Matrix Isolation

Type of physical science: Chemistry

Field of study: Chemical methods

Matrix isolation is a technique in which highly reactive species are trapped, or chemical reactions are allowed to take place, in a cryogenic matrix of an inert material. This technique facilitates study of highly reactive, unstable chemical species not easily examined by conventional analytical techniques.

Overview

Matrix isolation is a technique in which, as the name implies, certain types of highly reactive or unstable chemical species are isolated, or trapped, within a matrix of some other, inert, material in order to study these species or, in some cases, to prepare such species for subsequent use in another application or process.

This technique requires highly specialized equipment and much modern technology, especially cryogenic and high-vacuum technology. Production of a matrix requires cryogenic conditions, with the temperature of the isolated matrix and chemical species typically being at or below that of liquid nitrogen. Most matrix isolation work is performed at temperatures corresponding to liquid helium, which is only about 4 degrees Celsius above absolute zero, the coldest possible temperature. To reach this temperature, liquid helium or a specialized refrigerator must be utilized, either of which is an expensive proposition calling for much specialized equipment and plumbing. The matrix material is usually an inert element, with argon being a particularly useful example, in order to minimize the interaction of the isolated chemical species with the matrix itself. Matrix formation typically takes place via deposition of the matrix gas and one or more chemical species of interest onto some inert solid surface, such as an unreactive metal that has been cooled to the desired temperature in a high vacuum. The vacuum is necessary both to maintain the cold temperature of the deposition surface by acting as an insulator, much as the vacuum chamber of a glass thermos bottle, and to allow the stream of matrix gas and reactant(s) to reach the deposition surface.

Materials trapped in the matrix can be subsequently analyzed by a battery of spectroscopic methods. In addition, especially if more than one reactant is codeposited in the matrix, chemical reactions that take place in the vapor phase prior to deposition can be probed by studying the reaction products subsequently trapped. Matrix isolation is particularly valuable for the production, isolation, and study of highly unstable or reactive chemical substances that do not exist for extended periods of time at ambient conditions.

Matrix isolation techniques aid in understanding the structures and properties of highly reactive materials. As an example, consider the pyrolysis of coal which yields, ultimately, coke, along with various volatile organic gases and liquids as well as highly reactive species such as free radicals. In order to understand both the mechanism of coal pyrolysis and factors that can improve the efficiency of this process, the pyrolysis products may be trapped in an argon matrix and analyzed by various spectroscopic techniques. Such a study provides information about the structure and chemical bonding in both the pyrolysis products and the original coal. This knowledge aids the development of more efficient energy production from the coal as well as providing insight applicable to coal gasification techniques as an alternative energy source to petroleum.

There are two major areas of concentration in the field of matrix isolation. Most reported work combines matrix isolation with some sort of analytical technique to learn about the structure and bonding forces existing in trapped samples. Spectroscopic techniques utilize the interaction of electromagnetic radiation, or radiant energy, with matter and are essentially the only analytical tools that can be utilized on matrix isolated samples resulting from the requirements of high vacuum and low temperature to maintain the matrix. Most spectroscopic techniques including infrared, electron paramagnetic resonance, and others may be applied to samples in a matrix. In these methods, the absorption of radiant energy causes some sort of transition from one energy state to another in the studied molecule. The spectroscopist measures the energies absorbed by the species, which in turn supplies information about the type of bonds present, the electronic structure of the species, or other information depending on the spectroscopic method utilized.

In order to apply spectroscopic techniques to matrix isolated samples, some sort of window, which is transparent to the electromagnetic radiation utilized in the spectroscopic technique, must be provided so that the sample can be studied while still at very low temperature and pressure. This requirement is central to the design of any matrix isolation spectroscopy apparatus. To maintain low pressures, the apparatus must have a chamber that can be evacuated.

Inside this chamber is the actual surface on which the deposition takes place. The deposition surface must withstand the extreme cold and must not interfere with the spectroscopic technique employed. In spectroscopic methods utilizing magnetic fields, the deposition surface utilized is usually a sapphire rod rather than a metal to eliminate interference by the deposition surface. The surface is in contact with a good thermal conductor which is, in turn, in contact with the refrigerant. The thermal conductor is necessary to allow speedy removal of heat from the deposition surface, as the substances depositing on the surface are at a higher temperature. One cannot assume that the deposition surface will remain at a fixed temperature. In fact, in certain types of experiments it is desirable to allow the surface to warm slightly to stimulate chemical reactions within the deposited material. For these reasons, it is necessary to provide some means of monitoring the temperature of the deposition surface and thus of the matrix. This is typically accomplished by a thermocouple, in which temperature is measured as a function of the potential that develops when the junctions of the thermocouple are at different temperatures.

Production of a matrix requires the codeposition of an inert matrix material along with the species to be studied. The matrix material used is typically a noble gas such as argon, although for certain applications, other gases such as nitrogen or methane may be utilized. This matrix gas is typically introduced to the chamber via a nozzle directed at the deposition surface.

Matrix properties will vary depending on its temperature. In addition, any impurities will alter the structural character of the matrix so that it is very important that there be no leaks allowing entry of air, water, or other materials into the chamber. A semiordered structure characterizes the pure matrix material, in which localized areas of highly ordered structure exist. These ordered areas are randomly oriented relative to one another so that the structure is best described as semicrystalline.

When another material is codeposited with the matrix gas, it must somehow be incorporated into this structure. As it is desired actually to isolate reactive species in the matrix, ideally they will be incorporated into the ordered structure of the matrix by substitution of the reactive species for one or more of the atoms or molecules in the matrix material. Larger molecules deposited within the matrix will necessarily occupy sites normally occupied by many matrix atoms or molecules. Ideally, the reactive substance is completely surrounded by the inert matrix material so that it cannot interact with other reactive species. In order to ensure complete isolation of the deposited species, the reactive species must be present in a very small amount relative to the inert matrix material. This ensures wide separation of reactive species within the matrix so that chemical reactivity is minimized. In addition, the matrix must be kept well below its freezing point, maintaining rigidity and preventing diffusion of the trapped species through the matrix where they may react with one another. This is particularly important when a highly reactive species is generated in the gas phase and trapped in the matrix in order to be studied, as in the case of coal pyrolysis. As the matrix warms, it becomes less rigid, allowing the components to migrate into slightly more stable packing configurations, a process known as annealing. If the temperature rises above the annealing range, the species in the matrix become quite mobile, resulting in the reactive species actually diffusing toward and mingling with one another so that they are no longer trapped and will react to form products that were not present when the matrix was in its initial rigid state. The higher the temperature of the matrix, the greater this reactivity will be.

Sometimes, however, this is a desirable characteristic. It is possible to deposit one reactant in the matrix, then to add a second reactant. In a rigid matrix, no reaction of the two takes place. By slowly warming the matrix, migration occurs and the two substances will chemically react. The formation of products and intermediates can be monitored spectroscopically. Such experiments allow much information about the actual pathway, or mechanism, of a chemical reaction to be determined.

In order to introduce a substance into the matrix, it must be in the gas phase. The final component necessary in a matrix isolation apparatus is thus some device (typically an oven of some sort) to vaporize the substance being studied. In the case of the coal study mentioned earlier, this is accomplished by a simple furnace in which the coal is heated as a result of resistance to electrical current flow in much the same way that the eye of an electric range functions. Other techniques, such as laser vaporization, may be utilized if an oven is not adequate.

Applications

Matrix isolation is a versatile tool for the study of chemical reaction pathways and highly reactive materials. Typically, chemical reactions take place by a sequence of steps called a mechanism. Often, substances are produced in one step of the mechanism which are highly reactive and react rapidly in a subsequent step. These materials, which typically have very brief lifetimes, are called intermediates. Intermediates may be studied in the gas phase with techniques that can make very rapid measurements. Matrix isolation provides a means whereby these reactive intermediates may actually be trapped in such a fashion that, even though they are highly unstable and reactive, they will exist for long periods of time simply because there is nothing in the matrix with which they can react. In addition, the very cold matrix actually lowers the energy of these species so that they will not decompose as readily.

When coal is thermally decomposed via pyrolysis, very reactive intermediates are observed when the pyrolysis products are isolated in a matrix and analyzed. Coal, however, is unusual in that these species are generated simply by heating the coal. To study reactive intermediates, the stable starting material is usually deposited in a matrix, then the reactive species is generated by some method such as bombardment with high-energy electromagnetic radiation that can break chemical bonds to form these reactive materials in the matrix. Typically, ultraviolet light or lasers are used in this process, which is called photolysis. Generating the intermediate in the matrix facilitates study of the subsequent reactivity of the intermediate. This may include simple decomposition to more stable substances. If the matrix is slightly warmed, these intermediate species can migrate and react with other materials with which they come in contact in the matrix to yield new products. These processes can all be monitored over time spectroscopically.

Photolysis techniques are especially useful in studies of organometallic compounds containing carbon-hydrogen bonds. Reactions of organometallic compounds often proceed via an intermediate in which the bonding capacity of an atom, often the metal, is unfilled. Such a species is said to be unsaturated. Photolysis may be used to generate these unsaturated species in a matrix. Since the intermediate is not interacting chemically with any surrounding materials, its properties are typically assumed to be very similar to what its properties would be in the gas phase. Thus, study of these intermediates in the matrix allows understanding of their probable structure to be gained, which in turn will lead to increased understanding of the actual mechanism of the chemical reaction in which the intermediate is generated.

An understanding of the properties of unsaturated species is very important in developing new and more effective catalysts. Many chemical reactions of commercial importance rely on a catalyst, which speeds up a net chemical reaction but undergoes no net change in the process. Often, reactions are catalyzed by metal surfaces, in which many of the metal atoms are unsaturated. In order to understand more thoroughly the functioning of catalysts, it is desirable to understand the properties of unsaturated metal atoms. This is readily accomplished in a matrix isolation apparatus. The metal of interest is vaporized to generate metal atoms, which are codeposited in the matrix with other chemical species that can form bonds with the metal. Depending on the relative concentrations of materials, individual metal atoms may be deposited in the matrix or aggregates of several metal atoms, called clusters, may be deposited.

Codeposition of the metal and another reactant results in the formation of unsaturated species.

Spectroscopic analysis then allows determination of the properties and bonding modes that are typical of a metal surface. This allows greater understanding of the bonding modes typical of catalyst surfaces and ultimately may lead to development of a more effective catalyst.

A more limited application of matrix isolation is in the actual preparative scale synthesis of novel chemical substances. Usually, substances produced by matrix isolation are not easily extracted from the matrix without decomposing. If such species could be isolated, there would be a wide range of potential uses for them. For example, clusters of metal atoms may be isolated in a matrix. If the clusters can be placed, without decomposing, onto a suitable support, they could then be used industrially as catalysts.

Context

Scientists had toyed with the basic idea of matrix isolation for a long time before actually being able to put it into practice. It is a classic example of a theory that is simple in concept but difficult in practice. Generally, as things get colder, they become more chemically inert. Thus, the idea of essentially "freezing out" highly reactive reaction products or intermediates was a straightforward intellectual advancement. In order to do this effectively, however, scientists had to wait until cryogenic technology was available actually to produce surfaces that were cold enough to trap these species. While some experiments relevant to the development of modern matrix isolation methods were performed in the mid-1920's, they were not repeated or developed because of the limited availability of liquid helium.

In the 1950's, technology was developed which made possible the systematic utilization of matrix isolation. With the increasing availability of cryogenic liquids, more matrix isolation experiments were performed. The greatest boon to the technique was not so much availability of liquid helium as the development of refrigerator technology that allowed generation and long-term maintenance of extremely low temperatures in the matrix isolation apparatus. This increased the length of time available for a single experiment, which previously had depended on the amount of liquid helium available. With the extended time frame, it was possible to deposit greater quantities of the desired materials in the matrix, so that more information could be gleaned. In addition, the versatility of the method was increased. With more precise temperature control of the matrix, experiments dependent on diffusion through the matrix could be performed in a controlled fashion. With the development of modern computer controlled data collection and analysis equipment, the amount of information and the sensitivity of measurements have greatly increased.

Matrix isolation is an important technique in the study and isolation of atomic clusters.

As the importance of clusters in the theoretical and practical framework of chemistry increases, matrix isolation methods will offer the key to thorough study of these species and may very likely afford an easy synthetic route for these highly reactive materials.

Another area of increasing importance is the application of matrix isolation to chemical vapor deposition. This technique is used in industry to produce thin semiconductor films. Matrix isolation provides an easy way to gain understanding of the film formation process, which will help improve the production techniques for these films. Ultimately, this will lead to improved semiconductors.

In summary, matrix isolation is an experimental technique that not only has justified its existence but also is continually found to be applicable to problems on the forefront of modern science.

Principal terms

ANNEALING: a process in which structural irregularities in a crystalline or semicrystalline material are minimized by warming

FREE RADICAL: a highly reactive chemical substance possessing a lone unshared electron

INERT ELEMENT: a chemical element such as neon or argon that is chemically unreactive because of its stable electron configuration

INTERMEDIATE: a reactive species that is produced and subsequently consumed during the course of a chemical reaction

MECHANISM: a sequence of chemical steps describing the route by which a chemical change takes place, including the net change observed and the existence of any intermediates in the net reaction

PHOTOLYSIS: a process in which a chemical species is split into reactive components as a result of bombardment with high-energy light

PYROLYSIS: the thermal decomposition of a chemical compound

SPECTROSCOPY: the study of the interaction of electromagnetic radiation of various energies with chemical substances

Bibliography

Cradock, Stephen, and A. J. Hinchcliffe. MATRIX ISOLATION. London: Cambridge University Press, 1975. A very well-written work giving excellent comprehensive coverage of matrix isolation. While containing a fair amount of technical information, the text is largely narrative, very readable, and illustrated with excellent, well-captioned drawings. The work includes chapters on matrix materials, with a discussion of probable matrix structure, and the technology of modern matrix isolation with detailed discussion of refrigerator development and vacuum generation. This is probably the most comprehensive work on the general aspects of matrix isolation available.

Howard, J. A., and B. Mile. "Study of the Genesis, Structure, and Reactions of Small Metal Clusters Using a Rotating Cryostat." ACCOUNTS OF CHEMICAL RESEARCH 20 (1987): 173. This is one of the more readable scientific journals, as it contains summaries of specific types of research. This article describes a preparative scale technique for highly reactive compounds utilizing a reactor similar to that in the Timms reference and true matrix isolation techniques. Includes a summary of the product types generated as well as the types of reactions utilized to generate these products.

Moskovits, Martin. "Applications of Matrix Isolation to the Study of Metal Clusters with a Postscript on the Reactivity of Clusters in Supersonic Beams." In METAL CLUSTERS, edited by Martin Moskovits. New York: John Wiley & Sons, 1986. Although this chapter is technical in nature, the determined reader may glean from it an understanding of how matrix isolation may be used to study metal clusters. Includes a good overview of the spectroscopic techniques that are applicable and a brief discussion on cluster generation.

Perutz, Robin N. "C-H Activation by Organometallics: The Role of Matrix Isolation Studies." PURE AND APPLIED CHEMISTRY 62 (1990): 1539. Describes in-depth photolysis as applied to matrix isolation studies, specifically in the area of carbon-hydrogen bond activation, important in the study of catalysis.

Timms, P. L. "Low Temperature Condensation of High Temperature Species as a Synthetic Method." ADVANCES IN INORGANIC CHEMISTRY AND RADIOCHEMISTRY 14 (1972): 121. While not strictly dealing with matrix isolation, this work describes synthesis methods in which products are collected by deposition on the sides of a cooled vessel. Describes techniques for generating the gas phase reactive species and includes a drawing of an apparatus to condense the products. The bulk of the work is a review of the types of compounds synthesized using this method.

Clusters of Atoms

Photochemistry, Plasma Chemistry and Radiation Chemistry

Photon Interactions with Molecules