¹⁴ quantum cm⁻² s⁻¹) in a vacuum (1 × 10⁻⁵ Pa) at 293 K led to the formation of a new long-wave region of spectral sensitivity. The products of photolysis of TlN₃(A) thallium and nitrogen formed in the stoichiometric ratio on the sample surface. The topography and kinetics of thallium accumulation were determined, and the effective rate constants for photolysis evaluated. Measurements of the contact voltage, current-voltage characteristics, and photocurrent showed that the photolysis of thallium azide resulted in the formation of TlN₃(A)-Tl (photolysis product) microheterogeneous systems. A model of photolysis of TlN₃(A) was suggested. According to this model, photolysis included the generation, recombination, and redistribution of nonequilibrium charge carriers in the contact field and the formation of the end products of photolysis. The limiting stage of photolysis of TlN₃(A) was the diffusion of interstitial thallium cations toward the neutral (T _n Tl _m )⁰ center. Original Russian Text ? E.P. Surovoi, L.I. Shurygina, L.N. Bugerko, N.V. Borisova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 4, pp. 784–790.     

Temperature Dependence Kinetic Studies of the Reaction of O(3P) with CHI3 and C2H5I and the 298 K Reaction of OH(X2Π) with CHI3

A flash photolysis–resonance fluorescence technique was used to investigate the kinetics of the OH(X²Π) radical and O(³P) atom‐initiated reactions with CHI₃ and the kinetics of the O(³P) atom‐initiated reaction with C₂H₅I. The reactions of the O(³P) atom with CHI₃ and C₂H₅I were studied over the temperature range of 296 to 373 K in 14 Torr of helium, and the reaction of the OH (X²Π) radical with CHI₃ was studied at T = 298 K in 186 Torr of helium. The experiments involved time‐resolved resonance fluorescence detection of OH (A²Σ⁺ → X²Π transition at λ = 308 nm) and of O(³P) (λ = 130.2, 130.5, and 130.6 nm) following flash photolysis of the H₂O/He, H₂O/CHI₃/He, O₃/He, and O₃/C₂H₅I/He mixtures. A xenon vacuum UV (VUV) flash lamp (λ > 120 nm) served as a photolysis light source. The OH radicals were produced by the VUV flash photolysis of water, and the O(³P) atoms were produced by the VUV flash photolysis of ozone. Decays of OH radicals and O(³P) atoms in the presence of CHI₃ and C₂H₅I were observed to be exponential, and the decay rates were found to be linearly dependent on the CHI₃ and C₂H₅I concentrations. Measured rate coefficients for the reaction of O(³P) atoms with CHI₃ and C₂H₅I are described by the following Arrhenius expressions (units are cm³ s^?1): k_O+C2H5I(T) = (17.2 ± 7.4) × 10^?12 exp[?(190 ± 140)K/T] and k_O+CHI3(T) = (1.80 ± 2.70) × 10^?12 exp[?(440 ± 500)K/T]; the 298 K rate coefficient for the reaction of the OH radical with CHI₃ is k_OH+CHI3(298 K) = (1.65 ± 0.06) × 10^?11 cm³ s^?1. The listed uncertainty values of the Arrhenius parameters are 2σ‐standard errors of the calculated slopes by linear regression.     

Quenching of triplet states by molecular oxygen and the role of charge-transfer interactions

The rate constants for oxygen quenching in benzene solution of the triplet states of several organic compounds with relatively high triplet energies have been measured in laser photolysis and pulse radiolysis experiments. The previously observed trend for aromatic hydrocarbons where the quenching rate constants decrease from a limiting value of about one ninth of that expected for a diffusion controlled reaction to lower values for triplet states with increasing triplet energy was not observed for the triplet states of certain aromatic ketones and amines. The higher rate constants observed, e.g. oxygen quenching of triplet N-methyl indole has k_Q = 1.4 × 10¹⁰ dm³ mol^?1 s^?1, are interpreted as being due to the presence of low lying triplet charge-transfer states which enhance the efficiency of quenching.     

没食子酸的激光光解研究

The transient species of gallic acid(GA)have been studied by 266 nm nanosecond laser flash photolysis inaqueous solution and acetonitrile.The intermediate with absorption at 320 nm was identified as excited triplet state(~3GA~*),the decay rates of which were obtained in aqueous solution and acetonitrile respectively.Energy transferfrom ~3GA~* to β-carotene was observed and the energy transfer rate constant k_(ent)was determined to be 2.2×10~9mol~(-1)·L·s~(-1).GA underwent photoionization during photolysis and the quantum yield of photoionization was de-termined to be 0.12 at room temperature with KI as a reference.     

The kinetics of photostabilization of cyanocobalamin in liposomal preparations

Cyanocobalamin (B₁₂) is a photosensitive vitamin, and its photodegradation to hydroxocobalamin (B_12b) in liposomes has been investigated. The values of apparent first-order rate constants (k_obs) for the photodegradation of B₁₂ in liposomes (nine preparations) are in the range of (0.52-2.24) × 10^–3 min^–1, compared to 3.21 × 10^–3 min^–1 for B₁₂ in aqueous solution (pH 5.0). The entrapment efficiency of B₁₂ in liposomes is 26.4-38.8%. The values of k_obs show a linear relation with phosphatidylcholine (PC) content in liposomes, indicating the influence of PC in inhibiting the rate of photolysis of B₁₂. The value of the bimolecular rate constant for photochemical interaction of B₁₂ and PC is 0.32 M^–1 min^–1, indicating the stabilizing effect of PC on the photolysis of B₁₂. The ratio of B₁₂ stabilization in liposomal preparations is in the range 2-6 compared to that of the unentrapped vitamin The stabilization of B₁₂ is mediated by a photoinduced charge-transfer B₁₂-PC complex that leads to the reduction of B₁₂ to B_12r, which is then oxidized to B_12b that has low susceptibility to photolysis. The extent of stabilization of B₁₂ probably depends on the degree of interaction between the two compounds under the reaction conditions, indicated by the loss of B₁₂ fluorescence.     

Reaction of excited iodine atoms with methyl iodide: Rate constant determinations

The rate constant for the reaction I(²P_1/2) + CH₃I → I₂ + CH₃ has been reevaluated taking into account both collisional deactivation of excited iodine atoms and loss of I₂ by I₂ + CH₃ → I + CH₃I. The reevaluation is based upon data obtained (R. T. Meyer), J. Chem. Phys., 46 , 4146 (1967) from the flash photolysis of CH₃I using time-resolved mass spectrometry to measure the rate of I₂ formation. Computer simulations of the complete kinetic system and a closed-form solution of a simplified set of the differential equations yielded a value of 6(± 4) × 10⁶ 1./mole-sec for the excited iodine atom reaction in the temperature region of 316 to 447 K. A slight temperature dependence was observed, but an activation energy could not be evaluated quantitatively due to the small temperature range studied. An upper limit for the collisional deactivation of I(²P_1/2) with CH₃I was also determined (2.4 × 10⁷ 1./mole-sec).     

The photochemistry of a potential diphenylsilylene precursor. Time-resolved spectroscopic studies of the reactivity of a transient vinylsilirane

The photochemistry of 3,4-dimethyl-1,1-diphenylsilacyclopent-3-ene (2), a potential precursor to diphenylsilylene (SiPh₂), has been studied in hydrocarbon and methanol solution by steady state and laser flash photolysis techniques. The results are consistent with the formation of three major photoproducts: SiPh₂ (ca. 20%), 2,3-dimethyl-1,3-butadiene (DMB; ca. 20%), and 2-methyl-2-(methylethenyl)-1,1-diphenylsilirane (9; ca. 75%), the product of formal photochemical (1,3)-sigmatropic rearrangement of 2. Attempts to detect the silylene by laser flash photolysis were unsuccessful, but the vinylsilirane could be easily detected as a long-lived transient exhibiting λ_max=285 nm and lifetime τ=1.3 s in hexane at 25 ^{^}C. The same spectrum is observed in neat methanol solution, where the lifetime of the species is shortened to τ=36 ms and it is quenched by sodium methoxide and methanesulfonic acid with absolute rate constants of k _MeO-=2.0×10⁶ M⁻¹ s⁻¹ and k_H+ =2.6×10³ M⁻¹ s ⁻¹, respectively. The photochemistry of 2 is compared to that of the corresponding germanium homologue (1), whose photolysis in solution has been previously shown to afford diphenylgermylene and DMB cleanly and in high yields. The corresponding vinylgermirane (11a), though not detectable as a primary photoproduct from 1 in solution, is the exclusive product of photolysis of 1 in a 3-methylpentane glass at 78 K, where it exhibits an essentially identical UV/vis spectrum to that of 9. The rate constant for its unimolecular thermal isomerization to regenerate 1 is three orders of magnitude larger than that of the corresponding process in the homologous vinylsilirane (9) in hydrocarbon solution at 25 ^C.     

Emission spectrum of CH3S radical

The emission spectrum of the CH₃S radical is obtained in the photolysis of CH₃SCH₃ by using a xenon resonance lamp. The emission is also obtained in the photolysis of CH₃SSCH₃ at ca. 200 nm irradiation as well as at 147.0 nm irradiation. The excitation threshold to produce CH₃S fluorescence in the photolysis of CH₃SSCH₃ is determined to be 202 ± 3 nm or 6.14 ± 0.09 eV. The ratios of electronic quenching rate to fluorescence rate of CH₃S^* with N₂, H₂, D₂ and CH₄ are determined.     

Photodissociation laser isotope effects

Marked differences in the laser action (1.315 μm) observed following the flash photolysis of CD₃I and CH₃I are reported (substantially greater outputs are observed with CD₃I). These differences result from the significantly smaller cross section for quenching of I(5 ²P ) by CD₃I, relative to that for CH₃I. Absolute values for the quenching cross sections have been determined using time resolved atomic absorption spectrophotometry. These data were employed in a computer simulated model which satisfactorily reproduced the light output from CH₃I, CD₃I and CF₃I photochemical laser systems. It is concluded that isotopic substitution can markedly influence the cross section for quenching of an excited state and thus influence partitioning between the various available channels.     

Global Reaction Pathways in the Photodissociation of I3− Ions in Solution at 267 and 400 nm Studied by Picosecond X-ray Liquidography

The mechanism of a photochemical reaction involves the formation and dissociation of various short-lived species on ultrafast timescales and therefore its characterization requires detailed structural information on the transient species. By making use of a structurally sensitive X-ray probe, time-resolved X-ray liquidography (TRXL) can directly elucidate the structures of reacting molecules in the solution phase and thus determine the comprehensive reaction mechanism with high accuracy. In this work, by performing TRXL measurements at two different wavelengths (400 and 267 nm), the reaction mechanism of I₃⁻ photolysis, which changes subtly depending on the excitation wavelength, is elucidated. Upon 400 nm photoexcitation, the I₃⁻ ion dissociates into I₂⁻ and I. By contrast, upon 267 nm photoexcitation, the I₃⁻ ion undergoes both two-body dissociation (I₂⁻+I) and three-body dissociation (I⁻+2I) with 7:3 molar ratio. At both excitation wavelengths, all the transient species ultimately disappear in 80 ns by recombining to form the I₃⁻ ion nongeminately. In addition to the reaction dynamics of solute species, the results reveal the transient structure of the solute/solvent cage and the changes in solvent density and temperature as a function of time.     

Electron spin resonance studies of the anaerobic and aerobic photolysis of alkylcobaloximes

Aerobic and anaerobic photolysis of methyl(pyridine)cobaloxime, benzyl(pyridine)cobaloxime and analogous compounds in CHCl₃ results only in an electron transfer reaction from an equatorial ligand producing photo-reduction of Co^III to Co^II, the complex retaining its axial ligands.If after the anaerobic photolysis of benzyl(pyridine)cobaloxime the oxygen is introduced without any further photolysis we obtain an ESR spectrum of nitroxide, arising from the attack of a benzyl radical on the dimethylglyoxime equatorial ligand.For the other complexes, homolytic cleavage of the CoC bond occurs and in the presence of oxygen gives rise to the superoxide cobalt complex adduct Py(Co^IIIO₂^?.During photolysis of methyl(pyridine)cobaloxime in isopropanol homolytic cleavage of the CoC bond occurs in preference to electron transfer reaction from the equatorial ligands.The anaerobic photolysis of benzyl(pyridine)cobaloxime in isopropanol or in water at 113–133 K results in an electron transfer reaction. However, at 170 K we observe the formation of the Co^II complex arising from CoC bond cleavage.A mechanism for photo-induced insertion of oxygen in the CoC bond is proposed.     

Photoassisted synthesis of mixed-metal clusters: [PPN] [CoOs3(CO)13], H2RuOs3(CO)13, and H2FeOs3(CO)13

The utility of photochemical methods for the directed synthesis of mixed-metal metal clusters has been explored. The 366 nm photolysis of a solution containing [PPN] [Co(CO)₄] (PPN = (Ph₃P)₂N⁺) and Os₃(CO)₁₂ gives the new cluster [PPN][CoOs₃(CO)₁₃] in 33% yield. Irradiation of a mixture of Fe(CO)₅ and H₂Os₃(CO)₁₀ yields H₂FeOs₃(CO)₁₃ in 95% yield, and photolysis of Ru₃(CO)₁₂ in the presence of H₂Os₃(CO)₁₀ gives the new cluster H₂RuOs₃(CO)₁₃. Details of these syntheses, their probable mechanisms, and the characterization of the new compounds are discussed.     

Synthesis,photolysis, and redox properties of 2-dialkylamino-3-methoxy-1,4-naphthoquinones

A synthesis is reported for 2-dialkylamino-3-methoxy-1,4-naphthoquinones. The photolysis of these compounds in the liquid phase was studied and their halfwave potentials (E_1/2) were determined. The introduction of a methoxy group at C³ leads to a shift in the spectral absorption toward longer wavelengths, shift in E_1/2 toward positive values, and a slight change in the photolysis quantum yield. The physicochemical parameters were found to be dependent on the nature of the dialkylamino group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 657–661, March, 1990.