Reactions of hydrogen atoms

In this section, we discuss reactions of hydrogen atoms, understanding that the isotopes of hydrogen are also included. A summary of various experiments measuring energy disposal in the products of hydrogen atom reactions is given in Table 1. 

Three-dimensional quantum mechanical calculations [219] on a semi-empirical surface [220] predict that the product rotational distributions 

There is indirect evidence from hydrogen maser studies [221] that the reactions H + H2, HD, D2 (v = 1) show a preference for resonant exchange reactions in the case of H + H2 (v = 1) and H + HD(i = 1) and for non-resonant exchange for H + D2 (v = 1) in accord with theoretical calculations [222]. With recent experimental developments, particularly UV lasers, it can be expected that spectroscopic methods will be applied to measuring energy disposal for these reactions. 

A preliminary account [261] of the observation of chemiluminescence due to electronically excited Li atoms from the reaction 

If there is no difference in fractional energy disposal with isotope, then a discrepancy arises between the molecular beam measurements of (Ft) and the values of (FT derived from the chemiluminescence experiments. For D + C12, the beams result is FX = 0.40 which is much lower than the chemiluminescence estimate, FX = 0.60, obtained under comparable initial conditions [196], No satisfactory explanation for this difference has yet been advanced. The good agreement between the H, D + Cl2 infrared chemiluminescence results measured by different groups suggests that a remeasurement of the product translational energy distribution might be required to resolve this discrepancy. 

The mean fraction of the available energy going into product vibration, (Fy , shows a minimum for the reaction H + CI2 in the sequence H + Fj (0.58), H 4- Cl2(0.38) and H + Br2(0.55). The molecular beam measurements of the fraction of the energy in product translation, iFi), show a uniform decrease in the sequence D + CI2 (0.44), D + Br2(0.31) and D +12(0.28). The energy disposal appears to be independent of the hydrogen isotope. The results for H and D -I- CI2 are virtually identical and, for H and D -I- F2, (Fy) = 0.58 [226, 227] and 0.62 [230], respectively. (The value for D + F2 comes from measured relaxation studies and is placed in some doubt by recent time-resolved infrared chemiluminescence mapping experiments [252] on D -I- F2 which give (Fy) = 

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Rettner C T 1992 Dynamics of the direct reaction of hydrogen atoms adsorbed on Cu(111) with hydrogen atoms incident from the gas phase Phys.Rev. Lett. 69 383... 

Formation of Hydrogen Tetroxide. The reaction of hydrogen atoms withHquid ozone at — 196°C proceeds through the intermediate formation of hydroperoxyl radicals forming hydrogen tetroxide, which decomposes on warming to produce equimolar amounts of and O2 (53). 

Alkoxyl radicals are very active in reactions of hydrogen atom abstraction (see Table 2.20). The problems of their reactivity will be discussed in Chapter 6. 

Another important characteristic of radical abstraction reactions is the force constants of the ruptured and the generated bonds. The dependence of the activation energy for the reactions of the type R + R X > RX + R1, where X = H, Cl, Br, or I, on the coefficients Ai and Af was demonstrated experimentally [17]. It was found that parameter re = const in these reactions, while the square root of the activation energy for a thermally neutral reaction is directly proportional to the force constant of the ruptured bond. The smaller the force constant of the C—X bond, the lower the Ee0, and the relationship Feo12 to A(1 I a) 1 is linear (see Figure 6.4). The same result was also obtained for the reactions of hydrogen atoms with RC1, RBr, and RI [17]. 

AEba = —45 kJ mol 1 for the HO + SiH4 reaction and AEba = —43 kJ mol-1 in the reaction of hydrogen atom with water. The repulsion of the electron orbitals of the atoms forming the reaction center AER plays an important role in all the radical abstraction reactions. In the interaction of radicals with molecules the contribution of this repulsion ranges from 25 to 46 kJ mol-1. In reactions of molecules with hydrogen atoms the contribution is naturally smaller, varying from 8 to 16kJ mol-1. 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table 6.12. Isomerization proceeds via the formation of a six-membered activated complex, and the activation energy for the thermally neutral isomerization of alkoxyl radicals is equal to 53.4 kJ mol-1. These parameters were used for the calculation of the activation energies for isomerization of several alkoxyl radicals via Eqns. (6.7, 6.8, 6.12) (see Table 6.14). The activation energies for the bimolecular reaction of hydrogen atom (H-atom) abstraction by the alkoxyl radical and intramolecular isomerization are virtually the same. 

The reaction of hydrogen atom abstraction by the alkylhydroxyperoxyl radical from alcohol limits chain propagation in oxidized alcohol [8,9]. 

FIGURE 16.1 The dependence of activation energy E on reaction enthalpy A He for reaction of hydrogen atom abstraction by aminyl radical from the C—H bond of alkylperoxyl radical and O—H bond of hydroperoxyl radical calculated by IPM method (see Chapter 6). The points fix the reactions with minimum and maximum enthalpy among known aromatic aminyl radicals. 

The reaction of hydrogen atoms with hydrazine has only been studied at temperatures between 25 and 200 °C53 54. At these temperatures the reaction proceeds via (5). This is confirmed by the observation that in the reaction D+N2H4 no NH2D could be detected this rules out reaction (4) at low temperatures. For the rate coefficient of reaction (5) Schiavello and Volpi54 quoted the Arrhenius expression... 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

THE REACTION OF HYDROGEN ATOMS WITH OXYGEN AND THE HYDROGEN CHLORINE REA CTION... 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

Difluorine is an extremely active reagent and reacts with organic molecules at low (200 and lower) temperatures. Such a high activity of difluorine is due to the very high BDE of the formed hydrogen fluoride molecule (-DF H = 570 kJ mol ) and the relatively low BDE in the difluorine molecule (DF F = 158.7 kJ mol-1). Due to this great difference in the BDE of the reactants and the products, bimolecular reactions of hydrogen atom abstraction... 

The formation of formaldehyde by reaction (17) is not consistent with the experimental data. H02 yielding peroxide is obtained besides formaldehyde. The reaction of the CH2OH radical with 02 was investigated63 under conditions comparable to those for the reaction O + CH3OH. The CH2OH radical was obtained by the reaction of hydrogen atoms with methyl alcohol. It was found that hydrogen atoms reacted with alcohol only, by abstraction of the H atom. The radical obtained yielded... 

Cashion and Polanyi83 and Clement and Ramsay96 examined the red emission from about 6000 to 10,000 A resulting from the chemiluminescent reaction of hydrogen atoms with nitric oxide. The former authors observed emission from both vibrationally excited (2.9-3.6 p.)... 

As discussed in the section concerning the reactions of hydrogen atoms with NO, the NO is an efficient catalyst for H2 formation. Thus, the HNO initially formed reacts with another H atom... 

D. Rates of Reaction of Hydrogen Atoms with Olefins. 158... 

Boddy and Robb have also studied the reactions of hydrogen atoms with propylene (11) and with 2-butene and isobutene (12). In all these reactions they observed decomposition of the hot alkyl radicals and also suggested an enhanced abstraction of hydrogen from the parent olefins by the hot radicals. The reaction with propylene appeared to be complicated by the ocurrence of a number of side reactions. One of the isolated products was 4-methyl-l-pentene, indicating the presence of allyl radicals, which the authors postulated to be formed in the reaction... 

In 1952 Darwent and Roberts (33) used an entirely different technique, by which rates of reaction of hydrogen atoms with olefins could be determined relative to the rate of abstraction from H2S. Hydrogen atoms were generated by photolysis of H2S and the reactions of interest were... 

Neta P (1972) Reactions of hydrogen atoms in aqueous solutions. Chem Rev 72 533-543 Neta P, Schuler RH (1972) Rate constants for reaction of hydrogen atoms with aromatic and heterocyclic compounds. The electrophilic nature of hydrogen atoms. J Am Chem Soc 95 1056-1059 Neta P, Fessenden RW, Schuler RH (1971) An electron spin resonance study of the rate constants for reaction of hydroqen atoms with organic compounds in aqueous solutions. J Phys Chem 75 1654-1666... 

C(s) + H, takes place at a temperature of about 950 °C. As in the previous case, the reaction of hydrogen atoms with solid carbon is fast at this temperature67). The decomposition of the hydrocarbons like CH-radical takes place by dissociative deexcitation of the hydrogen metastables44) ... 

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