Hydrogen atoms, addition

Perhaps the most fruitful of these studies was the radiolysis of HCo(C0)4 in a Kr matrix (61,62). Free radicals detected in the irradiated material corresponded to processes of H-Co fission, electron capture, H-atom additions and clustering. Initial examination at 77 K or lower temperatures revealed the presence of two radicals, Co(C0)4 and HCo(C0)4 , having similar geometries (IV and V) and electronic structures. Both have practically all of the unpaired spin-density confined to nuclei located on the three-fold axis, in Co 3dz2, C 2s or H Is orbitals. Under certain conditions, a radical product of hydrogen-atom addition, H2Co(C0)3, was observed this species is believed to have a distorted trigonal bipyramidal structure in which the H-atoms occupy apical positions. 

Stabilization of the (incipient) C 6) carbocation developing in the C-O heterolysis. The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidine-6-yl radicals. On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition [27]. 

The poly (amino acid)s with aromatic side chains behave somewhat differently. In poly(phenylalanine) the a-carbon radical is the major radical species observed, but radicals formed by hydrogen atom addition to the ring are also found. Benzyl radicals formed by side-chain cleavage are present, but only in very low yield. In poly (tyrosine) the only radical species observed is the tyrosyl phenoxyl radical formed by loss of the hydroxyl hydrogen. There is no evidence for formation of significant concentrations of a-carbon radicals. Thus, the nature of the substituents can strongly influence the radiation sensitivity of the backbone chain. 

The formation of the n-propyl radical from propylene suggests that this hydrogen transfer should proceed through some energetic process caused by silent discharge because a normal hydrogen atom addition to propylene proceeds almost exclusively to the terminal carbon (15). Thus, two molecular processes similar to those in the reactions of cyclohexene may be plausible to interpret the hydrogen transfer. 

Removal of hydrogen atoms Addition of hydrogen atoms... 

Racemization of brinzolamide to the S isomer occurs under heat and light (pH independent) conditions (Fig. 113). This can occur via a radical mechanism from radical formation at the chiral center to form a resonance stabilized planar radical with hydrogen atom addition occurring on both sides of the planar carbon centered-radical to racemize the stereocenter (163). 

The proposed pathway is supported by the results obtained from Hg /NH3 and cyclohexane. The major amine product is the interesting tertiary amine shown. This probably results from recombination of cyclohexyl with the radical derived from hydrogen atom addition to the imine (Scheme 5). 

In such an insertion the benzophenone is excited to its triplet state, resembling a C-0 diradical. The oxygen atom of the ketone then removes a hydrogen atom from the methylene group, leading to a carbinyl radical and the hydroxy diphenylmethyl radical formed by hydrogen atom addition to the benzophenone. This pair of radicals couples... 

Relative Rates of Hydrogen Atom Addition to Asymmetric Olefinio Carbon... 

The rate of hydrogen atom addition to alkenes grows in the order ethylene < propene 1-butene < 2-methylpropene. 

Given the /3-scission hypothesis, an examination of possible dodecenyl radicals indicates no reasonable mechanism of formation of Ci0H20 or CnH22. These products are formed only from the radical addition path. They can be used then to predict the relative importance of this path, XA. Decene is used to do this since it is better separated by GC from the corresponding diolefin than is undecene. XA is given by the ratio of the predicted value for decene from Table IV to the actual experimental yield. (Values for hydrogen atom addition are used here they are fairly close to those for CH3 and C2H5- addition, and available rate data (13, 14) indicate that H- atoms are the most likely radicals to add to dodecene.)... 

Beckert D, Mehler K. (1983) Investigations of hydrogen atom addition to vinyl monomers by time resolved ESR spectroscopy. Ber Bunsen es Phys Chem 87 587-591. 

Typical theoretical results obtained for a wide variety of base radicals will be represented through a discussion of the results for five radicals (Table 1). The first data block in Table 1 represents the experimental and theoretical HFCCs obtained for the radical formed through net hydrogen atom addition to C5 in... 

Table 5 Comparison of theoretical and experimental isotropic HFCCs (G) in radicals formed by net hydrogen atom addition to a double bond in the DNA bases."...

Despite great efforts put forth by experimentalists, the exact identity of most radical products generated in irradiated DNA is still unknown. The first ESR studies on fuU DNA only provided evidence for the formation of a thjonine centered radical [54,55]. Work performed on DNA irradiated by ultraviolet light [56] and on oriented fibers [57] confirmed this radical to be formed through net hydrogen atom addition to C6 in thymine [T(C6H)]. The definitive... 

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