Sulfur, natural abundance

The only sulfur isotope with a nuclear spin is which is quadrupolar (/ = 3/2) and of low natural abundance (0.76%). In view of these inherent difficulties and the low symmetry around the sulfur nuclei in most S-N compounds, S NMR spectroscopy has found very limited application in S-N chemistry. However, it is likely that reasonably narrow resonances could be obtained for sulfur in a tetrahedral environment, e.g. [S(N Bu)4], cf. [S04] . On the other hand both selenium and tellurium have isotopes with I = Vi with significant natural abundances ( Se, 7.6% and Te, 7.0%). Consequently, NMR studies using these nuclei can provide useful information for Se-N and Te-N systems. 

Atomic masses calculated in this manner, using data obtained with a mass spectrometer can in principle be precise to seven or eight significant figures. The accuracy of tabulated atomic masses is limited mostly by variations in natural abundances. Sulfur is an interesting case in point. It consists largely of two isotopes, fiS and fgS. The abundance of sulfur-34 varies from about 4.18% in sulfur deposits in Texas and Louisiana to 4.34% in volcanic sulfur from Italy. This leads to an uncertainty of 0.006 amu in the atomic mass of sulfur. 

Despite the presence of the A + 1 isotope for hydrogen and the A + 4 isotope for sulfur these elements are considered A and A + 2 correspondingly, as the natural abundances of these isotopes are very low and may be detected only in the case of a large number of H and S atoms in a sample molecule. 

Figure 15.15 Natural abundance 2H MAS spectra observed in a series of vulcanised natural rubbers with various vulcaniser (sulfur 1 or 3 wt%) and/or filler (carbon black 0 or 40 wt%) contents. The spinning speed is 0.5 kHz. The number of scans is about 300000. Spectra are simulated with two components (a mobile and a rigid one) with various residual quadrupolar interactions...

Carlson, P.R., and Forrest, J. (1982) Uptake of dissolved sulfide by Spartina altemiflora evidence from natural sulfur isotope abundance ratios. Science 216, 633-635. 

Sulfur-33 is the only naturally occurring isotope of sulfur with a nonzero spin (7 = 3/2). Since it has a moderate quadrupole moment (—5.5 x 10 m ), a low natural abundance (0.76%), and a low magnetogyric ratio (2.055 x 10 rad s ), it is clearly an intrinsically insensitive nucleus. 

The only sulfur isotope with a nuclear spin is S, which is quadrupolar (/ = 3/2) and oflow natural abundance (0.76%). Consequently, S NMR spectroscopy has found very limited apphcations in S-N chemistry. 

Figure 4 Te NMR spectrum of the ternary sulfur-selenium-tellurium melt containing 1.5 mol% of both Se-enriched selenium and Te-enriched tellurium (enrichment 92%). The spectral assignments and the identification of the molecular species were made on the basis of combined information from the natural abundance and from the Se- and Te-emiched samples taking the known trends in the Se and Te chemical shifts into accoimt. (Reprinted with permission from Ref. 26. 1993 American Chemical Society)...

Heavy water, D2O, is of interest as a nuclear reactor moderator. A typical power reactor requires some 200 tons of heavy water. Since the natural abundance of deuterium is 1/7000, an annual production of 200 tons of D2O requires the processing of a hydrogenous feed at a rate between 2 million ton/yr., for 100% extraction, to 20 million ton/yr., for 10% extraction. Only water, sulfuric acid, and petrochemicals are processed in this amount and can supply such quantity of feed material. 

Intensive investigations of the sulfur dynamics of forest ecosystems in the last decade can be attributed to the dominant role of sulfur as a component of acidic deposition. Studies in forested catchments include Fuller et al. (1986), Mitchell et al. (1989), Stam et al. (1992), and Andersson et al. (1992). Sulfur with a distinctive isotopic composition has been used to identify pollution sources (Krouse et al., 1984), and has been added as a tracer (Legge and Krouse, 1992 Mayer et al., 1992, 1993). Differences in the natural abundances can also be used in systems where there is sufhcient variation in the 5 " S of ecosystem components. Rocky Mountain lakes (USA), thought to be dominated by atmospheric sources of sulfate, have different 5 " S values than lakes believed to be dominated by watershed sources of sulfate (Turk et al., 1993). 

Early work by Thode et al. (1949) established that there are large variations in the isotopic composition of sulfur compounds in nature. Since then sulfur isotope abundance data have been frequently used to elucidate many terrestrial processes including the genesis of sulfide ore bodies. The mechanisms of isotopic fractionation (alteration of relative isotopic abundances) can be broadly categorised under exchange processes or the kinetic isotope effects discussed on pp. 324ff. Isotopic exchange may be represented by the reaction ... 

Fig. 6.2.7. Sulfur isotope abundance variations in nature (evaporitic curve after Kaplan, 1975).

In view of this universal knowledge about proton and carbon NMR spectroscopy within the chemical community, it is remarkable that heteronuclear NMR is still looked npon as something of a curiosity. Admittedly, most organic compounds contain only nitrogen, oxygen, and sulfur atoms, as well as the obligatory hydrogen and carbon atoms, elements that have an unfavourable isotope distribution when it comes to NMR spectroscopy. Each of these three elements has a dominant isotope i N (99.63% natural abundance), 0 (99.76%), and (95.02%), with O, S, and... 

---