Hydrogen Bonds Involving Sulphur

In the solid and liquid states, and in concentrated solutions, phosphinodithioic acids RR P(S)SH give broad IR absorptions at about 2400 cm which is indicative of association through S-H-S bonding. On dilution of a solution or raising its temperature, this broad absorption is replaced by a single sharp absorption at 2560 cm which is characteristic of unbonded (P)-S-H (13.39). 

Spectroscopic data indicate that the trifluoromethyl derivative (CF3)2P(S)SH is exceptional and remains monomeric over a wide range of conditions. There is no H bonding in thioacetic acid, MeC(S)SH and in dithiobenzoic acid, PhC(S)SH it is very slight, thus suggesting that (C)-S-H-S-(C) bonds are less polar and weaker than (P)-S-H-S-(P) bonds. 

In the solid state some phosphinodithioic acids occur as dimers (13.40a) while others appear as continuous spiralling chains (13.40b), like the oxy analogues (13.7c). The S-H-S bonds are unsym-metrical and non-linear with S/H/S 180°. 

Association of phosphoro and phosphono dithioate esters into dimeric units may also occur under suitable conditions. 

Spectroscopic data indicate hydrogen bonding takes place in difluoro-dithiophosphoric acid. The acid is monomeric in the vapour state and may have structure (13.42a), but in the liquid state it is associated and may have structure (13.42b). 

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It was assumed that disulphide bond breakdown was a prerequisite for supercontraction. Later work by Elod and Zahn Melliand Textilber, 1949, 30,17) and others has thrown doubt upon the assumption because phenols, formamide, and lithium bromide can all cause supercontraction, and it is well known that they have no effect upon the cystine sulphur. Lithium bromide, in particular, as well as the others, is capable of dissociating hydrogen bonds which points to the probability that the latter are involved. 

Ethylene and thioformaldehyde are the products of irradiation of matrix-isolated thietane at lOK. Sulphur-carbon bond homolysis has also been observed on irradiation of the nucleoside membrane transport inhibitor, 6-[ (4-nitrobenzyl) thio] -9- (/8-D-ribofuranosyl) -purine (94), and the oxazolidin-2-one (95) has been converted into the allyl derivative (96) by photochemically induced radical allylation.Efficient conversion of cyclic thioacetals into the corresponding carbonyl compounds under neutral conditions has been achieved by 2,4,6-triphenylpyrylium tetrafluoroborate-sensitised irradiation in moist dichloromethane,and diaryl sulphides and the corresponding sulphoxides and sulphones have been reported to undergo anion-promoted carbon-sulphur bond photocleavageboth processes appear to involve an initial electron transfer. Sulphur-hydrogen bond horaolysis has been reported in t-butanethiol and is also responsible for the photoinitiated thiylation of fluorobromoethylenes and of trialkylethynylsilanes and t-butylacetylene. 

Much of the kinetic work has been concerned with the oxidation of isopropyl alcohol to acetone in aqueous sulphuric or acetic acids. The rate-determining step definitely involves the breaking of a carbon-hydrogen bond, as isopropyl alcohol is oxidised 6-7 times faster than its 2-deutero derivative . At all acidities, the rate of oxidation shows a first-order dependence on both the concentration of HCr04 and the alcohol, but the dejiendence on [H] varies. In dilute acidic solution the rate is proportional to [H] , to [H] in more concentrated solution and to Ho in 20-60% aqueous sulphuric acid . These observations are consistent with two mechanisms (i) a bimolecular electron transfer process (218) ,... 

A complementary view is given by X-ray photoelectron studies which provide evidence for hydrogen bond formation as well as preferential attachment by one or two nitrogen atoms per cyanine molecule to the hydroxyl groups in the substrates. In the thiacarbocyanine case, hydrogen bond formation was detected for microcrystalline cellulose and /S-cyclodextrin cases which involved the sulphur atom of the cyanine. 

A neutron diffraction study of sodium thiosulphate pentahydrate has shown that the large differences in S-O distances found in an earlier A"-ray study were due to errors in the oxygen positions. The three S-O distances are 1.45—1.47 A, and the S-S length is 1.95 A. The terminal sulphur atom is involved in the extensive hydrogen-bonding scheme which is present. 

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