H Atoms, H2, and

H Atoms, H2, and H3.—Reactions of ground-state H atoms with the following molecules and radicals have been reported HBr, HN3, C2H4, i-C4H8,  

372- 375 various fluorine- and bromine-containing compounds.Hot-atom studies of the H -I- CH3CI reaction have determined the apparent energy threshold for the process as 47 + 14kJmol and D atom reactions include measurements of the internal energy distribution in OD formed from D -t- N02, and of the rates and kinetic isotope effects in the D + CI2 and Br2 systems.Reaction (27) is believed to produce NF(a A) highly selectively, with a 

The contribution of the reaction channel (29) to the removal rate of O3 by H atoms has been estimated by several workers. High yields of 0( P) have 

Kajimoto, T. Kawajira, and T. Fueno, Chem. Pins. Lett., 1980, 76, 315. 

Sugawara, K. Okazaki, and S. Sato, Client. Pins. Lett., 1981, 78, 259. 

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Introducing the electron pulses into dilute aqueous solutions under anaerobic conditions causes, as stated above, primary changes in the solvent (12). In such experiments, water molecules undergo conversion mainly into OH radicals and hydrated electrons [e (aq)] and, to a lesser extent, H atoms, H2 and H2O2 molecules. 

Even the H atom itself can form compounds in which its coordination number (CN) is not just 1 (as expected) but also 2, 3, 4, 5 or even 6. A rich and unexpectedly varied coordination chemistry is thus emerging. We shall deal with the H atom first and then with the H2 molecule. 

Many of these compounds (MH ) show large deviations from ideal stoichiometry (n= 1, 2, 3) and can exist as multi-phase systems. The lattice structure is that of a typical metal with atoms of hydrogen on the interstitial sites for this reason, they are also called interstitial hydrides. This type of structure has the limiting compositions M H, M H2 and M H 3 the hydrogen atoms fit into octahedral or tetrahedral holes in the metal lattice or a combination of the two types (Figure 5.21). 

The initial ionization of a water molecule produces an electron and the water radical cation. The water radical cation is a strong acid and rapidly loses a proton to the nearest available water molecule to produce an HO radical and HsO. The electron will lose energy by causing further ionizations and excitations until it solvates (to produce the solvated electron Ca ). In addition to the two radical species HO and e q, a smaller quantity of H-atoms, H2O2, and H2 are also produced. 

About 20% of the HO radicals interact with the sugar phosphate by H-atom abstraction and about 80% react by addition to the nucleobases. In model sugar compounds, the H abstraction would occur evenly between the hydrogens on Cl, C2, C3, C4, and C5. In DNA, H abstraction occurs mainly at C4 since the C4 -H is in the minor groove and to some extent with the C5 -H2. 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

Much work has been done using the neutral residual H atoms (G 0.5). Allan, Robinson, and Scholes (1) measured G(H2) in alkaline solutions of 2-propanol, ethanol, or methanol, in the presence of 10 4 M acetone. In such a system, G(H2) can be used to determine the relative rate constants h+rh,/ h+oh-, where RH2 is the organic compound which forms H2 by reaction with H atoms. Nehari and Rabani (42) used a similar technique with different organic solutes. 

At pressures less than 1 Pa, hydrogen atoms are formed by heated wires of W, Pd, or Pt. Under such conditions, concentration hydrogen atoms up to 95% may be obtained using a discharge tube. The half-life of the H atom is about 1.0 s at 30 Pa, which allows it to be transported out of the discharge and its reactions examined. The presence of hydrogen atoms in such experiments is established by observation of the Balmer series of the H atom spectrum and the diminution in intensity of the H2 molecules band spectrum. The very long half-life shows that only about 1 in 10 collisions between atoms leads to recombination. 

Figure 5 Molecular graphs for adamarrtane (LHS) and He C6H10. The He atom is shown encased by its four interatomic surfaces. In the text a methine carbon is labelled Cl and its bonded H atom HI a methylene carbon is labelled C2 and its two bonded H atoms H2. Critical points are denoted by dots red for bond, yellow for ring and green for cage. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this book.)...

Cyclohexane and Benzene. At conventional dose rates the yields of the three major products decrease with increasing benzene concentration. At — 0.35M benzene the hydrogen yield is G(H2) — 3.0, a reduction of 2.5 G units (17). This is greater than the yield of thermal H atoms (11), and Stone and Dyne (17) have showed that the major part of this reduction is caused by some physical process such as energy transfer or charge transfer. 

In 1929, shortly after the emergence of quantum mechanics, Paul Dirac made his famous statement that in principle the physical laws necessary to understand all of chemistry were at that point known—the only difficulty was that their application to chemical systems generally led to equations that were too difficult to solve. Consequently, at that time quantnm principles could be rigorously applied only to simple atoms and molecules, such as H, He, H2+, and H2. 

These studies showed that water decomposes, yielding radicals (69) and molecular entities (5). Later studies have shown that the two molecular products, H2 and H202, are formed with different yields (30), Gh2 < Gh202 The radical products were assumed to be H and OH radicals (69). Later, it was shown that the reducing radical may exist in two forms (12), one being a H atom (20) and the other a hydrated electron (19, 24). It was also shown that an eaq reacts with H+ to yield a H atom (23, 43), while the opposite reaction—the conversion of H atoms by OH" into eaq—has been demonstrated as well (50). 

There is no parallel in the work of Sitharamarao (37) on the steady-state radiolysis of salicylate solutions to the observation by Sakumoto and Tsuchihashi (35) that diphenylcarboxylic acids are formed from benzoate solutions, but in view of the complexity of the analytical problems and the lack of total material balances this remains an open question. If indeed diphenylcarboxylic acids or their hydroxyl derivatives are formed in irradiated salicylate solutions, it may be that the H atom adduct and the protonated electron adduct behave differently, one giving dihydroxy-diphenyl, C02 and H2 and the other diphenylcarboxylic acids and water. More data are required on the steady-state radiolysis to clear up this point. 

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