Gabriel Stein

Energy gap law in the solvent isotope effect on radiationless transitions of rare earth ions

Data on the fluorescent yields of tervalent ions of Pr, Sm, Eu, Gd, Tb, Dy, and Tm in H2O and D2O, containing perchlorate ions, and isotope effects on the fluorescence are reported. The results are interpreted in terms of the mechanisms responsible for radiationless processes in solutions of rare earth ions [Y. Haas and G. Stein, J. Phys. Chem. 75, 3668 (1971); 76, 1093 (1972)]. It is shown that along the series of the closely related rare earth ions, all of which exhibit well shielded f−f electronic transitions, the results can be correlated in terms of a single variable: the energy gap between the lowest fluorescent and highest nonfluorescent level. For several rare earth ions, the matching of this gap by a single high energy (OH or OD) vibration of one solvent molecule is the decisive factor accounting for observed yields and isotope ratios.

Intensity dependence in laser flash photolysis experiments: Hydrated electron formation from ferrocyanide, tyrosine, and tryptophan

The light intensity dependence in monophotonic and consecutive biphotonic processes under laser flash photolysis conditions is derived. In a monophotonic process the relation between photoproduct yield and exciting light intensity is linear but there is saturation at high intensity. In a biphotonic process there are three different regions. The relation is quadratic at low intensity, linear at medium intensity, and there is saturation at high intensity. The difference between the two linear relations is discussed. These results are applied for analyzing the process of hydrated electron formation from excited states of various substances. The process is found to be monophotonic for ferrocyanide. For tyrosine and tryptophan in aqueous neutral solutions, under the conditions of the laser experiments, a biphotonic process is observed. For the tyrosinate ion (alkaline solution pH 12) the findings are not conclusive. Other work on this topic is discussed with reference to the analysis.

Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. I. Formation of the Hydrated Electron

Excitation of the Fe(CN)64− ion in aqueous solution into the 1T1u or 1T2g excited singlet state, below 313 nm, leads to hydrated electron formation in competition with internal conversion to the lowest excited singlet 1T1g state. The limiting quantum yield of hydrated electron formation is a linear function of quantum energy between 313 and 228 nm, reaching φe≃ 0.9 at 228 and 214 nm. Above 313 nm hydrated electron formation is not observed, but photoaquation is. Dependence of Φe on scavenger (N2O) concentration is observed. The involvement of CTTS character in the process, and the role of rapid solvent rearrangement in the dissociation of the excited state, are discussed.

Radiationless transitions in solutions: Isotope and proximity effects on Dy3+ by C–H and C–N bonds

The lifetime and quantum yield of fluorescence from the 4F9/2 level of Dy3+ have been measured in perprotonated and perdeuterated dimethylsulfoxide and methylcyanide under various excitation conditions. The results are compared to those obtained in light and heavy water and with other rare earth ions and discussed with reference to the modified energy gap law in the theory of radiationless transitions. The specific energy gap in Dy3+ is such that efficiency of quenching depends on matching the electronic energy gap of the ion by a single vibration of C–H, C–D, or C–N and on the distance of the bond from the ion.

ESR Study of Complex Formation and Electronic Relaxation of Fe3+ in Aqueous Solutions

ESR measurements of aqueous solutions of Fe3+ containing various electrolytes were made at X‐ and Q‐band frequencies. It was found that the line shape and integrated intensity depend on the pH and on the type and concentrations of counteranions in the solution. Addition of LiCl and NH4NCS result in reduction in the integrated intensity and subsequent disappearance of the ESR signal. This is interpreted in terms of the formation of monosubstituted complexes which give signals too broad to observe. From the results, formation constants have been calculated. Successive addition of NH4F results in the formation of complexes of the type [Fe(H2O)6−nFn]3−n having different ESR linewidth ranging from 10 to 1000 G. There is a strong reduction in the linewidth on going from X‐ band to Q‐band frequencies. The results are explained in terms of electronic relaxation via the modulation of the zero field splitting (zfs) parameter. From the analysis of the data it was possible to derive values for the zfs interactions an...

Conversion of Solvated Electrons into Hydrogen Atoms in the Photo‐ and Radiation Chemistry of Aqueous Solutions

Radiation chemical and photochemical experiments are reported which show that the conversion of solvated electrons to H atoms is not specific to the H3O+ ion. Proton donors in general may react in the conversion eaq—→H. The relative reaction rates correlate with the pK values of these acids as implied by Bronsted general acid catalysis law. The implications of this result for the nature of the solvated electron in water are briefly considered.

Yield and Reactivity of Electrons and H Atoms in Irradiated Aqueous Solutions

Using specific scavengers of solvated electrons eaq—, and of atomic hydrogen H in aqueous solutions irradiated with x rays, evidence is obtained that the hydrogen yield Ghydrogen=3.7±0.1 between pH 2–4. It is shown that this yield corresponds to the sum of the yields of molecular hydrogen GH2=0.5, the primary yield of atomic hydrogen GH=0.55 and yield of solvated electrons Geaq—=2.65. Evidence is obtained showing that H atoms formed as such (GH=0.55) appear in the bulk of the solution even at neutral pH. The velocity constants of the reactions of H atoms with acetate, isopropanol, glucose, glycerol, and nitrite were obtained. The reactivity of eaq— with Haq+, acetone and ferricyanide was studied and relative velocity constants obtained. An upper limit, keaq—+H2O≤103 liter mole—1 sec—1 is found for the reaction of eaq— with water to yield hydrogen.

Reaction of cythchrome c with one-electron redox reagents. I. Reduction of ferricytochrome c by the hydrated electron produced by pulse radiolysis☆

Abstract Pulse radiolysis-kinetic spectrometry has been used to investigate the reaction of hydrated electrons with ferricytochrome c in dilute aqueous solution at pH 6.5–7.0. Time resolutions from 2·10 −7 to 1 s were employed. Transient spectra from 320 to 580 nm were characterized with a wavelength resolution of ±0.5 nm. 1 In neutral salt-free solution, k (ferricytochrome c + e − aq )=(6.0±0.9)·10 10 M −1 ·s −1 and k (ferricytochrome c +H)=(1.2±0.2)·10 10 M −1 ·s −1 . The reaction of ferricytochrome c with hydrated electrons is sensitive to ionic strength; in 0.1 M NaClO 4 , k (ferricytochrome c + e − aq )=(2.4±0.4)·10 10 M −1 ·s −1 . In contrast, k (ferricytochrome c +H) is insensitive to ionic strength. Time resolution of three spectral stages has been accomplished. The primary spectrum is the first observable spectrum detectable after irradiation and is formed in a second-order process. Its rate of formation is indisting-uishable from the rate of disappearance of the electron spectrum. The secondary spectrum is generated in a true first order intramolecular process, k (p→s)=(1.2±0.1)·10 5 s −1 . The tertiary spectrum is also generated in a true first-order process, k (s→t)=(1.3±0.2)·10 2 s −1 . The specific rates of both transformations are independent of the wavelength of measurement. The tertiary spectrum, observable 50 ms after initial reaction and remaining unchanged thereafter for at least 1 s, shows that relaxed ferrocytochrome c is the only detectable product. This product is not autoxidizable, as expected for native reduced enzyme. It is more probable that the intramolecular changes responsible for the p→s and s→t spectral transformations involve the influence of conformational relaxation of ferrocytochrome c upon electronic energy states then that they are intramolecular transmission of reducing equivalents from primary sites of electron attachment.

Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. II. Photoaquation

Seven‐nsec laser pulses at 337 nm from an N2 gas laser and continuous illumination at 254, 313, and 365 nm were used to investigate the formation of (Fe(CN)5H2O)3− from photoexcited states of Fe(CN)64− and its protonated form in aqueous solution. The same pentacyano‐aquo complex is formed at all wave‐lengths. It is formed in ≤ 1 nsec as shown by the laser results. The quantum yield is constant at 337 nm between pH 3.8–10.5. The spectrum of the product changes and the quantum yield appears to decrease at lower pH. The quantum yield at pH 9 is the same at 365 and 313 nm and decreases to half at 254 nm. Photoaquation occurs via the lowest singlet 1T1g state. Excitation to higher singlet states results in competition between decay to the lowest singlet and transition from the short‐lived higher singlets to a CTTS state yielding the hydrated electron.

Reduction of Ferricytochrome c by Some Free Radical Agents

Fast pulse radiolysis and kinetic spectroscopy were used to rapidly generate a variety of free radicals in situ and study their reactions with ferricytochrome c in the time range 10-6 to 1 second. The radicals included t-butanol, which is inert to ferricytochrome c; malate, lactate, and ethanol, which react with it relatively slowly but are completely utilized in reducing it to ferrocytochrome c; and hydrated electrons and hydrogen atoms, which react with it very rapidly but yield ferrocytochrome c only in part, showing intramolecular consecutive reactions and further attack on the ferrocytochrome c protein. From a detailed comparison between malate and hydrogen atoms it is argued that malate reacts directly and selectively with a specific part of the ferricytochrome c surface while hydrogen atoms react with other parts of the protein too, yielding radicals which in part transfer intramolecularly to yield ferrocytochrome c.