Example of structure determination

As a simple example, we will consider an intermediate phase which occurs in the cadmium-tellurium system. Chemical analysis of the specimen, which appeared essentially one phase under the microscope, showed it to contain 46.6 weight percent Cd and 53.4 weight percent Te. This is equivalent to 49.8 atomic percent Cd and can be represented by the formula CdTe. The specimen was reduced to powder and a diffraction pattern obtained with a Debye-Scherrer camera and CuKa. radiation. 

The observed values of sin 0 for the first 16 lines are listed in Table 10-5, together with the visually estimated relative line intensities. This pattern can be indexed on the basis of a cubic unit cell, and the indices of the observed lines are given in the table. The lattice parameter, calculated from the sin 6 value for the highest-angle line, is 6.46 A. 

The density of the specimen, as determined by weighing a quantity of the powder in a pyknometer bottle, was 5.82 gm/cm. We then find, fVom Eq. (3-7), that 

Since the molecular weight of CdTe is 240.02, the number of molecules per unit cell is 945/240.02 = 3.94, or 4, within experimental error. 

At this point, we know that the unit cell of CdTe is cubic and that it contains 4 molecules of CdTe, i.e., 4 atoms of cadmium and 4 atoms of tellurium. We must now consider possible arrangements of these atoms in the unit cell. First we examine the indices listed in Table 10-5 for evidence of the Bravais lattice. Since the indices of the observed lines are all unmixed, the Bravais lattice must be face-centered. (Not all possible sets of unmixed indices are present, however 200,420, 

---

Examples of Structure Determination Through the Combined Use of Advanced Spectroscopic Techniques... 

Examples of Structure Determination from Powder XRD Data. 158... 

Long-chain polymers. To conclude this series of examples of structure determination by trial, accounts will be given of the elucidation of the structures of two long-chain polymers. Substances of this type are of increasing practical importance, and moreover their molecules are very interesting stereochemicaJly. The experimental data available for the study of their crystal structures is more scanty than in the case of crystals composed of small molecules there is no morphological evidence on crystal symmetry, and only limited optical evidence... 

Why was MS slow to gain favour in peptide structure determination After all, by 1965, when the early mass spectra of peptides were being collected and assessed, sophisticated examples of structure determination by MS were commonplace in other areas of the organic chemistry of natural products. 

In fact, some of the early examples of structure determination were particularly favourable in terms of practical MS (even though they appear to be more structurally complex examples than would be expected for pioneering studies) because they were sufficiently volatile without derivatisation. They could be described as naturally derivatised peptides (many natural peptides are TV-acylated and ester-ified). Dolastatin 15 (Figure 4.7) has no zwitterionic characteristics, since it is TV-methylated and the C-terminus is cyclised as part of a lactone. It is already suitable for MS since it has adequate volatility without derivatisation. 

Phosphorus-31 n.m.r. spectra have been used to determine the structure of many co-ordination compounds. In addition, the spectral parameters have often been used in discussions of the nature and strength of co-ordinate bonds in which phosphorus is the donor atom. In Section II of this review, we have presented some examples of structure determination involving n.m.r., and we hope that future work of this type will also be assisted by the Tables of shifts and coupling constants given in the later Sections. The Tables contain a selection of the available data for all the main types of compound in which phosphorus is normally considered to behave as an electron-pair donor. 

The X-ray technique has proved particularly valuable in indicating the formation of heteropentalenes, either as intermediates or final products, in several reactions of thiadiazoles. These applications are referred to in their proper place101,278,279 throughout the text, as are examples of structural determinations of 1,2,4-thiadiazoles,84 thiadiazolines (A2 73 A3 28°), thiadia-zolidines,29,30,52,96,98 as well as condensed structures.105,164... 

Practical examples of structural determination achieved with the aid of spectral analyses including two-dimensional NMR techniques are presented below. 

Chapter 12 A Recent Example of Structure Determination Through Total Synthesis and Convergent Syntheses Lasonolide A... 

---