Techniques for structural determination

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

The opening sentence above says it all. NMR is by far the most valuable spectroscopic technique for structure determination. Although wei) just give an overview of the subject in this chapter, focusing on NMR applications to small molecules, more advanced NMR techniques are also used in biological chemistry to study protein structure and folding. 

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

The use of infrared (IR) as a technique for structure determination is not very common in recent times. The reviews by Kurzer <1965AHC(5)119, 1982AHC285> contain a table of IR spectral absorptions of 1,2,4-thiadiazoles which covers spectra published before 1982. Additional spectral data was published in CHEC(1984) <1984CHEC(6)463>. 

Probably the most important recent advance in the chemistry of dibenzothiophene has been the adoption of NMR techniques for structural determination. This will eliminate much of the tedious synthetic work which was previously needed to establish the structure of new derivatives. 

An advanced discussion of the identification of lipids by fast atom bombardment (FAB) mass spectrometry, a powerful technique for structure determination. 

In the preceding example several types of spectroscopy are brought to bear. While the product structure could probably be deduced from IR spectroscopy or NMR (either 1II or 13C), the use of all three methods confirms the assignment. It is often prudent to use more than a single technique for structure determination so that the results reinforce each other. If a structure assignment is not consistent with all the data, the structure is probably incorrect. 

A complete discussion of modem instrumental techniques for structure determination is found in R. M. Silverstein, G. C. Bassler, and T. C. Morrill, Spectrometric Identification of Organic Compounds, 4th ed., Wiley, New York, 1991. Another excellent reference is P. Crews, J. Rodriguez, and M. Jaspars, Organic Structure Analysis, Oxford University Press, New York, 1998. 

The status of Mossbauer spectroscopy is akin to that of NQR. It is restricted to solids, and it yields information about the electronic and chemical environment about certain nuclei. Where diffraction methods are inappropriate, Mossbauer may be the best technique for structure determination. 

If the retention times of the analytes are known, or there is an efficient method for their detection on-line, such as UV, MS or radioactivity, stop-flow HPLC-NMR becomes a viable option. In the stop-flow technique, all the usual techniques available for high-resolution NMR spectroscopy can be used. In particular, these include valuable techniques for structure determination such as 2-dimensional NMR experiments which provide correlation between NMR resonances based on mutual spin-spin coupling such as the well-known COSY or TOCSY techniques. In practice, it is possible to acquire NMR data on a number of peaks in a chromatogram by using a series of stops during elution without on-column diffusion causing an unacceptable loss of chromatographic resolution. 

NMR spectroscopy has been an extraordinary important technique for structure determination of hydrogenated fullerenes ever since the first synthesis of hydrogenated fullerenes almost two decades ago. During this time, technical advances have pushed the limits regarding both sensitivity and spectral resolution. Also, sophisticated 2D NMR experiments have been implemented more and more frequently in recent years. Consequently, NMR spectroscopy has now been used to successfully characterize complex hydrogenated fullerenes whose structure for a long time... 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

LC-MS is a quick and accurate analytical tool for determining the composition of 99mTc radiopharmaceuticals. It also supplements structural information obtained from other analytical methods such as NMR. Therefore, LC-MS may also be a complimentary technique for structure determination of radio-labeled compounds by analyzing the fragmentation patterns of the mass spectra. LC-MS has the potential to become a routine analytical tool for radiopharmaceuticals labeled with 99mTc or other radioisotopes such as 186Re. 

By the end of the nineteenth century, molecular weight determinations carried out by means of vapor density measurements proved that phosphorus(V) and phosphorus(III) oxide in the gaseous state consist of molecules of the compositions P406 (3,86) and P4O10 (87), respectively. However, their molecular structures remained uncertain (see, for example, Ref. 88) until adequate techniques for structure determination such as electron and X-ray diffraction became available (6, 7, 72). 

As techniques for structure determination by X-ray diffraction i evolved, molecules of even greater complexity could be studied. Two ilDolecules of special biological interest were penicillin, the important ntibiotic discovered by Alexander Fleming, and vitamin B12, used tin the treatment of pernicious anemia. Several possible structures had kbeen proposed for penicillin, but the / -lactam structure was considered wmlikely by Robert Robinson and John Cornforth from the point of view... 

In principle, HREM is a very single technique for structure determination and with electron wavelengths currently used, atomic detail should, in theory, be easily resolvable, without any "phase problem" However, the practical difficulties are by now well known and characterised (2) and a brief summary of the relevant points will be given here. 

It is clear that electron microscopy is not the most favourable technique for structure determination of new (superconducting) phases X-ray diffraction and particularly neutron diffraction do a far better job in the ab initio structure determination. Electron microscopy and electron diffraction are extremely powerful however to determine the local structure i.e. to detect deviations from the average structure, as determined by X-rays or neutrons. In this way several new phases have been first identified by electron microscopy some of them have been later made into bulk superconductors. In other cases the identification of isolated defects in an existing material have inspired chemists to produce new superconducting materials this was, for example, the case for the occurrence of double HgO layers in a one-layer Hg-1223 superconductor. 

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