Mounting of the Crystal

For unit cell determination and data collection, the crystal must be mounted into the diffractometer on the goniometer head (see Fig. 9.1, blow-up part), this is done by using a specially constructed brass pin crystal holder (Figs. 9.3. and 9.4). 

If the crystal does not survive cooling (if cracks appear and/or the intensity of the reflections is lost) or it cannot be transferred intact into the loop (or seems to react or decompose inside the oil as happens occasionally), the crystal can be sealed into a capillary crystal holder (see Figs. 9.3 and 9.4, right). 

The advantages of this procedure is that the crystal is in contact only with the mother solvent (actually surrounded by it all the time) and that the unit cell determination and data collection can also be performed at room temperature. Disadvantages are the extreme care needed to seal and manipulate the fragile capillary crystal holder, which is unique for each crystal, and the possible movement of the crystal inside the capillary tube during measurement. After successfully placing the crystal into the diffractometer, unit cell determination can commence as the next step. 

3 Unit Cell Determination and Preliminary Space Croup Selection 

Before the unit cell determination and the actual data collection, the diffraction pattern image from the crystal should be very carefully evaluated for three factors (i) Does the crystal diffract at all (even very nice looking crystals might not dif- 

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If the detection system is an electronic, area detector, the crystal may be mounted with a convenient crystal direction parallel to an axis about which it may be rotated under tlie control of a computer that also records the diffracted intensities. Because tlie orientation of the crystal is known at the time an x-ray photon or neutron is detected at a particular point on the detector, the indices of the crystal planes causing the diffraction are uniquely detemiined. If... 

Fig. 12.5. Section view of the mounting of the Te02 crystal inside the copper frame. The whole experiment was surrounded by a copper thermal shield, not shown in figure.

The single crystal catalysts, -1 cm in diameter and 1 mm thick, are typically aligned within 0.5 of the desired orientation. Thermocouples are generally spot-welded to the edge of the crystal for temperature measurement. Details of sample mounting, cleaning procedures, reactant purification, and product detection techniques are given in the related references. The catalytic rate normalized to the number of exposed metal sites is the specific activity, which can be expressed as a turnover frequency (TOF), or number of molecules of product produced per metal atom site per second. 

Figure 5.8 A Debye-Scherrer powder camera for X-ray diffraction. The camera (a) consists of a long strip of photographic film fitted inside a disk. The sample (usually contained within a quartz capillary tube) is mounted vertically at the center of the camera and rotated slowly around its vertical axis. X-rays enter from the left, are scattered by the sample, and the undeflected part of the beam exits at the right. After about 24 hours the film is removed (b), and, following development, shows the diffraction pattern as a series of pairs of dark lines, symmetric about the exit slit. The diffraction angle (20) is measured from the film, and used to calculate the d spacings of the crystal from Bragg s law.

The experiments were performed in stainless steel UHV chambers which were equipped with the instrumentation necessary to perform Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), UV Photoelectron Spectroscopy (UPS), Low Energy Electron Diffraction (LEED), work function measurements (A( )), High Resolution Electron Energy Loss Spectroscopy (HREELS), and Temperature Programmed Desorption (TPD). The Au(lll) crystal was heated resist vely and cooled by direct contact of the crystal mounting block with a liquid nitrogen reservoir. The temperature of the Au(lll) crystal was monitored directly by means of a... 

The specimen P (Fig. 61), however it is made, should be not more than 0 5 mm wide. It is best to rotate it to ensure random orientation of the crystals (otherwise discontinuous spotty arcs may be produced on the photograph), and for this purpose it is mounted on a rotating holder. Centring may be done by hand, or better, by using a holder fitted with adjusting screws. 

The next step is for a protein crystallographer to mount a small perfect crystal in a closed silica capillary tube and to use an X-ray camera to record diffraction patterns such as that in Fig. 3-20. These patterns indicate how perfectly the crystal is formed and how well it diffracts X-rays. The patterns are also used to calculate the dimensions of the unit cell and to assign the crystal to one of the seven crystal systems and one of the 65 enantiomorphic space groups. This provides important information about the relationship of one molecule to another within the unit cell of the crystal. The unit cell (Fig. 3-21) is a parallelopiped... 

The classical method of mounting crystals is to transfer them into a fine glass capillary along with a droplet of the mother liquor. The capillary is then sealed at both ends and mounted onto a goniometer head (see Fig. 4.20, and Chapter 4, Section III.D), a device that allows control of the crystal s orientation in the X-ray beam. The droplet of mother liquor keeps the crystal hydrated. 

In all forms of data collection, the crystal is mounted on a goniometer head, a device that allows the crystallographer to set the crystal orientation precisely. The goniometer head (Fig. 4.20) consists of a holder for a capillary tube containing the crystal two arcs (marked by angle scales), which permit rotation of the crystal by 40° in each of two perpendicular planes and two dovetailed sledges, which permit small translations of the arcs for centering the crystal on the rotation axis of the head. 

Protein crystals, either sealed in capillary tubes with mother liquor or flash-frozen in a fiber loop, are mounted on the goniometer head, which is adjusted to center one face of the crystal perpendicular to the X-ray beam and to allow rotation of the crystal while maintaining centering. Flash-frozen crystals are held in a stream of cold nitrogen gas emerging from a reservoir of liquid nitrogen. 

In the diffractometer, the goniometer head and crystal are mounted in a system of movable circles called a goniostat, which allows automated movement of the crystal into almost any orientation with respect to the X-ray beam and the detector (see Figs. 4.21 and 4.22). 

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