Reactions with H atoms

Wojnarovits L, Takacs E, Dajka K, Emmi SS, Russo M, D Angelantonio M (2004) Re-evaluation of the rate constant for the H atom reaction with fm-butanol in aqueous solution. Radiat Phys Chem... 

H atoms reactions with solutes can be carried out alone below pH 2 in or Ar saturated solutions containing 0.2 - 1 mol I" tert-butanol. 

A number of investigators have studied the H-atom reaction with... 

H Atom Reaction with Residual Carbon or Carbon Species in Reactor... 

The experimental results on the HD/Ar system can be explained as follows. The H and D atoms initially formed in the irradiated HD/Ar sample may encounter an HD molecule located in a substitutional site of solid Ar in the course of their thermal migration. The D atoms may react with HD to form an H atom and a D2 molecule in an exothermic process (AG < 0). On the other hand the H atom reaction with HD is an endothermic process (AG > 0) and is unlikely to occur [77, 94] ... 

There are two main reaction types of H atom reactions with most organic molecules H atom abstraction (from saturated molecules) and H atom addition (to the double bonds of unsaturated molecules). For instance ... 

Alam MS, Rao BSM, Janata E (2001) A pulse radiolysis study of H atom reactions with aliphatic alcohols evaluation of kinetics by direct optical absorption measurement. Phys Chem Chem Phys 3 2622-2624... 

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

An overview of the time-dependent wavepacket propagation approach for four-atom reactions together with the construction of ab initio potential energy surfaces sufficiently accurate for quantum dynamics calculations has been presented. Today, we are able to perform the full-dimensional (six degrees-of-freedom) quantum dynamics calculations for four-atom reactions. With the most accurate YZCL2 surface for the benchmark four-atom reaction H2 + OH <-> H+H2O and its isotopic analogs, we were able to show the following ... 

It is interesting to contrast the rate ratio for reactions 10.1 and 10.4 where either H or D atoms react with H2 with that for reactions 10.1 and 10.7 where common H atoms react with either H2 or D2 (compare Figs. 10.1a and b). In the first case, (kH,HH/kD,HH), there is a ZPE difference in the transition state but not the ground state consequently the high temperature KIE is inverse. In the second (kH,HH/kH,DD), however, there are zero point energy differences in both the transition and ground states. We expect the vibrational force constants to be smaller in the more loosely bound transition as compared to the ground state. The isotope effects scale with the force constant differences. Consequently RT[ln(kH,HH/kH,DD)] =... 

The mechanism of H2 formation involves both unimolecular H2 elimination, when both of the H atoms of a hydrogen molecule originate from the same alkane molecule, and H atom elimination with subsequent H atom abstraction reaction. We show the mechanism on the example of cyclohexane photolysis hv = 7.6 eV) [91] ... 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

Besides the oxygen atom reactions discussed earlier, we studied those involving I.2-C2H4CI2,66 NH3,66 and acetylene, cyclohexane, and benzene. At first attempts were made to find an O atom reaction that would be similar and at the same time essential for all substances. This is the ease, for example, for hydrogen atoms and hydroxyl. Hydrogen reacts with saturated hydrocarbons by abstraction of the H atom, and with unsaturated hydrocarbons by addition as well. Hydroxyl is believed to react with hydrocarbons by abstraction of the H atom and formation of water. 

The basic conclusion on the mechanism of oxygen atom reactions is that the main primary steps of these reactions at temperatures up to 300 °C. are C—C, or C=C bond scissions. The formation of H20 and incorporation of the O atom at the C—H bond occurs less readily. The H atom abstraction with OH formation are least probable. 

Direct reaction between an adsorbed species A(s) and a gas-phase molecule B is sometimes proposed. This reaction pathway is called the Eley-Rideal mechanism. Although such a mechanism may seem as reasonable as the Langmuir-Hinshelwood model discussed above, very few heterogeneous reactions are still thought to occur by the Eley-Rideal mechanism. (An exception seems to be when species B is a very reactive radical species, e.g., a gas-phase H-atom reacting with an adsorbed species, as is discussed in Problem 11.10, in which an Eley-Rideal pathway initiates the growth process.)... 

At normal temperatures H atoms are very mobile on metal surfaces. We take this into account by the possibility of diffusion steps for the H atoms. A H atom jumps with rate D/z onto the nearest neighbour sites on the lattice. If this site is occupied by N, reaction occurs and an A particle (NH particle) is formed. The same holds if the site is occupied by A or B (NH2) where the products B or NH3 = 0 are formed, respectively. NH3 desorbs immediately from the surface and an empty site is formed. That is we deal with the diffusion-limited reaction system. It is important to note that all the reaction steps discussed above (with the exception of the N2-adsorption) are independent of 7/ and 7m. 

Paramagnetic species formed by reactions of the radiation-produced transient species in ice and frozen aqueous systems have been studied by ESR technique. The radiation-produced electrons have been found to react e.g. with acidic solutes to form H-atoms and with group 11(b) metal ions to give the corresponding univalent radical ions, while the holes can react with anions such as S04 2 and H2P04 giving the radical ions S04 and HP04. Evidence that the electron and hole are coupled to each other, and may in fact exist in irradiated pure ice primarily in an (exciton-like) bound state has been discussed. The present work provides evidence for the reactions of the radiation-produced positive holes apart from the reactions of the electrons. 

The yield of D-glutamic acid from poly-a,L-glutamic acid was very low in the presence of O2 (1 atm.). This can be accounted for by either of the mechanisms discussed above since O2 can combine both with H atoms and with the free radicals formed at the a-carbon atom (see Reactions 20-27). 

The reaction of silylene centers with dihydrogen has been studied in some detail346. It appears to proceed by a free-radical chain mechanism, initiated by radical sites on the silica surface which abstract hydrogen from H2. The H atoms react with the silylene center to give a free radical, and a reaction chain can then ensue (equations 103-105). The hydrogen addition to silylene centers is reversible the hydrogen is completely removed at temperatures near 1000 K. 

X.M. Yang, D.H. Zhang, Dynamical resonances in the fluorine atom reaction with the hydrogen molecule, Acct. Chem. Res. 41 (2008) 981. 

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