Reactions of thermal hydrogen atoms in ethane and propane at 10 K: Secondary site selectivity in hydrogen abstraction from propane

1994 ◽  
Vol 100 (11) ◽  
pp. 8010-8013 ◽  
Author(s):  
Frank J. Adrian ◽  
Joseph Bohandy ◽  
Boris F. Kim
Keyword(s):  
Hydrogen Abstraction ◽  
Secondary Site ◽  
Hydrogen Atoms ◽  
Site Selectivity

Quantum chemistry calculation of reaction pathways of carboxyl groups during coal self-heating

2017 ◽  
Vol 95 (8) ◽  
pp. 824-829 ◽  
Author(s):  
Xuyao Qi ◽  
Haibo Xue ◽  
Haihui Xin ◽  
Ziming Bai
Keyword(s):  
Quantum Chemistry ◽  
Oxidation Process ◽  
Hydrogen Abstraction ◽  
Thermal Parameters ◽  
Reaction Pathways ◽  
Carboxyl Groups ◽  
Heating Process ◽  
Hydrogen Atoms ◽  
Self Heating ◽  
Quantum Chemistry Calculations

During coal self-heating, reactions of carboxyl groups feature in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways during this process still have not been revealed. This paper selected the Ar–CH2–COOH as a typical carboxyl group containing structure for the analysis of the reaction pathways and enhancement effect on the coal self-heating process by quantum chemistry calculations. The results indicate that the hydrogen atoms in carboxyl groups are the active sites, which undergo the oxidation process and self-reaction process during coal self-heating. They both have two elementary reactions, namely (i) the hydrogen abstraction of –COOH by oxygen and the decarboxylation of the –COO· free radical and (ii) the hydrogen abstraction of –COOH and its pyrolysis. The total enthalpy change and activation energy of the oxidation process are 76.93 kJ/mol and 127.85 kJ/mol, respectively, which indicate that this process is endothermic and will occur at medium temperatures. For the hydrogen abstraction of –COOH by hydrocarbon free radicals, the thermal parameters are 53.53 kJ/mol and 56.13 kJ/mol, respectively, which has the same thermodynamic properties as the oxidation process. However, for the pyrolysis, the thermal parameters are –42.53 kJ/mol and 493.68 kJ/mol, respectively, and is thus exothermic and would not occur until the coal reaches high temperatures. They affect heat accumulation greatly, generate carbon dioxide, and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

Reaction pathways of hydroxyl groups during coal spontaneous combustion

2016 ◽  
Vol 94 (5) ◽  
pp. 494-500 ◽  
Author(s):  
Xuyao Qi ◽  
Haibo Xue ◽  
Haihui Xin ◽  
Cunxiang Wei
Keyword(s):  
Activation Energy ◽  
Free Radicals ◽  
Hydrogen Abstraction ◽  
Enthalpy Change ◽  
Reaction Pathways ◽  
Hydroxyl Groups ◽  
Hydrogen Atoms ◽  
Typical Structure ◽  
Self Heating ◽  
Hydroxyl Free Radicals

Hydroxyl groups are one of the key factors for the development of coal self-heating, although their detailed reaction pathways are still unclear. This study investigated the reaction pathways in coal self-heating by the method of quantum chemistry calculation. The Ar–CH2–CH(CH3)–OH was selected as a typical structure unit for the calculation. The results indicate that the hydrogen atoms in hydroxyl groups and R3–CH are the active sites. For the hydrogen atoms in hydroxyl groups, they are directly abstracted by oxygen. For hydrogen atoms in R3–CH, they are abstracted by oxygen at first and generate peroxy-hydroxyl free radicals, which abstract the hydrogen atoms in hydroxyl groups later. The reaction of R3–CH contains three elementary reactions, i.e., the hydrogen abstraction of R3–CH by oxygen, the conjugation reaction between the R3C■ and oxygen atom, and the hydrogen abstraction of –OH by hydroxyl free radicals. Then, the microstructure parameters, IRC pathways, and reaction dynamic parameters were respectively analyzed for the four reactions. For the hydrogen abstraction of –OH by oxygen, the enthalpy change and activation energy are 137.63 and 334.44 kJ/mol, respectively, which will occur at medium temperatures and the corresponding heat effect is great. For the reaction of R3–CH, the enthalpy change and the activation energy are −3.45 and 55.79 kJ/mol, respectively, which will occur at low temperatures while the corresponding heat influence is weak. They both affect heat accumulation and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

Laser Photochemical Vapour Deposition of Epitaxial Ge Films on GaAs from GeH4 Using NH3 as a Sensitiser

1988 ◽  
Vol 129 ◽  
Author(s):  
C.J. Kiely ◽  
C. Jones ◽  
V. Tavitian ◽  
J.G. Eden
Keyword(s):  
Growth Rate ◽  
Photoelectron Spectroscopy ◽  
Vapour Deposition ◽  
Film Growth ◽  
Hydrogen Abstraction ◽  
Auger Spectroscopy ◽  
Hydrogen Atoms ◽  
Production Of Hydrogen ◽  
Substrate Temperatures ◽  
Secondary Ion

ABSTRACTThe viability of ammonia as a sensitiser for the epitaxial growth of Ge on GaAs by laser photochemical vapour deposition (LPVD) has been investigated. Specifically NH3/GeH4/He (0.8/5/95 sccm, 5.5 Torr total pressure) mixtures have been irradiated by a 193nm ArF excimer laser in parallel geometry for substrate temperatures, Ts<400°C. As evidenced by a dramatic acceleration in Ge film growth rate, the NH3 efficiently couples the laser radiation to the GeH4 precursor molecule. The microstructures of LPVD Ge films grown with and without NH3 have been examined by TEM, and the epitaxial nature of both types of films has been verified, although some subtle differences are noted. Chemical analysis of the deposited films has been carried out using Auger spectroscopy, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy. Our results show that there is little or no nitrogen incorporation into the Ge films grown in the presence of NH3, and that hydrogen contamination in our films is minimal. The beneficial effect of NH3 on the growth rate of LPVD Ge films is attributed to the photolytic production of hydrogen atoms which efficiently decompose GeH4 by hydrogen abstraction collisions.

The Mercury-Sensitized Photolysis of Pentamethyldisilane

1996 ◽  
Vol 51 (1-2) ◽  
pp. 105-115 ◽  
Author(s):  
C. Kerst ◽  
P. Potzinger ◽  
H. Gg. Wagner
Keyword(s):  
Quantum Yield ◽  
Rate Constants ◽  
Hydrogen Abstraction ◽  
Branching Ratios ◽  
Bond Breaking ◽  
Hydrogen Atoms ◽  
Substitution Reactions ◽  
Abstraction Reaction

Abstract Two primary processes were observed in the Hg-sensitized photolysis of Me 5 Si 2 H: (I) hydrogen abstraction from the Si-H bond with a quantum yield of 0(1) = 0.85, (V) Si-Si bond breaking with 0(V) = 0.04. The hydrogen atoms formed in (/) undergo an H atom abstraction reaction (k(3)), as well as substitution reactions at the Si centers resulting in the formation of dimethylsilane and trimethylsilyl radical (k(4)) or trimethylsilane and dimethylsilyl radical (k(5)). The following branching ratios have been determined:[xxx]The ratio of disproportionation (k(2)) to combination (k(1)) for the pentamethyldisilyl radical has been determined with MeOH as the scavenger for 1-methyl-l-trimethylsilylsilene, 0.046 < k(2)/A: C1) < 0.071. A mechanism with pertinent rate constants has been proposed which accounts for theresults.

Site of H Atom Attack on Uracil and Its Derivatives in Aqueous Solution

1985 ◽  
Vol 40 (3-4) ◽  
pp. 292-294 ◽  
Author(s):  
Suresh Das ◽  
David J. Deeble ◽  
Clemens von Sonntag
Keyword(s):  
Aqueous Solution ◽  
Double Bond ◽  
Pulse Radiolysis ◽  
Hydrogen Abstraction ◽  
Methyl Groups ◽  
Oh Radicals ◽  
Hydrogen Atoms ◽  
Sugar Moiety ◽  
Reducing Properties

Hydrogen atoms from the radiolysis of water at pH 1.6 add to the 5,6-double bond of pyrimidines. The preferen­tial site of attack is the C(5) position (values in brackets) in the case of 6-methyluracil (87%), 1,3-dimethyluracil (71%), uracil (69%) and poly(U) (60%). This reaction yields a radical of reducing properties which can be monitored by its reaction with tetranitromethane in a pulse radiolysis experiment. In thymine (37%), thymidine (32%) and 1,3-dimethylthymine (25%) H-addition no longer pre­ferentially occurs at C(5), but addition is now mainly at C(6). Hydrogen abstraction from the methyl groups or the sugar moiety is negligible (≦ 5.5%). A comparison is made with literature values for the equivalent reactions of OH radicals.

The Reaction of Hydrogen Atoms with Silyl Radicals; the Decomposition Pathways of Chemically Activated Silanes

1983 ◽  
Vol 38 (8) ◽  
pp. 896-908 ◽  
Author(s):  
K. Wörsdorfer ◽  
B. Reimann ◽  
P. Potzinger
Keyword(s):  
Vibrational Energy ◽  
Flow Reactor ◽  
Hydrogen Abstraction ◽  
Hydrogen Atoms ◽  
Experimental Conditions ◽  
Silyl Radicals ◽  
Subsequent Step ◽  
Temperature Rate ◽  
Fast Flow ◽  
Fast Flow Reactor

Abstract The reactions of hydrogen atoms with silane and the methylated silanes - with the exception of tetramethylsilane -have been investigated in a fast flow reactor. Under our experimental conditions hydrogen abstraction from the Si-H bond is followed by combination of hydrogen atoms with the corresponding silyl radicals. The molecules formed in this way are activated by about 375 kJ/mol of vibrational energy. Two decomposition channels have been unequivocally identified, namely the elimination of molecular hydrogen and of methane, both with concomittant formation of the respective silylenes. In a subsequent step, silylene inserts into the substrate under formation of disilanes. With increasing degree of methylation. stabilization of the activated molecule competes with decomposition and dominates the kinetics in the case of trimethylsilane. With methyl -and dimethyl-silane, methyl radicals are observed as an additional reaction product. On the basis of RRKM calculations it is unlikely that they originate from a direct decomposition of the activated molecules.Absolute values for the room temperature rate constants of the abstraction reactions are given; for H+CH3SiH3, Arrhenius parameters have been determined.

Studies of the reactions of ethynyl radicals with hydrocarbons

1973 ◽  
Vol 335 (1603) ◽  
pp. 525-545 ◽  
Keyword(s):  
Nitric Oxide ◽  
Hydrogen Abstraction ◽  
Inert Gas ◽  
Translational Energy ◽  
Methyl Radical ◽  
Hydrogen Atoms ◽  
Kinetic Measurements ◽  
Primary Products ◽  
Ethynyl Radical ◽  
Nitrogen Mixture

It has been shown that ethynyl radicals may be satisfactorily generated by the photolysis, at 253.7 nm, of bromoacetylene in the presence of nitric oxide. Acetylene and butadiyne are primary products, being formed exclusively by the reactions C 2 H . + C 2 HBr→C 2 H 2 + C 2 Br . , C 2 H . + C 2 HBr→C 4 H 2 + Br . . Nitric oxide decreases the rates of formation of both products, indicating the effective scavenging of ethynyl radicals by this compound. Addition of an inert gas (nitrogen or carbon dioxide) increases the ratio [C 4 H 2 ]/[C 2 H 2 ] from 3.5 (no inert gas) to 7 (total pressure 80 kPa (1 Pa = 1 N m -2 )), the ratio thereafter remaining constant. The most obvious explanation for this behaviour is that, during photolysis, ethynyl radicals produced in the absence of inert gas have excess translational energy and, probably, enhanced reactivity. With increasing inert gas pressure, fewer ‘hot’ radicals react and the change in the ratio [C 4 H 2 ]/[C 2 H 2 ] reflects the change in selectivity of ‘thermalized’ ethynyl radicals. On account of this, investigations of the reactions of C 2 H . with added hydrocarbons were carried out with a standard 1:1:100 bromoacetylene-nitric oxide-nitrogen mixture. Results obtained with added alkanes (methane, ethane, 2,2 dimethylpropane) showed that ethynyl radicals abstract hydrogen atoms to form acetylene: C 2 H . + RH→C 2 H 2 + R . , The relative importance of reactions (1) and (2) has been estimated and values for k 1 / k 2 of 0.016 ± 0.005, 0.54 ± 0.04 and 0 .91 ± 0.04 have been obtained for methane, and ethane 2,2-dimethylpropane respectively. The ratio k 1 / k 2 did not vary over the temperature range 298 to 478 K in the case of 2,2-dimethylpropane but with methane, values for E 1 — E 2 and A 2 / A 1 of 12.54 ± 1.27 kJ mol -1 and 0.54 ± 0.25, respectively, were obtained. Studies of the reactions of ethynyl radicals with alkynes (acetylene, butadiyne and propyne) have shown that the radicals abstract hydrogen atoms (to form acetylene), displace hydrogen atoms (to form a di- or triyne) and, in the case of propyne, displace a methyl radical. For propyne, the relevant reactions are C 2 H . + C 3 H 4 →C 2 H 2 + C 3 H 3 . , C 2 H . + C 3 H 4 →C 4 H 2 + CH 3 . , C 2 H . + C 3 H 4 →C 5 H 4 + H . , and Values of 25 ± 3, 5 ± 2, 9.9 ± 1 and 23 ± 3 at 298 K have been obtained for k 7 / k 9 , k 4 / k 9 , k 8 / k 9 and k 2 / k 9 respectively. In the presence of butadiyne, acetylene and hexatriyne are formed as primary products. Acetylene is formed by reactions (4) and (13), C 2 H . +C 4 H 2 → C 2 H 2 + C 4 H . , whilst hexatriyne is formed by the displacement reaction (14) C 2 H . + C 4 H 2 →C 6 H 2 +H . . Kinetic measurements have shown that at 298 K k 4 / k 14 =0.6 ± 0.1 and k 13 / k 14 = 1.1 ± 0.2. Addition of acetylene-d 2 to bromoacetylene-nitrogen mixtures yields acetylene-d 1 and butadiyne-d 1 C 2 H . + C 2 D 2 → C 2 HD +C 2 D . , C 2 H . + C 2 D 2 → C 4 HD + D . . The rate-constant ratios k 12 / k 11 and k 2 / k 12 are 2 .8 ± 2.5 and 1.5 ± 0.3 respectively. This work thus indicates that ethynyl radical addition-elimination reactions, leading to polyalkynes, occur to a comparable extent to hydrogen-abstraction reactions in acetylene-containing systems. These results are shown to be of significance in regard to the formation and subsequent reactions of polyalkynes in both the pyrolysis and flames of acetylene and other hydrocarbons.

DEUTERIUM ISOTOPE EFFECTS IN ABSTRACTION OF HYDROGEN ATOMS FROM PHENOLS

1962 ◽  
Vol 40 (4) ◽  
pp. 701-704 ◽  
Author(s):  
R. A. Bird ◽  
G. A. Harpell ◽  
K. E. Russell
Keyword(s):  
Isotope Effect ◽  
Rate Constants ◽  
Isotope Effects ◽  
Hydrogen Abstraction ◽  
Degree Of Polymerization ◽  
Hydrogen Atoms ◽  
Transfer Constant ◽  
Deuterium Isotope Effects

The effect of six deuterated phenols on the rate and degree of polymerization of styrene has been studied. The rate and degree of polymerization are decreased by deuterated phenols to a much less extent than by the corresponding phenols. Approximate transfer constants are estimated, and it is found that the transfer constant for hydrogen abstraction from the deuterated phenol is less than 0.2 of the transfer constant for the normal phenol. The rates of reaction of 2,2-diphenyl-1-picrylhydrazyl with three deuterated phenols have been determined. The rate constants for deuterated 2,6-di-t-butylphenol and 4-bromophenol are less than 0.15 of those for the corresponding phenols, but the isotope effect appears to be small with 4-nitrophenol.

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