R. E. Rebbert

Photolysis of Ethane at 11.6–11.8 eV

The photolysis of ethane, carried out with an argon resonance lamp, has been reinvestigated with the related purposes of (1) measuring quantum yields of all fragments formed in the dissociation of excited ethane and (2) associating these fragments with the primary processes occurring in the photolysis of ethane. These and their relative abundances are C2H6*→C2H6+, 5% →CH4+CH2, 16% →C2H5+H, 41% →C2H4+H2, 26% →CH3+CH3, 15%. These results are compared with conclusions reached in earlier studies on the photolysis of ethane with xenon and krypton lamps in order to determine the effect of energy on the relative probabilities of the primary processes. It is found that direct bond scission increases in importance with increasing energy, while processes involving rearrangement decrease in importance. The radical and molecular fragments formed in the dissociation of excited ethane were determined by (a) analyzing the products formed in C2H6–C2D6–NO (1:1:0.1) mixtures and (b) C2D6in the presence of H2S, which scaven...

Gas-phase photodecomposition of carbon tetrachloride

Abstract The gas-phase photolysis of CCl 4 was investigated at 213.9, 163.3 and 147.0 nm in the presence of HCl, HBr, and C 2 H 6 . Quantum yields of the products measured in these mixtures at a temperature of 300 K led to the conclusion that at 213.9 nm over 90% of the photodecomposition can be attributed to the photodissociative process: independent of pressure (5 - 60 Torr). At 163.3 nm, CCl 2 is formed via the photodecomposition process: Contrary to earlier suggestions, CCl 2 is unreactive towards CCl 4 . Combination with other radicals and insertion into HCl are the major modes of reaction of CCl 2 . Experiments carried out at 313.0 nm show evidence for the occurrence of photodissociation of CCl 4 . On the assumption that absorption of a photon by CCl 4 invariably leads to the detachment of a chlorine atom, the absorption cross-section at 313.0 nm is /s 3.7 ± 0.4 X 10 −26 cm 2 molecule −1 (300 K). This result indicates that photodecomposition of CCl 4 in the troposphere is of minor importance as compared to other processes including diffusion up to the earths stratosphere.

Pulse- and gamma ray-radiolysis of cyclohexane: Ion recombination mechanisms

Abstract The products formed in the γ-radiolysis and pulse-radiolysis of gaseous cyclohexane have been interpreted in terms of the ion fragmentation, ion-molecule reaction, and ion recombination mechanisms. It is shown that the fragmentation of the parent ion is partly quenched at a pressure of 55 torr. The products resulting from homogeneous neutralization of the major unreactive ions, c - C 6 H + 12 , c - C 6 H + 11 , and c - C 6 H + 10 , are deduced by comparing product yields at high dose rates, where the ions undergo homogenous neutralization, with yields in the gamma radiolysis, where the unreactive ions disappear mainly by reaction with impurities or neutralization on the wall. Ethylene and 1,3-butadiene are the major products resulting from electron neutralization of these ions. Fragmentation is strongly reduced when the neutralization process involves an atomic- or polyatomic- anion rather than an electron. For instance, addition of CCl 4 to cyclohexane results in a sharp drop of the yield of 1,3-butadiene, and a concurrent rise in the yield of 2-C 4 H 8 . In the gas phase, a value of 0.16±0.08 is suggested for the ratio of neutral excited molecule formation to ionization. In the liquid phase, it is seen that the relative importances of processes observed in the radiolysis are very different from the importances of these same processes in the far ultraviolet photolysis (8-11.6eV). In the radiolysis, where the mean energy taken up by the cyclohexane molecule may be higher than 11.6eV, solvent assisted fragmentation of the parent ion to give c -C 6 H + 11 , and the formation of triplet excited molecules in ion recombination processes are considered as explanations for the discrepancies. In the radiolysis, geminate neutralization of the vibrationally-relaxed parent ion to produce a singlet excited state of cyclohexane accounts for no more than about 25% of the cations.

Primary processes in the photolysis of propane: the use of HI as a radical scavenger

Abstract The photolysis of C3D8 has been investigated with 8.4, 10.0, and 11.6–11.8 eV photons, using HI to scavenge the radicals through the reaction: RD + HI → RDH + I (where RD is a fully deuterated alkyl or alkenyl radical). Comparison of the results with the results of analogous experiments using H2S as a scavenger leads to the conclusion that HI is a more efficient radical scavenger than H2S. The results are discussed with particular emphasis on determining whether the primary processes include direct C C and C H bond cleavage. An examination of the effects of HI concentration, conversion, and pressure on the yield of ethyl radicals intercepted indicates that the ethyl radicals are formed in the primary process: C3D8* → C2D5 + CD3. It is noted that the relative importances of this process and the other primary processes involving breaking of the C C bond (C3D8* → CD4 + C2D4 and C3D8* → C2D6 + CD2) do not change with energy, and it is thus suggested that they all occur from an excitation in the C C bond, and that RRKM considerations relating to equipartition of energy are not applicable to the dissociations of electronically excited alkanes. It is pointed out that, as in the photolysis of ethane, the dissociation leading to the formation of a molecule of hydrogen (deuterium) (C3D8* → D2 + C3D6), which apparently occurs as a result of an excitation in a C H (C D) bond, predominates in the 8.4 eV photolysis, but diminishes sharply in importance with respect to the C C bond cleavage processes when the energy is increased. The insertion of methylene into a primary C H bond of C3H8 to give n-butane is examined, and information concerning the internal energy of the CH2 species is derived and discussed in terms of the primary dissociation of propane.

DIRECT AND INERT-GAS-SENSITIZED RADIOLYSIS AND PHOTOLYSIS OF METHANE IN THE SOLID PHASE

The photolysis of CH4–CD4 mixtures has been briefly investigated at 20.4°K using the xenon and krypton resonance lines. From the isotopic composition of the hydrogen and ethane fractions, it could be derived that, in the solid phase, methylene and methyl radicals are produced. The methylene radicals, which are formed by the process CH4*→CH2+H2, insert into methane to form ethane.In the argon‐sensitized radiolysis at 20.4° and 77°K, the hydrogen‐molecule elimination process predominates, indicating that neutral excited‐methane molecules are formed nearly exclusively. In the xenon‐sensitized radiolysis, however, mainly hydrogen atoms and methyl radicals are observed. In all inert gas‐sensitized radiolyses, there is a gradual decrease in the efficiency of the energy transfer with increasing dilution.In the direct radiolysis, hydrogen atoms play a more important role than in the photolysis at 1236 A. Radiolysis of CD4−C2H4 (1:0.01) mixtures indicated that, at 77°K, hydrogen atoms react with ethylene and propy...

The gas phase photolysis of CHFCl2

Abstract The photolysis of CHFCl 2 at 300 K has been investigated at 213.9, 163.3 and 147 nm. Methane, Br 2 , HBr and HCl were added as free radical interceptors in order to unravel the primary photodecomposition processes. Analysis of the data shows that at 213.9 and 163.3 nm the photodissociative process occurs with a quantum yield of 0.9 – 1.0, giving stable CHFCl radicals. At shorter wavelengths the quantum yield of CHFCl shows a drastic decrease with concurrent appearances of species such as CFCl, CHF and CF. The laboratory experiments indicate that CF is mainly formed via the dissociative process The CF radicals react with CH 3 to yield C 2 H 2 (CF + CH 3 → C 2 H 2 + HF) while the CHF species insert readily into HCl to yield CH 2 FCl. In the presence of Br 2 , CF and CHF undergo reactions which result in the formation of CFBr 3 and CHFBr 2 respectively.

The photolysis of neopentane and isobutane with 7.6, 8.4, and 10.0 eV photons

Abstract The photolysis of neopentane has been studied using photons of energies 7.6, 8.4, and 10.0 eV, at pressures in the range 1 – 760 Torr and in the liquid phase. Quantum yields of all molecular and radical products smaller than C 5 have been determined in the gas phase experiments, and have been estimated in the liquid phase. In contrast to results obtained with other alkanes studied to date, hydrogen elimination is found to be an unimportant process in the photolysis of neopentane. The two predominant primary processes are elimination of methane (neo-C 5 H 12 → CH 4 + iso-C 4 H 8 ) and direct CC bond cleavage (neo-C 5 H 12 → CH 3 + t-C 4 H 9 ). A fraction of the t-C 4 H 9 radicals dissociate further unless collisionally stabilized, either by loss of a H atom or by loss of a methyl radical (presumably preceded by an initial rearrangement to the isobutyl structure). With an increase in photon energy, the importance of direct bond cleavage increases at the expense of the methane elimination process. In the liquid phase, secondary decomposition processes are quenched, and the estimated quantum yields of primary processes are similar, at all energies, to those found in the 7.6 eV gas phase photolysis at high pressures. Quantum yields of molecular and radical products formed in the 7.6 and 8.4 eV photolysis of isobutane are also reported and are discussed briefly, with particular emphasis on the effect of energy on the mechanisms of the molecular elimination processes: (iso-C 4 H 10 → CH 4 + C 3 H 6 ) and (iso-C 4 H 10 → iso-C 4 H 8 + H 2 ). For both of these primary processes, the lower energy pathway, in which the olefin is formed directly, predominates at 7.6 eV, but diminishes in importance relative to the higher energy channel (presumably involving carbene formation) when the photon energy is increased.

Photolysis of Methyl Iodide in Matrices of Organic Compounds at 20° and 77°K. Reactions of Hot Methyl Radicals

Methyl iodide‐hydrocarbon and methyl iodide‐alcohol matrices have been photolyzed with 2537‐A radiation both at 20° and 77°K. The probability of abstraction of an H atom by the hot CH3 radical formed upon photodissociation of CH3I is at least a factor of 10 larger in the solid phase than in the gas phase. However, the relative probabilities of abstracting an H atom from various organic compounds are about the same in both phases. Also, hot CH3 radicals are more reactive than CD3 radicals in both phases. Photolysis of CH3I or CD3I in the presence of (a) equimolar mixtures of perprotonated and perdeuterated hydrocarbons and (b) partially deuterium labeled hydrocarbons shows that the hot methyl radical abstracts an H atom or D atom with equal probability. The hot methyl radical does, however, exhibit a certain selectivity in the site of reaction. For instance, a secondary H atom is abstracted more readily than a primary H atom. This nonstatistical behavior is, however, less important in the solid phase than ...

Gas Phase Pulse Radiolysis of Hydrocarbon Mixtures; Determination of the Charge Recombination Rate Coefficient and Absolute Rate Constants of Ion‐Molecule Reactions of the t‐Butyl Ion through a Competitive Kinetic Method

The gas phase pulse radiolysis of several neopentane‐d12‐alkane mixtures has been studied with the purpose of examining the competition between the reaction of the fragment t‐butyl ion with the alkane to form isobutane, t‐C4D9+ +RH2→ i‐C4D9H+RH+, and the neutralization of the t‐butyl ion by an electron or SF6− ion, t‐C4D9++X−→products. Using a computer calculation which takes into account the pulse characteristics as a function of time, the yield of isobutane observed in the reaction with 2,3‐dimethylbutane (for which krn was accurately determined to be 5.7 ± 0.5 × 10−11 cm3/molecule·sec) and the dose per pulse, rate coefficients of the competing neutralization reactions were determined. These are α=1.92 ± 0.2 × 10−6 cm3/molecule· sec for neutralization by an electron, and 0.40± 0.04 × 10−6 cm3/molecule· sec for neutralization by the SF6− ion. The latter value is independent of SF6 concentration and both are independent of pressure between 40 and 200 torr. Finally, these values of the neutralization rate ...

Ion–Molecule Reactions in the Radiolysis of Ethane

The reactions of ions generated in ethane irradiated with gamma rays have been studied by analyzing the neutral products formed in reactions with ethane and with other molecules. In experiments in the presence of added (C2D5)2CDCD3, for example, it is shown that the following reactions take place: C2H5++C2H6→(C4H11+)*→sec‐C4H9++H2; sec‐C4H9++(C2D5)2CDCD3→n‐C4H9D+C6D13+. The intermediate (C4H11+)* ion can be stabilized by collisions and will then undergo an undetermined reaction (neutralization or proton transfer) to give n‐C4H10 as a product. The over‐all rate constant for reaction of the ethyl ion with ethane is shown to be ≤ 10−10 cm3/molecule·sec. Similarly, it is demonstrated that the reaction C2H3++C2H6→C4H9+ leads predominantly to the formation of t‐butyl ions under these conditions: C4H9++(C2D5)2CDCD3→(CH3)3CD+C6D13+. Supplementary experiments performed in a photoionization mass spectrometer demonstrate that ethylene ions undergo a “resonance H2− transfer” reaction with ethane: C2H4++C2D6→C2D4++C2H...