H. E. Gunning

The Photolysis of Ammonia at 1849A in a Flow System

An investigation has been made of the photolysis of ammonia at 1849A in a flow system at room temperature. The products of the reaction were found to be hydrazine, hydrogen, and nitrogen. The rate of decomposition of ammonia was independent of the linear flow rate but depended upon the ammonia pressure in the reaction zone. The fraction of ammonia decomposed which was recovered as hydrazine, while independent of ammonia pressure, increased markedly with increasing linear flow rate from zero in the static system to 0.84 at a linear flow rate of 1750 cm/sec.It was found that the quantum yield of ammonia consumption under flow conditions was consistently higher by a factor of approximately two than for the static conditions at the same pressures.A separate system was used to determine the extinction coefficient of ammonia at 1849A over the pressure range of the experiments. The value obtained was 1.21×103 liters‐mole‐1—cm—1.A mechanism has been proposed which is in qualitative agreement with the observed facts.

Reaction of Diborane with Hg 6(3P1) Atoms

An investigation has been made of the decomposition of diborane, photosensitized by mercury‐6(3P1) atoms under static conditions, at 29.30°C.The quantum yields of pressure increase and product formation have been found to be independent of substrate pressure in the complete quenching region. For runs of one hour duration at an incident light intensity of 2.5×10—6 einstein/min, the quantum yields are 0.50±0.02 for hydrogen, 0.25±0.02 for tetraborane, 0.02±0.02 pentaborane and 0.070±0.002 for pressure increase. The mean quantum yields of hydrogen and tetraborane formation were found to decrease with increasing duration of exposure during the first half‐hour of irradiation. Constant values were reached after 30–45 min of exposure. The ratio of the rates of hydrogen to tetraborane formation was found to maintain a value of two, independent of substrate pressure and duration of exposure, for runs exceeding five minutes in duration. Extensive decomposition was found to lead to solid products.A mechanism is prop...

Reaction of Cyclohexane with Mercury‐6(3P1) Atoms

An investigation has been made of the mercury‐6(3P1)‐photosensitized decomposition of cyclohexane at 29.30±0.01°C, under static conditions. The products of the reaction are hydrogen, bicyclohexyl, and cyclohexene, in order of decreasing importance. The mean quantum yields of cyclohexane disappearance, hydrogen formation, and cyclohexene formation were found to increase slightly with increasing substrate pressure; at 90‐mm cyclohexane pressure, the values are 0.83, 0.43, and 0.03, respectively. Extrapolation to initial conditions of data on the variation of mean quantum yield with time at constant substrate pressure gave 0.55 for the initial quantum yield of hydrogen formation, and 0.17 for cyclohexene formation, independent of substrate pressure. From the foregoing initial quantum yield data, taken in conjunction with the stoichiometry of the reaction, a value of 0.93 was obtained for the initial quantum yield of cyclohexane consumption.Absence of hexane, dodecanes, n‐hexylcyclohexane, and low molecular w...

Photochemical Separation of Mercury Isotopes. I. A Study of the Emission and Absorption Hyperfine Lines of Mercury at 2537 A

An experimental and theoretical investigation has been made of the emission and absorption parameters in the excitation of Hg202 atoms in natural mercury to the 6(3P1) state by irradiation with the resonance line from an electrodeless discharge containing Hg202. The absorption has been studied as a function of (a) absorption path length, (b) the concentration of mercury in the absorption cell, (c) the pressure of certain foreign gases in the absorption cell, (d) the operating temperature of the lamp, and (e) the type of excitor used with the lamp.The results of the study show that for the unique formation of Hg2026(3P1) atoms in natural mercury it is necessary to use a well‐cooled source operating at the minimum power level consistent with steady radiation output. Furthermore, the pressure of foreign gas in the absorption cell must be kept low to eliminate Lorentz broadening effects on the absorption hyperfine line.It has been found that the emission line from an electrodeless discharge operated at 26°C, ...

C13 Isotope Effect in the Thermodecomposition of Ethyl Bromide

The C12—C13 fractionation factor in the decomposition of gaseous ethyl bromide has been measured from 350—450°C, using samples of natural isotope abundance. The rate constants are defined as follows: CH3CH2Br→CH2=CH2+HBrk1,CH3C*H2Br→CH2=C*H2+HBrk2,C*H3CH2Br→C*H2=CH2+HBrk3. At 400°C, the C12 enrichment of the first fraction of ethylene from decomposition of the ethyl bromide is S0≡1+e0=2k1k2+k3=1.0079±0.0006, with a temperature coefficient of — 2.8×10‐5/°C.The primary and secondary isotope effects are defined, respectively, as β=k1/k2—1 and γ=k1/k3—1; thus, to a good approximation, β+γ=2e0. According to theory, β>γ≥0, so that e0 γ≥0. From the data of 400°C one then obtains as an upper limit (k1/k2)max≤1.0159±0.0012. This is significantly lower than the value k1/k2≥1.036 calculated for the rupture of an isolated C—Br bond. The present results, therefore, favor a mechanism involving the direct intramolecular elimination of HBr.

C13‐Isotope Effect in the Photolysis of Ethyl Bromide

The photolysis of ethyl bromide in a tenfold excess of cyclopentane has been studied over the temperature range, 30 to 250°C.Ethane is the principal volatile product of reaction. The quantum yield of ethane formation has been found to be unity at 30°C. An increase in apparent quantum yield to 1.5 at 250°C was observed. This increase is attributed to the greater absorption of the incident radiation at high temperatures.The C12 enrichment in the ethane product was 1.0070±0.0008, and invariant in temperature. The isotope fractionation effect is explained on the basis of the higher absorption of C12–C12–Br over C12–C13–Br in the long wavelength region.

The Reaction of Cyclobutane with Hg 6(3P1) Atoms

The reaction of cyclobutane with Hg 6(3P1) atoms has been studied in a static system at 30.10±0.01°C, over the pressure range from 2–250 mm. The primary products of the reaction are hydrogen, n‐butylcyclobutane, and a saturated C8H14 product which is assumed, from kinetic and mass spectral evidence, to be cyclobutylcyclobutane. In addition, prolonged exposure produced a saturated C12H22 product which is probably a dicyclobutylbutane.The quantum yields of pressure decrease (φP) hydrogen formation (φH2) and cyclobutane consumption (φC) all reach constant maximum values at cyclobutane pressures greater than approximately 100 mm. These values are φC=0.53, φH2=0.10, and φP=0.16. From the simple stoichiometry of the reaction the quantum yields of n‐butylcyclobutane (φBC) and cyclobutylcyclobutane (φC2) formation were calculated. From the data φBC=0.16 and φC2=0.10.The following mechanism was found to be consistent with the observed characteristics of the reaction: cyclo C4H8+Hg 6(3P1)→cyclo C4H7+H+Hg 6(1S0),H+c...

The Reaction of Methylcyclopentane with Hg 6(3P1) Atoms

The reaction of methylcyclopentane with photo‐excited Hg 6(3P1) atoms was investigated in a static system at 29.35±0.01°C, over a range of initial pressures of 5–110 mm. After an initial slight pressure rise, the reaction exhibited a linear pressure decrease, the magnitude of which increased linearly with initial pressures of methylcyclopentane above 15 mm. The average rate of hydrogen formation also increased with initial pressure of methylcyclopentane and decreased with time in the course of a run. The products found were hydrogen, methylcyclopentenes, and a heavy fraction with the formula C12H22. Upon the basis of chemical and physical properties, strengthened by kinetic considerations, it was concluded that the C12H22 compound was predominantly dimethyldicyclopentyls. Quantum yields reached a maximum of 0.44 for hydrogen formation and 0.42 for methyl‐cyclopentane consumption.The mechanism postulated proceeds through an initial carbon‐hydrogen bond split, thus: (1)  cyclo C6H12+Hg 6(3P1)  →cyclo C6H11+...

Further Studies on the Reaction of Cyclopentane with Mercury‐6(3P1) Atoms

A further investigation has been made of the reaction of cyclopentane with Hg6(3P1) atoms at 29.30±0.01°C, under static conditions. Analyses, using mass spectrometric techniques, confirm that the only direct products of the reaction are hydrogen, bicyclopentyl, and cyclopentene. No evidence was found for decomposition of the cyclopentane ring.The kinetics of the reaction can be satisfactorily explained in terms of the following general mechanism:(1) cyclo CnH2n+Hg6(3P1)→cyclo CnH2n−1+H+Hg6(1S0), (2) H+cyclo CnH2n→H2+cyclo CnH2n−1, (3) 2(cyclo CnH2n−1)→cyclo CnH2n−2+cyclo CnH2n, (4) 2(cyclo CnH2n−1)→bicyclo C2nH4n−2, (5) H+cyclo CnH2n−2→cyclo CnH2n−1. Reaction (5) can be ignored at high substrate pressures or low extents of decomposition. The limiting high‐pressure values for the quantum yields of cyclopentane disappearance, hydrogen formation, and cyclopentene formation are 0.90, 0.49, and 0.078, respectively.From the steady‐state treatment of the mechanism coupled with the quantum yield data, the ratio k...