Leon M. Dorfman

The standardization of hydroxyl radical rate data from radiation chemistry

Abstract In the literature, numerous discrepancies occur in rate constant data for reactions of the hydroxyl radical with inorganic and organic compounds. Some of these have been redetermined, using a pulse radiolysis competition technique in which the ferrocyanide ion is used as a reference solute. The validity of the method has been established by comparison of the data with rate constants determined by direct measurement using pulse radiolysis. The results have been used to correct literature data obtained by other indirect competition methods using thymine, paranitrosodimethylaniline and thiocyanate ion as reference solutes. Satisfactory self-consistency has been achieved.

Pulse radiolysis studies. XXI. Optical absorption spectrum of the solvated electron in ethers and in binary solutions of these ethers

The optical absorption spectrum of the solvated electron has been determined by pulse radiolysis in the pure liquid ethers: tetrahydrofuran, methyltetrahydrofuran, diethyl ether, dimethoxyethane, diglyme, triglyme, and tetraglyme. The absorption maxima are at 4720, 4650, 4350, 4880, 5220, 5440, and 5580 cm−1, respectively. The half‐widths of the bands have also been measured. The oscillator strength, determined for the first four ethers is approximately unity. The absorption bands have been determined in binary solutions with ethylenediamine and for tetrahydrofuran‐water over the entire concentration range. Calculations using a recent form of a cavity‐continuum model have been compared with the experimental results. The model shows agreement with the experimental values for the transition energy for an effective cavity radius of about 4 A and a coordination number of 6 or 8. Kinetics for the attachment of solvated electrons to pyrene and for the reaction of the solvated electron with the solvent counterio...

Pulse Radiolysis Studies. XX. Kinetics of Some Addition Reactions of Gaseous Hydrogen Atoms by Fast Lyman‐α Absorption Spectrophotometry

The pulse radiolysis technique combined with fast Lyman‐α absorption spectrophotometry, has been used to study the kinetics of several addition reactions of gaseous hydrogen atoms. The following absolute rate constants, at 298°K, have been obtained: H+O2+Ar=HO2+Ar, (5.9 ± 0.7) × 109M−2·sec−1, H+CO+H2=HCO+H2, (4.0 ± 0.6) × 107M−2·sec−1, H+CO+Ar=HCO+Ar, (2.6 ± 0.4) × 107M−2·sec−1, H+NO+H2=HNO+H2, (1.4 ± 0.2) × 1010M−2·sec−1. These results, and our earlier value for k1 with H2 as third body, give values of k1H2 / k1Ar = 2.9 and k2H2 / k2Ar = 1.5 for the relative third‐body efficiencies of hydrogen and argon. The high‐pressure limiting rate constant for H‐atom addition to ethylene, obtained directly in the high‐pressure region, was found to be H+C2H4=C2H5*, (5.5 ± 0.5) × 108M−1·sec−1, with both H2 and Ar as third body.

Pulse Radiolysis Studies. XVIII. Spectrum of the Solvated Electron in the Systems Ethylenediamine–Water and Ammonia–Water

The pulse radiolysis technique with fast infrared detection was used to determine the optical absorption spectrum of the solvated electron in the systems ethylenediamine–water and ammonia–water. These spectra, determined over the entire concentration range, show the following: In ammonia and in ethylenedamine, the bands at 1550 and 1350 nm, respectively, have the same shape and position as those attributed to the solvated electron in alkali metal solutions. In each of the two‐component systems a single band is seen with the peak position intermediate to those in the pure solvents. For ethylenediamine–water mixtures, the band shape (normalized) and half‐width (energy scale) are invariant with composition, while for ammonia–water mixtures the ratio of the peak position to the half‐width is invariant. These observations suggest a delocalized electron with optical characteristics determined by the aggregate properties of the solvent. Models which require that the optical properties be strongly influenced by s...

Pulse‐Radiolysis Observation of Molecular Cations of Aromatic Compounds in Halogenated Liquids

Electron irradiation of dilute solutions of aromatic compounds in liquid chlorinated hydrocarbon solvents results in the formation of the molecular cations of these aromatic compounds. The optical absorption spectra of the molecular cations of diphenyl, p‐terphenyl, t‐stilbene, and anthracene were observed by pulse‐radiolysis methods, and the identity of the reactive species established from the spectra and from various scavenger effects. At room temperature the half‐life of these cations in the particular solutions is on the order of a few microseconds. In solutions containing two different aromatic compounds as solutes, it was found that an electron transfer may take place between the neutral aromatic molecule of one compound and the molecular cation of the other.

Pulse Radiolysis Studies. XVII. Reaction Kinetics of Molecular Cations of Aromatic Compounds in Dichloroethane Solution

Fast reaction studies of the kinetics of molecular cations of aromatic molecules, formed by irradiation of the aromatic compounds in 1,2‐dichloroethane solution, have provided quantitative information about the modes of formation and of decay of these molecule ions. Specific reaction rates have been determined for the electron transfer from a neutral aromatic molecule to an aromatic cation for three donor–acceptor pairs. The rate of formation of the aromatic cation, thought to be a charge transfer from the aromatic molecule to the solvent cation, is so high as to suggest that migration of the solvent cation is not a process of molecular diffusion, but involves an electron jump process in the solvent. The decay of the aromatic cation appears to be due largely to recombination with the counterion. Absolute rate constants for electron transfer between the aromatic pairs studied are at or near diffusion controlled, with values ranging from 5.1 × 109 to 9.9 × 109M−1·sec−1 at 25°C.

PULSE RADIOLYSIS STUDIES. X. ELECTRON TRANSFER REACTIONS OF AROMATIC MOLECULES IN SOLUTION.

Absolute rate constants have been determined by the pulse radiolysis method for the electron transfer reaction from a series of aromatic anion radicals in isopropanol solution to a variety of different aromatic molecules: arenea−+areneb=arenea+areneb−. Nine donor—acceptor pairs were investigated in which the donor anion was diphenylide, p‐terphenylide, m‐terphenylide, and o‐terphenylide, and the acceptor molecule was naphthalene, phenanthrene, p‐terphenyl, pyrene, and anthracene. Several, but not all, of the reactions appear to be diffusion controlled with electron transfer rate constants ranging from 2.6×108 to 6.4×109 M−1·sec−1 at 25°C.

Pulse Radiolysis Studies. XVI. Kinetics of the Reaction of Gaseous Hydrogen Atoms with Molecular Oxygen by Fast Lyman‐α Absorption Spectrophotometry

The pulse radiolysis method has been used to produce ground‐state hydrogen atoms in gaseous hydrogen and has been combined with fast Lyman‐α absorption spectrophotometry to observe these atoms directly with microsecond time resolution. The technique has been used to study the kinetics of the reaction H + O2 + M = HO2 + M, where M is H2. The rate constant for this reaction was found to have the value (1.7 ± 0.4) × 1010M−2·sec−1 at 298°K. This value, together with earlier data for Ar as third body, gives kH2 / kAr = 2.0 at 298°K for the ratio of the efficiencies of H2 and Ar as third body in this reaction.

Pulse Radiolysis Studies. XIX. Solvent Effects in Electron Transfer and Proton Transfer Reactions of Aromatic Molecule Ions

Absolute rate constants were determined, by the pulse radiolysis technique, for the electron transfer reaction from an aromatic radical anion to a different neutral aromatic molecule: A1·−+A2=A1+A2·− for a number of aromatic molecule pairs in ethanol, in ethylenediamine, and in diethylamine. The dependence of these rate constants upon the standard free energy for reaction of the pair and upon the dielectric properties of the solvent, as predicted by the theory of Marcus for electron transfer reactions, is found to support this theory, at least semiquantitatively, over a limited range. The effect of solvent on the rate of protonation of the biphenyl radical anion by an alcohol: C12H10·−+ROH=C12H11·+RO− was studied for a number of mixed solvent systems, namely, ethanol–ethylenediamine, ethanol–cyclohexane, ethanol–triethylamine, ethanol–diethylamine, ethylene glycol–ethylenediamine, and ethylene glycol–triethylamine. In the alcohol–amine systems the first order rate constant for the protonation reaction was...

FAST REACTION STUDIES OF CARBOCATIONS IN SOLUTION

The pulse radiolysis method has been used to generate and study a series of five carbocations, consisting of phenylcarbeniurn ions and cyclopropylphenylcarbenium ions, in chlorocarbon solvents. Rate constants for the reactions of these substituted carbenium ions with various nucleophiles such as halide ions, alkyl amines, ammonia and water, have been determined. The observed reactivity trends (or, in some cases, lack thereof) with successive substitution are discussed in relation to steric and electronic effects. Comparison of these reactivities with observations from solvolysis experiments is discussed. Solvent effects are observed.