Harold A. Schwarz

Gas phase infrared spectra of oxonium hydrate ions from 2 to 5 μ

Infrared spectra of oxonium hydrate ions, generated by pulse radiolysis of argon containing small amounts of water vapor, have been measured using a signal averaging technique. The symmetric and antisymmetric OH stretch frequencies of the H3O+ part of H9O4+ are found at 3000 and 2660 cm−1. H11O5+ frequencies are found at 2180, 2860, and 3200 cm−1. In addition, bands at 3620 and 3710 cm−1, due to the outer water molecules are observed. Bands found during the growth of H9O4+ at low water pressures are at 3490, 3290, 3170, 2850, and 2700 cm−1. The 3490 cm−1 frequency is attributed to H3O+.

Gas phase infrared spectra of ammoniated ammonium ions

Infrared spectra of 2–5 μ are reported for NH4(NH3)n+, n=0 to 4. The ν3 mode is found at 3335 cm−1 in NH4+, 2867 cm−1 in NH4(NH3)4+. The ν3′ modes in NH4(NH3)3+ are found at 2682 and 3365 cm−1 and the ν1′ mode is found at 2790 cm−1. The overtone band 2ν4 is observed at 3087 cm−1 in the n=4 species and 3000 cm−1 in the n=3 species. Isotope effects are reported. Less detailed spectra are reported for n=1 and n=2 species.

Some Applications of Time‐Dependent Rate Constant Theory to Radiation Chemistry

It is well known that rate constants for diffusion‐limited reactions decrease with time after the start of an experiment, an effect due to the time required to establish a diffusion gradient of each reactant around the other. It is pointed out that slow reactions between ions also exhibit a time dependence because a finite time is required to establish the Boltzmann distribution of each type of ion around the other. The latter effect appears as an increase of rate constant with time for reactions between ions of opposite charge and a decrease for ions of similar charge. These time dependent effects are applied to the picosecond pulse radiolysis results of Wolff et al. on electron formation and decay in acetone and acid solutions, and to some competition experiments of Czapski and Peled. Effects attributed by them to reactions of a precursor of the hydrated electron are shown to be similar to expected consequences of time‐dependent phenomena of the hydrated electron itself.

Medium effects on reactions of the carbonate radical with thiocyanate, iodide, and ferrocyanide ions

Abstract Results are presented which show that there is no pH dependence of the carbonate radical reactivity toward SCN − , I − , and Fe(CN) 6 4− above pH 8.5. It is demonstrated that observations in the literature on these reactions which have been interpreted to show a p K a of 9.5 for the carbonate radical, in disagreement with other reports that the radical is not protonated in this pH region, can be explained by the medium effects. It is also shown that previous studies of the reaction between carbonate radical and thiocyanate are in error, and the mechanism of this reaction is elucidated.

Electron transfer barriers for ground- and excited-state redox couples: trans-dioxo(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) osmium(VI)/osmium(V)

Abstract Pulse radiolysis methods were used to measure rate and equilibrium constants for the reaction of the OsVI(TMC)(O)22+/ OsV(TMC)(O)2+ couple with semiquinone/quinone couples, OsVI(TMC)(O)22+ + Q− ⇋ OsV(TMC)(O)2+ + Q, where TMC = tetramethylcyclam and Q is 1,4,-nezoquinne of 2-methyl-1,4-benzoquinone. This work yields a self-exchange rate constant of 1.1 × 106 M−1 s−1 and a reduction potential of +0.048 V versus NHE for the dioxoosmium ground-state couple at 0.1 M ionic strength and 25°C. [Os(TMC)(O)2](PF6)2 crystallizes in the monoclinic space group, P2 1 /c, with a = 6.712(2) A , b = 17.756(6) A , c = 10.150(2) A , β = 95.59(2)° and Z = 2 . Single crystal X-ray diffraction results for the PF6− salt of the OsVI(TMC)(O)22+ indicate that the dominant isomer present in the crystal is the RSSR isomer in which one pair of N-methyl groups is ‘up’ while the other is ‘down’. The average OsO and OsN distances are 1.735(6) and 2.126(8) A, respectively.

Hydrogen Atom Reactivity toward Aqueous tert-Butyl Alcohol

Through a combination of pulse radiolysis, purification, and analysis techniques, the rate constant for the H + (CH(3))(3)COH → H(2) + (•)CH(2)C(CH(3))(2)OH reaction in aqueous solution is definitively determined to be (1.0 ± 0.15) × 10(5) M(-1) s(-1), which is about half of the tabulated number and 10 times lower than the more recently suggested revision. Our value fits on the Polanyi-type, rate-enthalpy linear correlation ln(k/n) = (0.80 ± 0.05)ΔH + (3.2 ± 0.8) that is found for the analogous reactions of other aqueous aliphatic alcohols with n equivalent abstractable H atoms. The existence of such a correlation and its large slope are interpreted as an indication of the mechanistic similarity of the H atom abstraction from α- and β-carbon atoms in alcohols occurring through the late, product-like transition state. tert-Butyl alcohol is commonly contaminated by much more reactive secondary and primary alcohols (2-propanol, 2-butanol, ethanol, and methanol), whose content can be sufficient for nearly quantitative scavenging of the H atoms, skewing the H atom reactivity pattern, and explaining the disparity of the literature data on the H + (CH(3))(3)COH rate constant. The ubiquitous use of tert-butyl alcohol in pulse radiolysis for investigating H atom reactivity and the results of this work suggest that many other previously reported rate constants for the H atom, particularly the smaller ones, may be in jeopardy.