Atomic H generation

Thus, the reduction of n-BuI is the equivalent of the addition of two hydrogen atoms [H ] (generated via the electrochemical reduction of the two hydronium ions). 

Treating the metals or the monohydrides with 10% HF yields NbHj and VHj. Atomic hydrogen is generated from the metal with HF and subsequently is absorbed by the metal or the monohydride. Atomic H generated by other sources, such as the thermal dissociation of Hj on heated W filaments, also may be used, but the hydrogen concentration is difficult to regulate. 

The diffusion of H and D atoms in the molecular crystals of hydrogen isotopes was explored with the EPR method. The atoms were generated by y-irradiation of crystals or by photolysis of a dopant. In the H2 crystals the initial concentration of the hydrogen atoms 4x 10 mol/cm is halved during 10 s at 4.2 K as well as at 1.9 K [Miyazaki et al. 1984 Itskovskii et al. 1986]. The bimolecular recombination (with rate constant /ch = 82cm mol s ) is limited by diffusion, where, because of the low concentration of H atoms, each encounter of the recombinating partners is preceded by 10 -10 hops between adjacent sites. 

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

Device quality a-Si H made by HWCVD (as they termed it) was first reported by Mahan et al. [19, 527], They obtained n-Si H with hydrogen concentrations as low as 1%. Deposition rates as high as 5 nm/s [528] and 7 nm/s [529] have been achieved for n-Si H of high quality. In order to obtain device quality material it was shown by Doyle et al. [525] that the radicals that are generated at the filament (atomic Si and atomic H) must react in the gas phase to yield a precursor with high surface mobility. Hence, the mean free path of silane molecules should be smaller than the distance between filament and substrate, d(s- Too many reactions between radicals and silane molecules, however, result in worse material. In fact, optimal film properties are found for values of pdf of about 0.06 mbar-cm [530, 531]. 

Nitrogen oxides. Nitric oxide (NO) itself, has been shown to be a poor nitrosating agent (28), probably because it is unable to abstract an amino-H atom to generate the dialkyl-amino radical, which might then combine with further NO. However, the presence of even a small amount of air results in complete conversion, presumably via oxidation of NO to NO2. Nitrosation by NO is catalyzed by metal salts, such as Znl2, CuCl, and CUSO4. The metal catalyzed reaction is inhibited in acid or aqueous media (29). 

Highly efficient C-H insertion adjacent to oxygen atoms to generate /9-hydroxy esters has been demonstrated [130, 143, 144]. Reactions with cyclic oxygenated systems have not been extensively explored with these carbenoids and only the reaction with tetrahy-drofuran has been reported [130]. The major sy -diastereomer 218 from the reaction of methyl phenyldiazoacetate with tetrahydrofuran at -50 °C was obtained in 67% yield with 97% enantiomeric excess (Eq. 29). 

Alkylidene borane 360 undergoes reaction with BH3-THF 361 to yield a dimerized cycloaddition product 362. BH3 hydroborates the B—C bond in two molecules of methylidene borane 360. The regioselectivity of hydroboration is governed by the electronic factors which facilitate the attack of boron (from BH3) on the two terminal carbon atoms, thus generating the B-C-B-C-B chain. The final product is obtained by the binding of two boryl ends via two B-H-B three-center, two-electron bridged bonds (Equation 20) <2004ZFA508>. 

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