Information from Complex Isotopic Patterns

If the isotopic distribution is very broad and/or there are elements encountered that have a lowest mass isotope of very low abundance, recognition of the mono-isotopic peak would become rather uncertain. However, there are ways to cope with that situation. 

Example I The Sn isotopic peak of tin compounds can easily be overlooked or simply could be superimposed by background signals (Fig. 3.8). Here, one should identify the °Sn peak from its unique position within the characteristic pattern before screening the spectrum from peak to peak. For all other elements contained in the respective ions still the lowest mass isotope would be used in calculations. 

Example II Ruthenium exhibits a wide isotopic distribution of which the ° Ru isotope can be used as a marker during assignment of mass differences. Moreover, the strong isotopic fingerprint of Ru makes it easily detectable from mass spectra and even compensates for a lack of information resulting from moderate mass accuracy (Fig. 3.11). 

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This analysis can be extended to the other small peaks in the mass spectrum, and one will find some ion resulting from fragmentation of the complexes during ionization. We will not go into detail here, but rather conclude that the careful analysis of the exact mass and the isotope pattern provides information on the elemental composition and with it on the stoichiometry of the complex (e.g. 2 2... 

P and 13C NMR data are more informative. Thus the 31P ttj NMR spectrum of the complex is a sharp singlet at 50°C, which converts into a pattern of lines characteristic of an AA BB spin system upon cooling at -40°C. On the other hand the P NMR of a sample of the complex, approximately 50% enriched in C02, clearly shows the three central lines of the 1 4 6 4 1 quintet expected for an isotopic mixture of the three possible isotopomers, while the C spectrum of this enriched sample displays a quintet at S 206 ppm (2Jcp=18Hz) due to the C02 ligands. A full description of these NMR properties can be found elsewhere (Alvarez et al, 1986). From these data, structure A can be proposed for this... 

Alas, things are not as simple as they may have appeared. From the discussions in Chapters 6 and 7 you might be under the impression that typical H NMR spectra exhibit just one sharp signal line for each 1H nucleus (or each set of equivalent H nuclei) and that the same thing is true for 13C spectra as well as for spectra of any other isotope. Actually, this is not usually the case. Instead, the individual signals expected on the basis of the molecule s symmetry are themselves often split into symmetrical patterns (multiplets) consisting of two or more lines. While these extra lines do make a spectrum appear more complex, they also offer valuable structural information that complements the chemical shift data. This chapter explains the source of these extra lines and shows how useful they can be for confirming the structures of molecules. 

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