James R. McNesby

Flash Photolysis of Methane in the Vacuum Ultraviolet. II. Absolute Rate Constants for Reactions of CH with Methane, Hydrogen, and Nitrogen

By means of kinetic spectroscopy, the concentration of CH has been measured in the flash photolysis of methane. Measurements were made by following the attenuation of C 2Σ+←X 2Π Q branch at 3143 A, where the disappearance of CH is of the first order in CH. Experiments were conducted with pure methane, methane+H2, and methane+N2. The reactions and the corresponding rate constants in mole−1 cm3 sec−1 are CH+CH4→C2H4+H,k=1.5×1012;CH2+H2→CH3,k=6.2×1011;CH+N2→products,k≃4.3×1010;CH+CH→C2H2,k≃1.2×1014.

Vacuum Ultraviolet Photochemistry. III. Primary Processes in the Vacuum Ultraviolet Photolysis of Water and Ammonia

Water and ammonia have each been photolyzed in the absence and presence of C2D4 which served to scavenge H atoms. Wavelengths used were: for ammonia, 1849 and 1236 A; for water, 1236 A. Under conditions where H atoms are efficiently scavenged by C2D4, the production of H2 signifies a primary photochemical process giving molecular H2 directly. It is found that at 1849 A, ammonia decomposes almost entirely to H+NH2. At 1236 A, two primary processes are observed. (a)NH3→H2+NH,(b)NH3→H+NH2. Process (a) is about ⅙ as probable as process (b). At 1236 A, the photolysis of water proceeds via two primary processes. (c)H2O→H+OH,(d)H2O→H2+O. The probability of process (c) being three times that of process (d). It is suggested that primary process (d) constitutes a reasonable photochemical mechanism for hydrogen formation in the earths upper atmosphere.

Vacuum Ultraviolet Photochemistry. II. Photolysis of Ethylene

The direct photolyses of CH2CD2 and trans‐CHDCHD were carried out at room temperature in the vacuum ultraviolet region. Kr, Xe, and Hg resonance radiations were used for photolyses. Products at the Kr lines were mainly hydrogen, acetylene, ethane, and n‐butane. The ratio of acetylene/hydrogen was 2.8±0.2 at 1236 A. The photolysis of CH2CD2 gives 40% H2, 40% HD, and 20% D2. The acetylene was 12% C2H2, 62% C2HD and 26% C2D2. The relative amounts of H2, HD and D2 depend upon whether a hydrogen molecule is eliminated from two (type I process) or one (type II process) carbon atom. The remaining vinylidene rapidly rearranges to acetylene.The photolysis of trans‐CHDCHD gives 17% H2, 73% HD, and 10% D2. This shows that hydrogen is formed from the trans as well as from the cis position and that the isotope effect for the type I process is slightly smaller than that for the type II process. Nearly free rotation of the molecule in its excited state is suggested. The percentages of the hydrogen isotopes produced are almost independent of exciting wavelength in both isotopic ethylenes. A comparison is made with Hg(3P1)‐sensitized photolysis.The direct photolyses of CH2CD2 and trans‐CHDCHD were carried out at room temperature in the vacuum ultraviolet region. Kr, Xe, and Hg resonance radiations were used for photolyses. Products at the Kr lines were mainly hydrogen, acetylene, ethane, and n‐butane. The ratio of acetylene/hydrogen was 2.8±0.2 at 1236 A. The photolysis of CH2CD2 gives 40% H2, 40% HD, and 20% D2. The acetylene was 12% C2H2, 62% C2HD and 26% C2D2. The relative amounts of H2, HD and D2 depend upon whether a hydrogen molecule is eliminated from two (type I process) or one (type II process) carbon atom. The remaining vinylidene rapidly rearranges to acetylene.The photolysis of trans‐CHDCHD gives 17% H2, 73% HD, and 10% D2. This shows that hydrogen is formed from the trans as well as from the cis position and that the isotope effect for the type I process is slightly smaller than that for the type II process. Nearly free rotation of the molecule in its excited state is suggested. The percentages of the hydrogen isotopes produced are ...

Vacuum Ultraviolet Photolysis of Ethane: Molecular Detachment of Hydrogen

The primary process of direct ethane photolysis by Xe radiation (1470 A and 1295 A) was studied at room temperature. The hydrogen and methane isotopic compositions from a mixture of C2H6—C2D6 and from CH3CD3 were measured mass‐spectrometrically. The results show that, contrary to the previously proposed mechanism, almost all (>95%) of the hydrogen is formed intramolecularly, and preferentially from the same carbon atom. Methane is also formed by a molecular process. Then the primary processes may be written as C2H6→CH3CH+H2 C2H6→C2H4+H2 C2H6→CH4+CH2. The absence of C2D6 in the products of the photolysis of CH3CD3 places an upper limit on [open phi]CD3/[open phi]CD3H+CH3D of about unity assuming all methyl radicals form ethane by recombination. The effect of the product ethylene on the composition of hydrogen isotopes from C2H6—C2D6 mixtures was examined, since any atomic hydrogen produced may be reacting rapidly with ethylene. The sample was purified and very low conversion (0.01%) was made. The result sh...

Flash Photolysis of Methane in the Vacuum Ultraviolet. I. End‐Product Analysis

Methane and mixtures of methane and methane‐d4 have been subjected to flash photolysis in the vacuum ultraviolet. Ethylene is the major hydrocarbon product. Isotopic distributions of the hydrogen, ethane, and ethylene fractions as well as complete product analysis suggest that CH plays a dominant role in the photolysis. The CH is formed either by direct dissociation of excited methane or secondary flash photolysis of CH2 in the flash. On the basis that acetylene is formed with unit efficiency by association of CH and ethylene is formed by reaction of CH with methane, the collision yield of the latter reaction is about 1/80.

Vacuum‐Ultraviolet Photolysis of Ethane at High Temperature

The rates of production of HD in the photolysis of C2H6+C2D6 at 1470 and 1236 A at temperatures sufficiently high to eliminate internal scavenging have been used as a measure of the relative importance of molecular elimination and atomic elimination processes. This, together with complete product analysis including isotopic determinations, gives the following relative weights for the primary processes 1470 A1236 AC2H6*→H2+C2H4†1.001.00C2H6*→2H+C2H40.150.88C2H6*→CH4+CH20.020.50. The probability of dissociation of C2H4† into H2 and C2H2 increases relative to collisional stabilization by a factor of about 2 when the energy of the exciting photon is increased from 1470 to 1236 A.

Vacuum Ultraviolet Photochemistry. IV. Photolysis of Propane

The direct photolysis of propane has been carried out in the vacuum ultraviolet region, at room temperature. Isotopic analyses lead to the conclusion that hydrogen and methane are formed almost entirely by molecular detachment processes. Among the three observed modes of hydrogen detachment CH3CH2CH3→CH3CCH3+H2, CH3CH2CH3→CHCH2CH3+H2, CH3CH2CH3→CH2CHCH3+H2. Process (1), the detachment from the central carbon atom, is most important when the exciting wavelengths are the Xe resonance lines, whereas process (2) increases in importance with the Kr lines. Two modes of methane detachment were observed, CH3CH2CH3→CH3CH+CH4, CH3CH2CH3→CH2CH2+CH4, in which process (5) is more important at both wavelengths. It has been shown that ethylene is formed almost equally from the molecular detachment process and from the disproportionation of of ethyl radicals. Ethane is formed from the disproportionation of ethyl radicals as well as from the association of methyl radicals, the former being more important.The processes (1)...

Photolysis of Methane at 1236‐Å: Quantum Yield of Hydrogen Formation

Pure CH4 and CD4 have been separately but simultaneously photolyzed at 1236 A by means of a split‐cell technique in order to obtain accurate relative quantum yields for the various modes of production of H2 and D2. The split‐cell technique has been used to obtain quantum yields by comparison with the CO2 actinometer. Molecular elimination yields for CD4 and CH4 (0.58) are found to be equal, while the H‐atom yield greatly exceeds the D‐atom yield. It is concluded from the results that a significant fraction of the atoms produced disappears by the association reaction M + H + CH3→CH4 + M. The effect of scavengers and inert gas also have been investigated. The total quantum yield for H and H2 formation is greater than unity and fragmentation of CH2 or CH3 formed in the primary process is suggested: CH3→H + CH2, CH2→H + CH. However, the lower limit for the quantum yield for D and D2 formation is 0.76 and indicates that fragmentation of CD2 or CD3 is less important than that of the corresponding protonated spe...

Mechanism of the Photolysis of Ethane at 1470 Å

A complete analysis of the products of the photolysis of ethane at 1470 A has been carried out. Isotopic analyses of products of the photolysis of C2H6+C2D6 and of CH3CD3 have led to the following conclusions.(1) It is possible to explain the data on the basis that nearly all molecular hydrogen produced in the primary process comes from the end carbon atom. CH3CD3→H2+CHDCD2*,CH3CD3→D2+CH2CHD*. The HD can be accounted for by the decomposition of excited CHDCD2* and CH2CHD*. CH2CHD*→HD+CHCH,CD2CHD*→HD+CDCD. (2) Other primary processes are CH3CD3→CH3+CD3,CH3CD3→H+D+CH2CD2. The importance of the latter, relative to the molecular elimination, is about 15:85.(3) Propane and butane arise from ethyl radicals formed by addition of H and D atoms to ethylene.(4) The excited state of ethane has a lifetime smaller than 10—9 sec.

Vacuum‐Ultraviolet Photolysis of Ethane at High Temperature. II. Collisional Deactivation of Excited Ethylene

The competition between decomposition and collisional deactivation of the excited ethylene produced by the photolysis of C2H6 at 1470 A has been studied. The pressure of C2H6 was varied in two series of photolyses at 25° and 310°C. Also, varying amounts of N2 and Ar were added to a fixed pressure of C2H6 in two series of photolyses at 310°C. The results are interpreted in terms of two excited states of ethylene, of which only one is capable of dissociating and has a dissociative lifetime of 6×10−10 sec (assuming unit efficiency for collisional stabilization). Nitrogen and argon are, respectively, 30% and 20% as effective as C2H6 in removing excess energy from excited ethylene.