Sulfur atom detection

Total sulfur in air, most of which is sulfur dioxide, can be measured by burning the sample in a hydrogen-rich flame and measuring the blue chemiluminescent emission from sulfur atom combination to excited S2 (313). Concentrations of about 0.01 ppm can be detected. 

Nitrogen compounds in middle distillates can be detected selectively by chemiluminescence. Individual nitrogen compounds can be detected down to 100 ppb nitrogen. Gas chromatography with either sulfur chemiluminescence detection or atomic emission detection has been used for sulfur-selective detection. 

Where the target analyte contains heteroatoms such as nitrogen, phosphorus and sulfur, atom-selective detectors can provide an ideal detection method. A number of examples appear in the literature of the use of a detector called a thermal energy analyser (TEA) for the measurement of A-nitroso compounds [14-17] and aromatic nitro compounds [18]. This has also been used as an HPLC detector [19, 20], and a modified TEA has been reported to be useful for analysis of amines and other nitrogen-containing compounds [17]. Unfortunately, this technique appears not to have gained in popularity, since no reports have appeared in the literature for over two decades. 

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

Additional work in our laboratory using a reconstituted monooxy-genase system containing purified cytochrome P-450 has indicated that the sulfur atom released from parathion covalently binds to at least three other, as yet unidentified, amino acids on the cytochrome P-450 molecule. It appears to be clear that the binding of the sulfur atom to cysteine or cysteines is responsible for the loss of cytochrome P-450 detecteable as its CO complex and the accompanying loss of monooxygenase activity (25). 

Using the method mentioned above, the effect of photoabsorption by sulfur atoms in amino acids was studied by Yokoya et al. [29]. They chose cystathionine as a model molecule because of the structural advantage that most of the decomposed fragments are easily detectable amino acids. A comparison of the distribution pattern of fragments between on-resonance and off-resonance lead to the conclusion that cystathionine was more frequently cleaved around a sulfur atom than at a bond apart from the sulfur. 

Factors which affect the oxepin-benzene oxide equilibrium positions are similarly expected to influence the thiepin-benzene episulfide distribution at equilibrium. Since however the thianorcaradiene tautomer has not to date been detected, the main evidence for this form is based upon the thermal instability and reactions of the thiepin system. Thus it is assumed that where the thianorcaradiene isomer is present, a spontaneous thermal decomposition involving extrusion of a sulfur atom will occur. Substitution at the 2,7-positions in the oxepin-arene oxide system leads to a preference for the seven-membered ring form and this effect was further enhanced by bulky substituents (e.g. Bu ). A similar effect was observed in thiepins and thus the remarkable thermal stability of (49) (2,7-r-butyl groups) and (51) (2,7-hydroxyisopropyl groups) contrasts with the behavior of thiepin (55)(2,7-isopropyl groups), which was thermally unstable even at -70 °C (78CL723). The stability of thiepin (49) results from the 2,7-steric (eclipsed) interactions which obtain in the thianorcaradiene form but which are diminished in the thiepin tautomeric form (relative to the episulfide tautomer). 

A number of reactions have been investigated which could form a thiophene ring by forming a bond y to the sulfur atom, but which have not been generalized. Photocyclization of phenylthioethylenes has led to benzothiophenes, usually in low yields, and sometimes accompanied by rearrangement (68JOC2218). Irradiation of (115 R = H or Me, and R1 = Ph) in the presence of iodine gave a mixture of (116) and (117) in about 11% maximum yield. If R1 = H or Me, no product (117) was found, and if R was Ph, only traces of cyclized products were detected. A complex mechanism was proposed to account for these results. 

---