Robert R. Hentz

Pulse Radiolysis of Liquids at High Pressures. III. Hydrated‐Electron Reactions Not Controlled by Diffusion

Specific rates of the reactions of eaq− with hydrogen ion, formamide, acetamide, acetoxime, benzyl alcohol, and 2‐chloroethanol have been measured by pulse radiolysis at 29°C and six pressures from atmospheric to 6.4 kbar. The specific rates increase monotonically with increase in pressure to 6.4 kbar except that for the H3O+ reaction for which there is no further significant increase above ∼3.5 kbar. Results for the H3O+ reaction are consistent with that reaction not being diffusion controlled. The activation volume of each reaction increases with increase in pressure. Such a result is attributed to a decrease in cavity volume of the electron with increase in pressure. Activation volumes, in units of milliliters per mole, at 1 atm and 29°C are −2.7, −10.3, −7.0, −9.5, −9.3, and −8.2 for the reactions with H3O+, formamide, acetamide, acetoxime, benzyl alcohol, and 2‐chloroethanol, respectively. Interpretation of the activation volumes gives a partial molal volume of 7 ml mole−1 and, with an electrostricti...

γ‐Radiolysis of Liquids at High Pressures. I. Aqueous Solutions of Ferrous Sulfate

The γ radiolysis of aerated and deaerated, acid solutions of ferrous sulfate has been studied at 20° and applied pressures in the range 0 to 6.34 kbar. Over the entire pressure range, G(Fe+++)=8.1±0.1 and G(H2)=4.1±0.1 in deaerated solutions containing 0.8N H2SO4, 1 mM NaCl and 2 mM Fe++. Radiolysis of deaerated, 0.8N H2SO4 solutions of dichromate gives G(O2)=0.78±0.02 over the whole pressure range. It is concluded from such results, and observation of a similar pressure independence of G(Ce+++) in the ceric sulfate dosimeter, that primary yields in water radiolysis are invariant as pressure varies from 0 to 6.34 kbar. In aerated ferrous solutions, G(H2), G(Fe+++), and G(—O2) vary with pressure, ferrous concentration, and acid concentration in such a manner as to indicate an effect of pressure on the competition between Reactions [1] and [2]: H+O2→HO2, H+H++Fe++→Fe++++H2. Analysis of the results, for aerated ferrous solutions containing 0.8N H2SO4 and 1 mM NaCl, gives k1/k2≈1000 at atmospheric pressure an...

Pulse Radiolysis of Liquids at High Pressures. II. Diffusion‐Controlled Reactions of the Hydrated Electron

For each of 11 presumably diffusion‐controlled reactions of eaq−, absolute specific rates were determined by pulse radiolysis at four to seven pressures from atmospheric to 6.4 kbar. The ratio of specific rate at 6.4 kbar to that at 1 atm, followed by the corresponding activation volume in milliliters mole−1, is given in parentheses for each of the reactants: O2 (1.05, −0.2), H2O2 (0.88, 0.5), acrylamide (0.90, 0.4), benzamide (0.95, 0.2), p‐nitrophenol (0.84, 0.7), NO3− (1.00, 0.0), S2O82− (1.07, −0.3), Cr(C2O4)33− (1.32, −1.1), Tl+ (0.74, 1.2), Cd2+ (0.75, 1.1), Sm3+ (0.86, 0.6). The Debye equation for specific rate of a diffusion‐controlled reaction is used for analysis of the results. Such an analysis suggests that the diffusion coefficient of eaq− changes negligibly (compared to normal ions and molecules) and possibly increases with increase in pressure to 6.4 kbar. With the result for reaction of eaq− with NO3−, certain activation volumes obtained in previous γ‐radiolysis studies (by assumption of a...

γ Radiolysis of Liquids at High Pressures. X. The Reaction H+OH− and the Partial Molal Volume of the Hydrated Electron

G(H2) is independent of pressure over the range 0–6.2 kbar in γ radiolysis of the following deaerated aqueous solutions at 23°C: (a) solutions with 0.001 M or 0.1 M hydroxide, isopropanol at concentrations in the range 9 × 10−4 M to 0.1 M, and nitrate at one‐fiftieth of the isopropanol concentration; (b) solutions with 0.01 M formate, 0.001 M nitrate, and 0.01 M or 0.1 M hydroxide; (c) near‐neutral solutions with 0.01 M formate and 0.01 M or 0.002 M benzyl alcohol. From pressure independence of G(H2) for such solutions and the established pressure independence of the primary yields of water radiolysis, it follows that activation volumes for the reactions of H with hydroxide, isopropanol, formate, and benzyl alcohol are equal. Consequently, ΔV1‡ = − 5.9 ml mole−1 is obtained for Reaction [1], H+OH−→H2O+eaq−. Combination of ΔV1‡ with the previously determined ΔV‡ = − 14.2 ml mole−1 for the back reaction gives ΔV for Reaction [1]. From ΔV, with a previous estimate of V(H) in the range 5.9–10.3 ml mole−1, th...

PULSE RADIOLYSIS OF LIQUIDS AT HIGH PRESSURES. I. ABSORPTION SPECTRUM OF THE HYDRATED ELECTRON AT PRESSURES UP TO 6.3 KBAR.

A system for study of the pulse radiolysis of liquids at pressures up to 6.9 kbar is described. With this system the optical absorption spectrum of the hydrated electron has been measured at 29 ± 3° C and applied pressures in the range 0–6.3 kbar. The spectrum shifts monotonically to shorter wavelengths with increase in applied pressure from 0 to 6.3 kbar. The transition energy at the absorption maximum, Emax, increases from 1.71 to 2.00 eV, and the bandwidth at half‐maximum increases by ∼ 28 %. It is concluded that the extinction coefficient at the absorption maximum decreases by ∼ 18 % and, therefore, that the oscillator strength shows little or no pressure dependence. Values of Emax obtained in the present work and from a published study of the temperature dependence show a monotonic increase with increase in the static dielectric constant, Ds. It is suggested that the correlation between Emax and Ds is a consequence of a predominant influence on the structure of eaq− of an attractive force determined ...

γ Radiolysis of Liquids at High Pressures. VI. Hydrogen‐Atom Reactions in Aqueous 0.8N H2SO4 Solutions

The competition between Reactions [1] and [2], H + S→no H2, H + RH→H2 + R, in aqueous 0.8N H2SO4 solutions was studied at 20° over the applied pressure range of 0–6.34 kbar by measurement of G(H2) from the gamma‐irradiated solutions. With O2 as S, in aerated solutions, relative specific rates k10 / k20 were determined at atmospheric pressure for methanol, glucose, glycerol, ethanol, and isopropanol as RH. From the pressure dependence of k1 / k2 and an estimated ΔV1‡ = 1.6 ml mole−1 for S ≡ O2, values of ΔV2‡ in the range −5.5 to −6.7 ml mole−1 are obtained for the five H donors. The average value ΔV2‡ = − 5.9 ml mole−1 is taken as characteristic for Reaction [2]. Values of k10 / k20 at atmospheric pressure also were determined for four deaerated 0.8N H2SO4 solutions: (1) RH ≡ glucose and S ≡ benzyl alcohol; (2) RH ≡ glucose and S ≡ BrCH2COOH; (3) RH ≡ S ≡ acetone; (4) RH ≡ S ≡ ClCH2COOH. Because little or no pressure dependence of G(H2) and, therefore, of k1 / k2 was observed with these four solutions, ΔV...

γ Radiolysis of Liquids at High Pressures. VIII. Primary Yields at 8.7 kbar and Reactions of the Hydrated Electron with H2O and H3O

Pressure invariance of the primary yields of water radiolysis (previously established for pressures up to 6.4 kbar) was shown to extend to 8.7 kbar in γ radiolysis of both neutral and 0.8N H2SO4 solutions. G(H2) was measured in γ radiolysis of the following deaerated solutions at 25° and pressures in the range 0–8.85 kbar: (a) 3 × 10−5M NaNO3 and 0.05M glucose (slightly alkaline); (b) 10−3M NaNO3, 2.3 × 10−4M HClO4, and 0.05M glucose. In solution (a) G(H2) is determined by a competition for eaq− between the diffusion‐controlled Reaction [3], eaq−+NO3−→no H or H2, and Reaction [1], eaq−+H2O→H+OH−. In solution (b) the competition is between Reactions [3] and [2], eaq−+H3O+→H+H2O. All H are converted to H2 by reaction with glucose. With the assumption that pressure dependence of k3 is determined approximately by that of water viscosity, kinetic analysis of the results gives ΔV1‡ = − 14.3 ml mole−1 and ΔV2‡ ≈ 0 ml mole−1. From the value of ΔV2‡, the absolute partial molal volume of eaq− (based on a value of −...

Pulse radiolysis of liquids at high pressures. V. Absorption spectrum and yield of the solvated electron in liquid ammonia at 23 °C and pressures up to 6.7 kbar

The absorption spectrum and primary yield of the solvated electron in liquid ammonia (eam−) have been determined by nanosecond pulse radiolysis of liquid ammonia at 23°C and pressures up to 6.7 kbar. With increase in pressure from 0.009 kbar (vapor pressure at 23°C) to 6.7 kbar, the following changes occur in the absorption spectrum: The transition energy at the absorption maximum (Emax) increases from 0.67 to 0.91 eV; the observable portion W of the bandwidth at half‐maximum, from Emax to the high‐energy side of the spectrum, increases by 35%; and optical density at the absorption maximum (ODmax) decreases by 32%. The pressure and temperature dependences of Emax are attributed to changes in (1) size of the electron cavity and (2) the local dielectric constant associated with the solvation shell of the electron. At 23°C and 0 kbar, (∂lnEmax/∂P)T for eam− exceeds that for solvated electrons in water, methanol, and ethanol by a factor of ∼4. Such a result indicates that the cavity of eam−(V=98 ml mol−1) is...

γ Radiolysis of Liquids at High Pressures. III. Aqueous Solutions of Sodium Bicarbonate

The γ radiolysis of deaerated aqueous solutions containing sodium bicarbonate and either glucose or isopropanol has been studied at ∼20° in the applied pressure range 0 to 7 kbar. For 0.05M bicarbonate and 0.036M isopropanol, G(H2) increases from 0.89 to 2.43 as applied pressure increases from 0 to 6.9 kbar. A 100‐fold change (0.005–0.5M) in bicarbonate concentration has little or no effect on the pressure dependence of G(H2). Because primary yields are invariant over the pressure range studied, the results are interpreted in terms of an effect of pressure on the competition between Reactions [2] and [3] eaq−+CO2→CO2−, eaq−+HCO3−→H+CO32−, with Reaction [3] followed by Reaction [4] H+RH→R+H2. Such an interpretation gives a difference in activation volumes of Reactions [2] and [3] as ΔV2‡—ΔV3‡≈18.4 ml mole−1 and a specific rate k30≈6×105M−1· sec−1 at atmospheric pressure. Assumption that Reaction [2] is diffusion controlled permits estimation of ΔV2‡≈1.6 ml mole−1, from the pressure dependence of water visc...

Pulse radiolysis of liquids at high pressures. IV. Hydrogen‐atom reactions in aqueous 0.1M HClO4 solutions

Absorption spectra were determined for various times after a 10‐nsec radiation pulse to 0.1M HClO4 solutions containing AgClO4 with and without tert‐butyl alcohol (TBA). The spectra and their time dependence, which show the same features at 1 atm and 6.6 kbar, are interpreted in terms of the following reactions: H + Ag+ → H+ + Ag [2], OH + Ag+ → OH− + Ag2+ [5], Ag + Ag+ → Ag2+ [6]. Scavenging of OH by TBA prevents formation of Ag2+ which has an absorption peak near 270 nm. For solutions containing TBA, a 60‐nsec spectrum with a peak at 370 nm is assigned to Ag and a 7‐μsec spectrum with a peak at 310 nm and shoulder at 260 nm is assigned to Ag2+. Growth and decay of Ag absorption at 410 nm were studied with solutions containing TBA at 29 °C and six pressures in the range 1 atm–6.7 kbar; the reactions of H with glycerol and ethylene glycol were studied by their competition with Reaction [2] in solutions also containing glycerol or ethylene glycol. The results give atmospheric‐pressure specific rates of 2.8...