Science
More U.S. chemists using neutron diffraction NSF cooperative program enables researchers to use facility at Argonne National Lab to study structural details of complex molecules Ward Worthy C&EN, Chicago Neutron diffraction crystallography is a powerful tool for elucidating the structures of complex molecules. Its use is growing rapidly, as more scientists become aware that neutron diffraction can open up entirely new fields of study.
Unfortunately, most of the recent growth has occurred outside the U.S.—for example, at new large facilities like the joint French-German-British high-flux reactor at Institut Laue-Langevin at Grenoble, France. Because of the need for large research reactors, complex instrumentation, and large-scale computers and software, neutron diffraction research is very costly. Its pursuit in the U.S. has generally been limited to the largest research laboratories, notably the national laboratories. And in recent years, relatively static funding of reactor-based research—coupled with rising reactor operating costs—has led to underutilization of existing facilities. Lately, however, a cooperative program funded by the structural chemistry
Technique allows direct observation of strong carbon-hydrogen-metal interaction
1.137 A
1.879 A
\ D
*s>
=c : H
P(OCH3)3
[Fe(P(OCH3)3)3(r,3-C 8 H 13 )]
26
C&EN Jan. 29, 1979
+
(OCH3)3
branch of the National Science Foundation has enabled an increasing number of investigators to take advantage of national neutron diffraction facilities. As Dr. Jack M. Williams, a senior chemist and neutron diffraction specialist at the Department of Energy's Argonne National Laboratory, points out, various mechanisms are available to fund university scientists using a large research facility. At one extreme, portions of individual grants are contributed to facility operating costs. At the other, a general "users group" is established and funded. P'or this particular program, an intermediate approach was deemed most beneficial—that is, to include all the individual research proposals to use the Argonne single-crystal neutron diffraction facility in a single proposal. That way, all the proposed research could undergo peer review at the same time. Also, the investigators—and Argonne—would know at the outset the total financial resources that would be available, and so could plan for most efficient use of the facility. As a fringe benefit, DOE levies no charges for the neutrons used in the program. Currently, Williams is working with 17 coprincipal investigators from 15 universities. They're tackling a wide variety of problems, including investigations of the hydrogen bond, studies of metal cluster compounds, determination of electronic charge distributions, and investigations of new superconductors. Neutron diffraction is only one of several tools available for structure determinations, of course. But one of its strong points is that it can often provide direct evidence of structural details that can only be inferred by other means. For example, Williams says, it's generally accepted—on the basis of x-ray diffraction and nuclear magnetic resonance experiments by Cotton, Muetterties, Parshall, and others—that metal complex-induced carbon-hydrogen bond activation, which is often important to the understanding of homogeneously catalyzed chemical reactions, often proceeds by an intermediate in which the hydrogen atom that is bonded to the carbon atom also is attracted to a transition metal. But the geometry of that intermediate, represented by C—H—M, has heretofore been unknown. Recently, however, Williams and Argonne coworkers Richard K. Brown and Arthur J. Schultz, together with Dr. Galen D. Stucky of the University of Illinois and Dr. Steven D. Ittel of Du Pont, used neutron diffraction to make the first direct observation of an unprecedentedly strong C—H—M interaction. In the metal-hy-
drocarbon complex [Fe(P(OCH3)3)3(773C8H 13 )] + [BF4]-, they found the H - F e separation (an indication of their interaction) to be only 1.879 A, the shortest ever observed. However, the carbonhydrogen bond separation, 1.137 A, wasn't significantly longer than the 1.10 A determined for aliphatic carbon-hydrogen bonds by spectroscopic methods. They suggest, therefore, that the C— H—Fe geometry they found is a precursor of a fully delocalized two-electron, three-center bond. It's tempting to speculate, they add, that in certain homogeneously catalyzed reactions, an otherwise inert carbon-hydrogen bond is activated (lengthened and weakened) as a result of the formation of such a bond. In another experiment, the Argonne team and Dr. F. Albert Cotton, of Texas A&M, have performed the first neutron diffraction studies of a quadruply bonded ditungsten complex, W 2 (C 8 Hi3)3. The work was done to determine accurately the carbon-carbon distances and to locate all the hydrogen atoms in the presence of the heavy tungsten atoms, Cotton says. Because tungsten both strongly scatters and absorbs x-rays, he explains, it wasn't possible to determine the carbon-carbon distances precisely using x-ray diffraction. The uncertainties were so large as to render meaningless many potentially interesting evaluations of distances and angles. The added precision of the neutron results has enabled the researchers to make more meaningful comparisons
Brown, Williams, Schultz align components of neutron diffractometer with analagous molybdenum and chromium complexes, and has also made it possible to make internal comparisons. These and many other neutron diffraction experiments have been done at Argonne's single-crystal neutron diffraction facility, although data are also collected on occasion at other national laboratories. Williams notes that the Argonne facility, which is controlled by a Xerox Sigma 5 computer, is one of the world's most highly automated. And Argonne's CP-5 neutron source
yields a high thermal neutron flux (about 3 X 106 neutrons per sq cm per second at the sample), comparable to those of similar instruments at Brookhaven and Oak Ridge National Laboratories. The high neutron flux is achieved by use of a veryhigh-quality beryllium single-crystal monochromator and by optimization of the instrumental configuration for single-crystal data collection. Because such beryllium crystals haven't been available for some years, most neutron diffraction facilities must use germanium or copper
Neutron diffraction analysis has some advantages over x-ray diffraction Although they are considered complementary techniques, neutron diffraction and x-ray diffraction are fundamentally quite similar. Both are based on the principle that a beam of incident radiation—whether electromagnetic as in the case of x-rays or particulate as in the case of neutrons—is scattered by the atoms in a crystal. When the wave length of the radiation is comparable to the distances between atoms, the scattered waves will variously combine and cancel, creating interferences. Strong reflections will occur only when "Bragg conditions" are satisfied—that is when nX = 2d sin 9 (where n is an integer, A the wave length, of the distance between two adjacent crystal planes, and 9 the angle between the incident beam and the reflecting plane). For a single-crystal sample, only a few Bragg reflections will occur at any given orientation. But as the crystal is rotated, there will emerge a diffraction pattern that can be recorded on film or with electronic detectors. Since the wave length and the angles are known, interplanar distances can be calculated. The intensity of the reflections is largely a function of the distribution of atoms within the crystal; study of the varying intensities can lead to determination of molecular structure.
Although the principles are fairly simple, the practice is not. Typically, great masses of intensity-vs.-angle data are generated in an experiment. Processing, refining, and interpreting the data require mathematical sophistication and powerful computers. X-rays are much cheaper to produce then neutrons. And, since x-rays are much more energetic than the neutrons used for diffraction studies, x-ray data can be obtained in less time and from crystals that would be too small to yield usable neutron diffraction data. So why bother with neutron diffraction? As it turns out, neutron diffraction works particularly well in areas in which x-ray diffraction is deficient. In x-ray diffraction, electrons are the principal cause of scattering. The more electrons an atom has, the more it scatters the x-rays. X-ray diffraction is usually the method of choice for initial determination of the locations of atoms of heavy elements. But accurately locating lighter atoms—especially hydrogen atoms—is often a difficult problem. Neutroh scattering, in contrast, involves nuclear rather than electronic processes. There's no regular relationship between atomic number and scattering amplitude. In general, light atoms are " s e e n " as well as heavy atoms.
Also, adjacent elements in the periodic table may have markedly different neutron-scattering amplitudes. As a result, neutron diffraction can easily distinguish between atoms, such as carbon and nitrogen in C = N, or nitrogen and oxygen in N = O, that would look much alike by x-ray diffraction. It can even distinguish between isotopes. Also, x-ray scattering factors must be calculated—and the values vary with the method of calculation—whereas neutron scattering amplitudes are determined experimentally. When light atoms are involved, bond lengths derived from neutron diffraction studies can be more accurate by nearly an order of magnitude than those obtained by x-ray diffraction. Another difference between the two techniques is that neutron intensities translate into true nuclear positions; atomic coordinates determined by x-ray diffraction are shifted somewhat by electron changes resulting from chemical bonding. However, this difference turns out to be an advantage in at least one field of study: By comparing the nuclear position determined by neutron diffraction with the electron "cloud" positions determined by x-ray diffraction, it's possible to obtain a better understanding of molecular bonding electron distribution.
Jan. 29, 1979 C&EN 27
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crystals, which generally yield lower neutron fluxes. But with the high neutron flux available at Argonne, Williams says, it's possible to work easily with crystals having unit cell parameters up to'30 A. The single-crystal neutron diffraction facility, a mile away from the Argonne chemistry labs, can be operated by teletype terminal either at the reactor or remotely from the lab. The ability to control data collection from the lab speeds up the collection process and allows more efficient use of manpower, Williams notes. With recently installed new instrumentation, it's now possible to align the crystal remotely, inspect data as they are collected, plot and contour Fourier maps (in seconds), plot drawings of crystal structures, and interact on a real-time basis to modify the data collection process. For routine data collection, a liquid nitrogen cold-stream device, designed by Dr. Charles Strouse of the University of California, Los Angeles, furnishes temperatures between about 100° K and room temperature. Since October 1977, all data have been collected at 110° K, Williams says, noting that at this temperature, the scattered neutron intensities for most crystals are approximately doubled, compared to room temperature. Neutron diffraction capabilities at Argonne will increase greatly when DOE's intense pulsed neutron system—now under construction—goes into operation. With the pulsed neutron source, there's no nuclear reactor. Instead, pulses of high-energy protons from an accelerator strike a target made of a heavy element such as tungsten or uranium, producing intense neutron bursts through a spallation process. The fast neutrons are then moderated to thermal or epithermal energies. According to current plans, peak thermal neutron fluxes of 2 X 10 15 neutrons per sq cm per second will be available by 1981. And by 1984, peak fluxes are expected to exceed 10 16 neutrons per sq cm per second—an increase of an order of magnitude over the highest available from any of today's reactor sources. The pulsed nature of the neutron beam also will help speed up data collection. The current approach, using a continuous beam, is to select a single wave length, then "scan" the crystal by rotating it mechanically. But with the new pulsed source, and a new single-crystal diffractometer defined by Argonne chemist Selmer W. Peterson, a "white" (multiwave length) neutron pulse will strike the sample, which will remain stationary. A large number of reciprocal lattice points will simultaneously satisfy the Bragg condition, each for a different wave length and/or scattering angle. A large-area position-sensitive detector will simultaneously register hundreds or thousands of reflections, each with x and y coordinates and also a time element. Time-of-flight analysis will correlate the scattering events with the different wave lengths. "Eventually," Williams says, "we expect it will be possible to collect complete data sets on x-ray-size single crystals in a matter of hours or days." •
Education African science infrastructure takes form Chemistry professors from a number of universities in Africa are gathering this month at the University of Benin, in Lome, Togo. They are there for a workshop on laboratory experiments in chemistry. That workshop is one of the first fruits of a developing science research and education infrastructure in Africa. And chemistry is playing a leading role in that development. Among other fruits of the African efforts are several environmental chemistry workshops that already have taken place. And plans have been laid for an international conference on chemistry to be held in 1980. The conference will take place from July 27 to Aug. 1 at the University of Nairobi, Kenya. The seed of the organizational structure promoting these activities was planted in 1974 in Dakar, Senegal, at an interministerial conference on science and technology for development, held under the auspices of the United Nations Educational, Scientific & Cultural Organization (UNESCO). That conference placed a strong emphasis on cooperation among scientific institutions in Africa as a way to mobilize Africa's scientific and technological resources for development. An outgrowth of that meeting is the Association of Faculties of Science of African Universities (AFSAU). This organization was formed in September 1977 at a constitutive assembly held in Nairobi, at which there were scientists representing almost half the universities in One of the first efforts of AFSAU, under the chairmanship of Prof. Douglas Odhiambo, vice-chancellor of the University of Nairobi, was to outline a plan for an African regional network for research and training in the basic sciences. This plan envisioned a group of subnetworks of scientific institutions in biology, chemistry, physics, geology, and mathematics. Scientists within each subnetwork would cooperate on such activities as workshops, exchange visits, training courses, and cooperative research. Without the subnetworks, there was no mechanism for cooperation except on an ad hoc basis. Meanwhile, specific plans already were moving ahead on environmental chemistry. Following the AFSAU constitutive assembly, 22 scientists from African universities and research institutions convened at an "expert meeting on environmental chemistry." At that session, the scientists drew up a proposal for an African regional network for environmental chemistry. Pending formation of a chemistry committee by AFSAU, the expert meeting established two subregional secretariats for the environmental chemistry regional
network. The one for western Africa, headed by chemistry professor O. Osibanjo, is at the chemistry department of the University of Ibadan in Nigeria. The secretariat for eastern Africa is at the chemistry department of the University of Nairobi and is headed by Dr. S. O. Wandiga. The secretariats lost no time in organizing workshops for chemistry specialists in their respective subregions. A workshop on analytical techniques in environmental chemistry was held in late March at the Ibadan chemistry department for 12 participants from five countries in western Africa and in early April at the Nairobi chemistry department for 22 specialists from six countries in eastern and southern Africa. Both workshops were able to draw on the expertise of a consultant supplied by UNESCO. The consultant, Prof. S0ren Jensen of the University of Stockholm, a noted specialist in chemical analysis of micro quantities of pesticides, worked in the lab with the participants, sharing his knowledge of sampling and analysis. The AFSAU chemistry committee was formally established at the second meeting of the AFSAU executive committee last July in Yaounde, Cameroon. The environmental chemistry network became an activity of the chemistry committee. Chairman and convener of the chemistry committee is Prof. M. Adjangba of the University of Benin. At its meeting in Yaounde, the chemistry committee established a number of tasks for itself, including efforts to: • Encourage chemistry departments to examine their curricula in chemistry with a view toward possible improvements and greater relevance to national situations. • Encourage departments to develop their postgraduate offerings and activities. • Explore the possibilities of publishing an African science journal. • Prepare a directory of African chemistry department personnel, equipment, courses, and research projects. With a formal organization, the chemistry committee now can enter into contracts; the first result is this month's workshop in Lome. It is being held under contract with UNESCO, and the resulting material will be published as Volume IV of the UNESCO Sourcebook in Laboratory Experiments in Chemistry. The new volume will be one of a series. Volume I resulted from a workshop in South Korea; Volume II came from a workshop in Jordan; and Volume III will result from a workshop in Mexico. The Lome workshop is focusing on material for first- and second-year university students. Both the workshop and the publication will be in French and Jan. 29, 1979 C&EN
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