Diffractometers, computer-controlled

The data measurement in an x-ray analysis is typically performed using a computer controlled diffractometer. Except for the crystal mounting and some preliminary procedures, the measurement of intensities is fully automated. It is the next... 

The crystals were transferred to a CAD4-Enraf Nonius PDP8/M computer-controlled diffractometer. In each case, 25 reflections were used in order to settle the crystals, to check their quality and to refine the cell dimensions. 

Single crystal X-ray diffraction data is nowadays collected using a computer controlled diffractometer, which measures the Bragg angle 6 and the intensity I for each Ai/reflection. Many modern diffractometers employ a flat-plate detector (CCD), so that all the reflections can be collected and measured at the same time. A full data set, which can be thousands of reflections, can be accumulated in hours rather than the days or weeks of earlier times. 

The third step in the structure determination is collection of the X-ray diffraction data. This may be done with a diffractometer in which a narrow collimated pencil source of X-rays is aimed at the crystal and the intensities and positions of the diffracted beams are measured automatically. The computer-controlled diffractometer is able to measure the angles to within less than one-hundredth of a degree. If sufficient time is allowed, very weak spots can be counted. Today, diffractometers are more likely to be used for preliminary measurements, while the major data collection is done with an area detector, an... 

The crystal structure of saccharin was determined (3) by 3-dimensional integrated intensity data collected on a computer-controlled diffractometer operated by an IBM 1620, in a closed-loop manner. The crystals were found to be monoclinic with a 9 552 3, b 6.919 3, c 11.803 + A, and B 103 9°, and the space group is p2] /c. 

X-ray diffraction studies were conducted with a Rigaku computer-controlled diffractometer equipped with a long fine-focus Cu X-ray tube, a receiving graphite monochromator to provide monochromatic Cu-Ka radiation, and a scintillation detector. 

As noted above, the traditional methods of quantitative analysis make use of one or a small number of non-overlapping peaks from the diffraction patterns of the different component (e.g. polymorphic) phases. With the advent of more powerful laboratory X-ray sources and synchrotron radiation, faster and more sensitive detectors, computer controlled diffractometers and the almost universal use of digitized data there is increasing use of the full diffraction pattern for quantitative X-ray diffraction analysis (Zevin and Kimmel 1995). 

Although crystal structure analysis was once a very time consuming and very expensive process, this has not been the case for a number of years. Structural results are usually available within two weeks of when a crystal is placed on the diffractometer instrument. This change has been due to a number of factors. Computer-controlled diffractometers have made data collection more accurate and much easier. The process of obtaining a trial structure has been much facilitated by the use of computerized direct methods and by computer graphics. 

A modern computer-controlled diffractometer allows a structural problem to be solved in a matter of days (and in some cases hours). The technique is ideally suited to complicated structures, and has been used quite extensively in the diterpene alkaloid group. The so-called "direct method" (using a single crystal of the free base) has been applied, for example to the C2o-alkaloid veatchine (1) (Pelletier, Mody, and W. H. DeCamp, J. Amer. chem. Soc., 1978, 100,... 

It is appropriate at this point to indicate our personal motivation for carrying out structural studies, the types of compounds we study, and the experimental conditions we employ. In a very general sense we are interested in the bonding of small molecules and ions, e.g., 02, N2, NO, N2 R+, olefins, and acetylenes, to transition-metal complexes. Because of our interest in bonding, we seek the best solutions we can attain. Rapid, qualitative answers to conformational problems are not our interest. Since those transition-metal systems that bind small molecules generally have the metal in a low oxidation state, and since a low oxidation state is usually stabilized by ligands of the type PR3 (R = alkyl or aryl), solution to our problems involves typically the determination of a large number of structural parameters. With only a few exceptions the intensity data are obtained at room temperature on a Picker FACS-1 computer-controlled diffractometer. Usually the ratio of observations to variables is at least 10, and it is often 20 to 30. 

COLLECTION OF DATA. It should be emphasized that the correct choice and preparation of the sample combined with careful collection of data will greatly aid in the total process. It is extremely important that the sample is pure, or if not, that all the impurity phases are known. Figure 1 shows a comparison of data collected from a typical 1970 s diffractometer, a modern computer controlled diffractometer and a high resolution diffractometer using synchrotron radiation at the National Synchrotron Light Source. The excellent resolution and high peak to background ratio (typically 1000 1) obtained from the synchrotron data enable very weak peaks to be easily observed. 

Figure 1 A comparison of data collected from typical diffractometers (a) a 1970 s diffractometer, (b) a modern computer controlled diffractometer and (c) a high resolution diffractometer using synchrotron radiation.

A Syntex four-circle computer-controlled diffractometer with a graphite monochromator and a pulse-height analyzer was used for preliminary experiments and for the subsequent collection of diffraction intensities. Molybdenum radiation (Ka, X = 0.70930 A ... 

For about 30 years from the 1960s, most single-crystal X-ray diffraction made use of computer-controlled diffractometers. Detectors were small scintillators coupled to photomultipliers, and measured one reflection at a time, providing information on both the direction and the intensity. For a given overall level of diffracted intensity, the time needed to collect a full data set was proportional to the size of the asymmetric unit of the structure a structure twice the size gave twice as many unique reflections. The most productive systems measured a few thousand reflections per day, and up to about an hour was required for the initial determination of the unit cell and crystal orientation, without which further measurements could not proceed since the correct... 

Another trend that is likely to continue is the improvement of traditional techniques and instruments and the discovery of new ones. In the late twentieth century computer-controlled diffractometers came into wide use because they facilitated the collection and processing of digitized diffraction patterns. According to Moore s law, the computing power of integrated circuits doubles about every eighteen months, and, if this law remains valid, the computerization of diffraction technologies should become... 

All reflections whose phases are to be determined have to be measured on the native protein and the isomorphous crystals. A computer-controlled diffractometer is used. Alternatively, photographic data and automatic photometers can be employed this has the advantage that the crystals do not have to be exposed to X rays for such a long time, reducing radiation damage. Recently, films have been replaced by area detectors which produce the intensities in digital form. If a synchrotron is used as radiation source, and the pattern is recorded by the Laue method (see Section 15.2.1.5), data acquisition takes only a few minutes. 

These film methods may appear old fashioned, but this is wrong. Any serious X-ray laboratory will have a few of these machines, which are always necessary to give a first impression of the diffraction problem on a relatively cheap apparatus. especially when the expensive computer-controlled diffractometers are not immediately available or when there are problems and a three-dimensional inspection of the reciprocal space is necessary to gain an impression of the difficulties. 

Diffracted X-ray intensities were measured on u computer-controlled diffractometer with a 9/29 scan mode 1081. The reference reflection was remeasured at the beginning and end of each cycle of 20 reflections. This reflection had to be carefully selected because it formed the basis for the relative scaling. At a maximum glancing angle of 28°. the resolution was 0.76xl0"" m l=/. /(2sin9)J. 

The most spectacular successes of X-ray methods, however, are in molecular and crystal structure analysis. Examples are the structures of insulin [130], hemoglobin (131], and vitamin B 2 [132]. Today, single-crystal X-ray structure analyses of relatively complex compounds (up to ca. 40-200 nonhydrogen atoms) are performed with computer-controlled diffractometers whose computers can be used simultaneously to solve the phase and structure problem within a few days or just a few hours.. The method has gained parity in investigation time with spectroscopic methods... 

Three-dimensional intensity data were collected on a Picker FACS-I computer-controlled diffractometer. Data from the crystals were rejected when there was more than 10% drop in the intensity of three standard reflections monitored every 2 hrs. Multiple observations were averaged and intensities for the various derivatives were scaled to the native data. 

Data were measured on an ENRAF-NONIUS CAD-4 and a PWl 100/20 Philips Four-Circle computer controlled diffractometers. CaKa (A = 1.54178 A) or Moif (A = 0.71069 A) radiation with a graphite crystal monochromator in the incident beam was used. Intensities were corrected for Lorentz, polarization and absorption effects. All nonhydrogen atoms were found by using the results of the SHELXS-86 direct method analysis. After several cycles of refinements the positions of the hydrogen atoms were calculated and added to the refinement process. 

Single-crystal X-ray diffraction is the most useful experimental technique available for the determination of the three-dimensional crystal arrangement. Improvements in automated data collection methods on computer-controlled diffractometers and the development of more-robust refinement software has led to structure solution becoming almost routine. Over the past 10 years the total number of structures stored in the Cambridge Crystallographic Database (often referred to as the Cambridge Structural... 

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