X-ray crystallography diffraction

Structural data are now available for the complete series of chlorofluorophosphoranes and are presented in Table 10. Phosphorus pentafluoride has been particularly well studied by electron diffraction, X-ray crystallography and microwave spectroscopy (Table 10), many studies being reinvestigations. As noted by Macho et al 255 and as can be seen from Table 10, the effect of successive substitution of fluorine in PF5 by chlorine... 

A multitude of concepts such as X-ray, neutron and electron diffraction, X-ray crystallography, low-angle scattering, powder diffraction, scattering by noncrystalline and amorphous solids, all refer to the same physical phenomenon. Whereas X-rays and electrons are scattered by extranuclear charge clouds, more massive particles like neutrons and a-particles are scattered on atomic nuclei. In principle, all of these processes are of the same type, as described for X-rays below. 

Physical, chemical, and biological properties are related to the 3D structure of a molecule. In essence, the experimental sources of 3D structure information are X-ray crystallography, electron diffraction, or NMR spectroscopy. For compounds without experimental data on their 3D structure, automatic methods for the conversion of the connectivity information into a 3D model are required (see Section 2.9 of this Textbook and Part 2, Chapter 7.1 of the Handbook) [16]. 

How is the diffraction pattern obtained in an x-ray experiment such as that shown in Figure 18.5b related to the crystal that caused the diffraction This question was addressed in the early days of x-ray crystallography by Sir Lawrence Bragg of Cambridge University, who showed that diffraction by a crystal can be regarded as the reflection of the primary beam by sets of parallel planes, rather like a set of mirrors, through the unit cells of the crystal (see Figure 18.6b and c). 

Each diffracted beam, which is recorded as a spot on the film, is defined by three properties the amplitude, which we can measure from the intensity of the spot the wavelength, which is set by the x-ray source and the phase, which is lost in x-ray experiments (Figure 18.8). We need to know all three properties for all of the diffracted beams to determine the position of the atoms giving rise to the diffracted beams. How do we find the phases of the diffracted beams This is the so-called phase problem in x-ray crystallography. 

As already explained in the relevant section, the use of X-ray crystallography (Section V,D,2), the possibility to determine the molecular structure from powder diffraction without the need to obtain monocrystals, and the many variants of solid-state NMR (Section VI,F) have profoundly enhanced the study of tautomerism in the solid state. 

TABLE 4. Selected molecular parameters of methyl phenyl sulfone (from gas-phase electron diffraction) and p-methylsulfonylbenzoic acid (from X-ray crystallography)... 

The crystal structures in Chapters 5 and 6 were determined by x-ray diffraction, and the papers illustrate Pauling s approach to this experimental technique, including his most notable methodological contributions—the coordination method (SP 42) and the stochastic method (SP 47). In its day, SP 47 was a tour de force in the determination of a complex crystal structure. SP 46 contains Pauling s famous discovery of two quite different crystal structures giving the same x-ray diffraction pattern, which violated the then-current conventional wisdom in x-ray crystallography. 

When a diffracted X-ray beam hits a data collection device, only the intensity of the reflection is recorded. The other vital piece of information is the phase of the reflected X-ray beam. It is the combination of the intensity and the phase of the reflections that is needed to unravel the contributions made to the diffraction by the electrons in different parts of the molecule in the crystal. This so-called phase problem has been a challenge for theoretical crystallographers for many decades. For practical crystallography, there are four main methods for phasing the data generated from a particular crystal. 

The electron diffraction analysis of l,2-bis(methylsulfonyl)ethane, CH3S02CH2CH2S02CH3 , yielded a limited amount of structural information. However, this substance has also been studied by X-ray crystallography , and the two sets of data offer a possibility for comparison. The molecular model is shown in Figure 14. 

---