Case studies small crystals

Many EDSA studies have shown that, even for small crystals, the observed intensities sometimes can considerably deviate from the kinematical values. This is the case for reflections corresponding to higher orders of strong reflections. These deviations cannot be described by two-beam scattering which implies existence of systematic many-beam interactions. This is a situation when higher and lower orders of a given reflection are excited simultaneously. 

A company producing olive oil has occasionally a lot that is unlit for normal use owing to off-flavors. Purification processes remove the off-flavors but also the highly desirable natural flavor of the oil. The company wants to find another outlet for these lots and considers that the very special composition of olive oil may lead to a useful solid fat by complete hydrogenation. The company s laboratory is asked to do some trials and they make blend of 20% fully hydrogenated and 80% unmodified oil, to study crystallization behavior. Suppose that you have to do experiments in which the aim is to obtain (a) very small crystals, (b) rapid crystallization, or (c) fully stable and largish crystals. What temperature regime would you apply in each of these cases The fatty acid composition of the oil is 70% 0,15% Li, 10% P, and 5% others, of which about half is 08. 

In most well-studied molecular crystals, such as anthracene, tetracene, and other crystals of the aromatic series, the intermolecular distances are quite large in all directions and, hence, the overlap of the wavefunctions of neighboring molecules is very small. In such crystals, the deviations from electroneutrality of the molecules in the lowest energy excitations in the majority of cases can be neglected. The Frenkel excitons and weak exciton-phonon interaction are typical for the lowest energy states of this crystals. 

Short stretches of DNA (oligonucleotides) can be synthesised and crystallised and therefore studied at near-atomic resolution. These studies reveal that there is a great deal of structural variability and flexibility in the DNA structure (Kennard and Hunter 1989). Most of these studies have used conventional X-ray sources. However, as will be mentioned in chapter 10, SR has been used in a few cases where the crystals have been small or radiation sensitive. Also, use has been made of a brominated form of an oligonucleotide to use multiple wavelength techniques for phase determination. 

One of the methods of synthesis of clusters of uniform size consisting of just several atoms is the intrusion of liquid phase (e.g., mercury) under high pressure into zeolites with voids of different volume. High pressure is necessary for overcoming the capillary pressure in order to achieve filling of small voids with a liquid. When the pressure drops, the column of liquid in the thin capillary ruptures, similarly to the column of mercury in the thermometer upon cooling, and monodispersed clusters become trapped in the zeolite voids. Computer modeling and experimental studies of such small clusters both indicated that they form unique crystalline structures, impossible in the case of macroscopic crystals. For example, such structures may contain the axes of symmetry of fifth order. 

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