HS radical

Free-Radical-Initiated Synthesis. Free-radical-initiated reactions of hydrogen sulfide to alkenes are commonly utilized to prepare primary thiols. These reactions, where uv light is used to initiate the formation of hydrosulfuryl (HS) radicals, are utilized to prepare thousands of metric tons of thiols per year. The same reaction can be performed using a radical initiator, but is not as readily controlled as the uv-initiated reaction. These types of reactions are considered to be anti-Markownikoff addition reactions. 

Trace amounts of disulfane have been obtained in the gas phase by recombination of HS radicals generated by oxidation of hydrogen sulfide by oxygen atoms which in turn are produced from molecular oxygen in a microwave discharge [25] ... 

Only HS- radicals are able to abstract hydrogen atoms from sulfanes HS+H2S4 H2S + HS4... 

The HS2 radical was detected by its infrared absorption spectrum and the S2 molecule by luminescence spectroscopy. In addition, infrared bands assigned to dimers of disulfane molecules were observed at higher H2S2 concentrations. The HS2- radicals may further be split into hydrogen atoms and S2 molecules during the photolysis since the concentration of HS2- first increases and then decreases while that of S2 steadily increases. No evidence for the thiosulfoxide H2S=S was found, and the probably formed HS- radicals are assumed to be unable to leave their cage in the matrix and either recombine to H2S2 or form H2+S2 [69]. 

The TOF measurements of the H atom produced in the photodissociation of H2S indicate that a substantial number of HS radicals are produced vibrationally excited at each of the wavelengths studied. Their results are summarized in Table 9. 

This proposed reaction would be facilitated because the leaving group would be the stable HS radical (.15), and arylthio radicals... 

Especially important for cleaning of aquatic systems are iron(III) complexes with HSs. These thermally very stable compounds undergo photoredox reactions, creating ferrous ion and free HS radicals. In a neutral or alkaline medium, the Fe11 species is reoxidized to Fe111 by molecular oxygen, whereas in acidic medium a more efficient oxidant (H202) is needed (Fenton reaction) [11,19]. 

However, in the case of butanethiol, the main fragmentation occurs through a cleavage of the adjacent bond and corresponds to the loss of an HS radical and the most intense ion is... 

In contrast to the behaviour observed with amines, the spectra of t-butanethiol and of n-butanethiol are dominated by the loss of the HS radical formed by the cleavage of the adjacent bond. The ion of m/z 33, HS+, is either absent or very weak. Thus the HS group is the one that removes the electron ... 

Among Fe(III) chelate complexes those with HS are of special relevance due to abimdant occiurence of these ligands in the natiu-e. They are thermally very stable compounds, whereas under sunlight undergo photoredox reactions yielding ferrous species and free HS radicals (see Section III.A) (6,132). 

The reaction between two HS radicals has been studied by Darwent et Darwent and Roberts , during the course of an investigation of the reactions of H atoms with hydrocarbons, had occasion to consider the fate of HS radicals produced by the photolysis of H2S. They estimated the rate coefficient ratio for... 

The HS radical reacts with either oxygen or ozone, the latter giving the HSO radical. The oxidation processes ultimately yield SO2. The typical lifetime for atmospheric H2S is 3d. Global sources and sinks of H2S are estimated as 7.72 1.25 Tg a and 8.50 2.80 Tg a respectively, with an imbalance that was indefinable (Watts, 2000). 

The role of O2 is here twofold first it acts as an electron acceptor, secondly it is involved in the anodic corrosion reaction leading to the formation of SO4. This is in agreement with the results obtained with CdS electrodes as discussed in Section 8.1.4. Interestingly, the corrosion rate was considerably reduced after treatment of the particles in a solution of Cd " ions, which led to a blocking of S or HS radical sites at the surface, as already described in Section 9.1.1 [39]. Obviously, the formation of S radicals by holes is hindered. 

The HS radical formed in this way reacts with molecular oxygen ... 

It would be interesting to compare these results with those for the HS radical, and recently spectra have been described which are said to relate to this radical (52, 55). Hadley et al. (55) irradiated frozen aqueous hydrogen sulfide with ultraviolet light and obtained a powder spectrum which was reasonably well fitted by the parameters of Table III. This analysis was supported by the results from deuterated samples. Gunning et al. (52) obtained spectra from both H2S and D2S after irradiation of the solids, which are quite similar in appearance (Table III). 

Some confusion has arisen because of the result for HS radicals in the gas-phase. The proton hyperfine coupling of about 5 gauss (67) has been taken by both groups to be the Fermi contact value, whereas, in fact, it does not relate directly to the contact term. 

Reaction of HS- radicals, produced by Reaction 3, with the carbon radicals formed by radiolysis, may lead to distorted tritium distributions if the carbon radicals located on a particular amino acid residue react more slowly with HST than those located on other amino acid residues, but rapidly with HS- radicals. 

The results of an experiment which indicate that the irreversible addition of HS- radicals to aromatic rings is not an important process for native ribonuclease is shown in Figure 7. y-Irradiated ribonuclease was exposed to H235S for 4 hours and then immediately separated from noncovalently bound 35S by chromatography on Sephadex G25. After... 

Evidence for the reaction of HS radicals with disulfide bridges was obtained by studying the following sequence of reactions ... 

From the specific activities of the 35S-labeled proteins and of H235S the number of HS radicals which react with ribonuclease can be calculated. Table I shows that the number of SH radicals covalently bound is approximately equal to the number of carbon radicals produced by y-radiolysis. The latter was calculated from T incorporation and from the ESR data of Hunt and Williams (16). 

It appears that almost all of the HS radicals formed in the labeling reaction react with the protein and do not recombine in the gas phase. In native RNase about 70% of the HS- radicals react with disulfide bridges (Reaction 8), while 30% add to carbon radicals (Reaction 6). The latter process could, under conditions previously discussed, cause a distortion of the tritium distribution. The reactions of HS- radicals in reduced proteins remain to be investigated. 

The mass spectra of a number of heterocyclic compounds have been reported. The fragmentation patterns of thiophosphoryl derivatives of phosphorinanes are sensitive to stereochemistry, thus for the series (95) facile loss of the HS radical is indicative of an axial PS bond. The seven-membered heterocycles (96 Ch = S, Se) undergo a remarkable migration of sulphur or selenium from phosphorus to carbon with ring cleavage. While exocyclic P-C bonds of five-and six-membered heterocycles in the phosphonic class may be readily cleaved with retention of the phosphorus ring system, the seven-membered ring (97) exhibits facile expulsion of a phosphorus radical. ... 

Because of the rapid dissociation of vibrationally excited HS radicals in the state, the determination of the HS(Z) vibrational distribution has not been made by the LIF method [128], Previous laser photofragmentation TOF mass spectrometric [129,131,132] and Doppler spectroscopic [130] experiments indicate that HS(X) formed by process 42 is vibrationally excited, up to v = 6, with the HS vibrational distribution peaked strongly at V = 0. Recently, Continetti et al. [132] have observed the 193-nm photodissociation of HS(X, v) (process 43) formed in process 42 at high photodissociation laser powers ... 

The HS radicals can also be converted into the prodncts H2 and S2 via the following sequence of two fast exothermic elementary reactions ... 

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