Dynamics of Chemical Reactions

Type of physical science: Chemistry

Field of study: Chemical reactions

The microscopic, or molecular, view of the way in which atoms exchange and energy flows during the progression of a single chemically reactive collision forms the basis of the study of chemical reaction dynamics. Such a detailed picture of a reaction provides insight into why certain species react, the nature of the product molecules that will be formed, and the fate of the inevitably released (or consumed) energy.

Overview

Chemical reaction dynamics is an encompassing concept or field of study that attempts to describe the snapshot series of events and changing forces that govern a chemically reactive encounter, from the point at which two molecules begin to approach each other through the intimate collision in which atoms, electrons, or molecular fragments are exchanged, and beyond this to the point at which product molecules separate with well-defined amounts of energy in each of their possible motions or degrees of freedom (vibration, rotation, and translation). As such, it is a diverse and somewhat elusive topic that focuses on a chemical reaction event from a truly microscopic perspective. A single reactive encounter lasts for less than one-billionth of a second in most cases, so experimental observation of the event itself is all but impossible, and the study of reaction dynamics must depend upon a combination of indirect experimental observations and theoretical computation in order to gain insight into the progressive action.

Bulk reaction studies, which are very important in the areas of chemical synthesis and manufacturing, environmental chemistry, and geochemistry, focus predominantly upon the gross identification of products formed and the overall rates at which the process proceeds (that is, how many kilograms of reactant are consumed per hour). In dynamic studies, scientists ask fundamental questions about the forces that must act between two reactants to allow the transformation to proceed as observed, which molecular motions assist this process, the fate of the energy that is either consumed or released, and how one might make use of this information in controlling or exploiting reactions for man's use or even to understand more fully the natural physical phenomena of chemical origin.

The aurora borealis, or northern lights, provides a prime example of the way in which reaction dynamics can control the outcome of a chemical reaction. Anyone who has been in the northern latitudes during periods of intense solar activity can attest to the magnificence of the flickering patches of red and green light that are emitted from the night sky during these periods.

The green light is a result of the fluorescence of molecular nitrogen produced by the recombination of two nitrogen atoms. If one takes a sample of air (approximately 80 percent molecular nitrogen) into the laboratory and inputs energy to it in the form of heat, one finds that no amount of physically realizable thermal energy is sufficient to reproduce this green emission.

During solar activity, however, streams of charged particles are thrown out that eventually strike the upper atmosphere, and an amount of net energy per liter of air equivalent to what might have been applied in the laboratory is deposited by these particles. Because of the different form of the energy (charged-particle bombardment versus thermal heating), a very small fraction of the nitrogen molecules dissociate to yield excited nitrogen atoms. When these collide and recombine with other nitrogen atoms, they produce molecular nitrogen in an electronically excited state that can relax by emitting the green auroral light. Thus, the net conversion of collisional energy into light emission is catalyzed by a very specific chemical reaction whose dynamics are intimately influenced by the nature of the reactant nitrogen atoms. This example is but one way in which the natural forces between reactant species affect the outcome of a chemical (or energetic) process.

The forces that govern the behavior of a reaction are quite complicated and varied, but in the chemical realm they are almost all caused by the interaction between the electrons surrounding, and holding together, the molecules that are approaching a collisional event. At long distances of separation, the interaction between the molecular electron clouds that surround every molecule is negligible, and the only important forces are those of the electrons in each molecule with the molecule's own nuclei--that is, the bonding or covalent forces internal to each species. As the two molecular partners approach closer, there is a weak interaction between them that is almost always attractive, its low magnitude, however, plays a minor role in the subsequent chemistry. As the two approach within one to three bond distances (a typical bond distance is about 10^-10 meters, or 1 angstrom), this picture changes drastically, and the electrons from one molecule begin to interact strongly with both the electrons and the nucleus of the other collision partner. This covalent interaction can be either strongly attractive or repulsive, depending upon the specific molecules. With most typical molecules, the interaction is first mildly repulsive, but as the two come closer, it becomes attractive. This energy barrier is called the activation barrier, and the two colliding species must contain enough kinetic or internal energy to surmount this energy hill if they are to reach the attractive or energy-releasing portion of their collision path. It is in the region of separation of approximately one bond distance that the top of this hill resides, and this is the region of intimate electron mixing, where atoms and electrons may be permanently transferred between molecules. It is this transfer that results in the energy decrease and that describes the release of energy (the downhill portion) following traversal of the activation barrier. The post activation region is described by a path in which the newly formed molecules fly apart, and the energy released is found to reside in some balance between outgoing kinetic energy (speed of separation) and the internal energy of the new products (as vibrational and rotational motion, or stored electronic energy, which can later be released as fluorescence).

The energy released is not typically shared equally among all of these degrees of freedom in the products. Although the total energy must be conserved (the sum of the reactant kinetic plus potential energy must equal the sum of all the forms of energy in the products), the distribution of energy in the products is not usually found to be random. This distribution of energy is strongly governed by the forces in action between the evolving collision complex very near the top of the activation barrier.

For example, in the case of the reaction H2 + F → H + HF the reaction of molecular hydrogen with a fluorine atom to produce a free hydrogen atom and hydrofluoric acid (hydrogen fluoride), the collision complex or "transition state" looks like H-H→F at the barrier top, with the central H atom moving rapidly over to bond with the fluorine atom. Shortly after this transfer, the reaction exothermicity (outgoing energy from the creation of a stronger bond) causes the HF to separate; and much of the motion (or attractive force) of the transition state is "remembered," and the newly formed HF molecule is found to contain a very large percentage of the released energy in the form of vibration within the molecule. It is as if a new spring (bond) has been formed that initially was produced with a large extension before it was freed and allowed to fly apart, oscillating wildly about its equilibrium or rest extension.

Many reactions may experience strong torques at the activation barrier and produce products with great rotational excitation. In the case of some electron transfer reactions or reactions that transfer their atoms very late along the reaction path, one finds correspondingly large proportions of energy showing up as released kinetic energy, causing the molecules to fly apart with uncharacteristic speed. Thus, the energetic outcome of a chemical reaction is almost solely determined by the nature of the forces between the colliding pair during the very short time during which the "actual" reaction occurs, that is, during the existence of the transition state.

In all chemical reactions, this energy shows up as some balance between the degrees of freedom; one never encounters a force so specific as to put all the energy in only one form of motion. The actual distribution can vary widely, and observation of this distribution by means of physical measurements on the newly born products can reveal much about the nature of the elusive transition state in which all the important chemistry occurs.

The picture of a chemical reaction just described has treated the atoms and molecules as almost purely classical species; that is, as balls connected by springs that behave according to the strict rules of classical mechanics first developed by Sir Isaac Newton in the eighteenth century. It has been demonstrated in the twentieth century that such light species as atoms and electrons may show behavior that is characteristic of waves, defying traditional ideas of the ways in which material bodies should act. This description has been provided by the new theory of quantum mechanics, and in molecular reaction dynamics it is sometimes necessary to invoke such behavior. Thus, in order to calculate the strength of the interaction between molecules at close distances (the covalent forces) and the height of the activation barrier, it is necessary to rely solely on quantum mechanics and treat the electron clouds as waves of charge around the nuclei.

Similarly, in very unusual cases, particularly when light hydrogen atoms are being transferred between colliding pairs, it is not always necessary for the system to pass "over" the activation barrier; certain collisions are found to "tunnel" through it. In other words, a collision with insufficient energy to climb over the energy hill can, through its wave properties, be found to emerge spontaneously on the other side without ever having gone over. It is seemingly mysterious behavior such as this, as well as other quantum phenomena, that continues to make the study of chemical reaction dynamics a fascinating and active area of research.

Applications

One of the more remarkable applications of reaction dynamics has been in the area of laser development. The dynamics of certain chemical reactions, such as the fluorine-plus-hydrogen reaction discussed in the last section, lead to the very specific disposal of energy into particular motion. Thus, much energy was found to reside in vibrational motion of the newly born, or nascent, HF molecule, and in such circumstances it is possible to make use of this specific energy. If this reaction were run in a bulb of gas, subsequent collisions of the excited HF molecule with neighboring molecules would rapidly randomize the energy, and the net result would be a large sample of molecules that all have a little thermal energy; that is, the reaction would merely appear to heat up the bulb of gas. Fluorescence studies demonstrate, however, that highly excited molecules can lose their energy in another way--by emission of light. In the case of vibrationally excited molecules, this light appears in the infrared portion of the electromagnetic spectrum (infrared light is invisible to human eyes but is the same energy one feels when one places one's hand near, but not in contact with, a hot object such as a red-hot poker, or a heap of hot coals). If the HF-producing reaction is run in a low-pressure chamber, it is possible to harvest this energy before it is randomized, and in fact stimulate the light emission, in order to produce directed beams of infrared radiation. This light, the sole product of the transformation of chemical potential energy through the reaction process, creates what is called a chemical laser. The laser emits light of a well-defined frequency corresponding to the vibrational frequency of the HF bond. The reaction being discussed produces vibrational excitation so efficiently as to allow the generation of intense beams of light capable of melting plates of steel.

In the 1960's and 1970's, this laser was in fact temporarily exploited as a field weapon for the military.

More humanitarian applications of dynamics studies have been used in the production of fuel-efficient jet-propulsion engines. The propulsion of a jet or rocket engine is derived from the conversion of chemical potential energy through combustion into thermal energy, which expands the gases and leads to push out the engine exit. If some of this energy remains in product vibrational or electronic internal energy, then it may not be available for gas expansion and will reduce the efficiency of the fuel. Chemical dynamicists have been actively seeking fuels and fuel additives that either can lead to reactions with a greater release of exothermicity in the form of kinetic energy or that will rapidly convert the product internal energy into translation and thereby boost the overall efficiency.

The extraterrestrial regions of the universe provide ideal environments of low pressure in which to study the light emissions that result from chemical reactions. Laboratory studies done on Earth have led to great advances in understanding precisely what conditions and what chemical species exist in these distant realms. The large clouds of molecular species that lie between the stars contain the vast majority of matter in the universe and are of great importance in understanding how the universe was formed and how it is currently evolving. Reaction studies under conditions that simulate these environments as closely as possible have explained many of the mysteries of molecular synthesis in outer space. For example, if one looks at the emissions from these clouds and the absorptions of light that passes through them, one will find that a much greater percentage of the molecules present contain deuterium, the heavy isotope of hydrogen, than should be allowed according to the known fraction of deuterium in the universe. The temperature in these clouds is, however, extremely low (-250 degrees Celsius, or nearly 20 degrees above absolute zero), and when one considers the importance of vibrational motion to the exothermicity of reactions, one can conclude that the chemical reactions between molecules and deuterium proceed significantly faster at low temperatures than those with the light isotope, hydrogen. Thus, although there is much less deuterium available, the vibrational dynamics of the reaction cause these deuterated products to be strongly favored. Since this is only significant at low temperature, it has been dubbed the chemical "refrigerator effect."

Another fascinating interstellar reaction that can occur only at low temperature and very low pressure, radiative association, accounts for the critical reaction between atomic carbon ions and molecular hydrogen, which makes possible the production of nearly all the organic molecules in space. This reaction proceeds by emission of light during the collision itself. This highly unlikely and disfavored process causes the colliding pair to stick together and never separate. In this way, in the lonely confines of space, two colliding species find a way to come together, produce a stable molecule, and still conserve energy by emitting the reaction exothermicity in the form of a photon of light.

Context

The field of molecular reaction dynamics was born out of the development of molecular quantum mechanics, which started in the 1930's but did not gain the necessary computational tools required to determine accurate descriptions of complex molecular forces until after the 1950's. With the advent of high-speed computers, quantum theories could be applied accurately to calculate potential energies and force fields for molecules at any configuration along a reaction path. Although this idea is simple in concept, these calculations remain very difficult research problems today, and chemically accurate reaction surfaces exist for only the simplest of chemical reactions. With the rate of growth in computational capabilities, as well as in the understanding of quantum chemistry itself, this field is undergoing a revolution.

At about the same time that computational abilities expanded, experimentalists (driven by the optimism that they would soon be able to explain their results with accurate modern theories) devised sophisticated methods to probe the energetic outcome of single reaction events.

The benchmark work of John C. Polanyi in the late 1950's and early 1960's with regard to fluorescence studies of nascent reaction products (HF, for example) and the wonderfully inventive development of crossed molecular beam techniques (whereby fine streams of molecules are crossed in a vacuum and the scattered products are carefully analyzed) by Dudley R. Herschbach and Yuan T. Lee in the 1960's and 1970's caused an explosive increase in the understanding of the subtle elegance of the simplest of chemical reactions. These three pioneers of modern reaction dynamics were jointly awarded the 1986 Nobel Prize in Chemistry for this groundbreaking work, which set the path of the field for the next decade.

With the development of the laser, with its ability to probe or excite the energy of molecules selectively and discretely, a whole new field of dynamics studies emerged that has begun to investigate how specific molecular motions take part in the intricate transfer of mass during reactive events. Thus, it has become increasingly clear that one can alter the natural path of reactive events by selecting not random but particular motions in the incoming pair of molecules. In addition, the ability to bring molecules together with well-defined orientations, rather than the random orientations that result naturally, allows investigation of the importance of particular forces in directing the chemistry. These studies not only lead to a greater insight into the nature of single collisions but also allow one to hope that someday it may be possible to direct chemical reactions to proceed in precisely the manner that is most desirable.

Perhaps the youngest, most exciting area of dynamic investigation is associated with studies aimed at looking directly at the elusive transition state, the midpoint in the reaction at which almost all is decided but which lasts less than one-trillionth of a second. Should this be possible, as ever more sophisticated experiments suggest, many of the mysteries of chemically reactive processes will be revealed. These revelations will undoubtedly govern the future directions of chemical research in many areas of both fundamental and applied chemistry and molecular physics.

Principal terms

ACTIVATION BARRIER: the point on a reactive molecular collision path at which the potential energy reaches a maximum and over which the system must pass for reaction to occur

DEGREES OF FREEDOM: the collection of available motions in a molecule in which translation, rotation, vibration, or electronic energies may be stored

EXOTHERMICITY: the energy released when a chemical reaction occurs between reactants with a higher potential energy than the products

TRANSITION STATE: the energetic species or collision complex that exists at the point at which a pair of reactants passes over the activation barrier, midway on the reaction path

TUNNELING: the quantum-mechanical or wavelike phenomenon whereby light atoms seem to move through reaction or activation barriers

Bibliography

Alvarino, J. M., and E. Martinez. "Two-Body and Three-Body Atomic Recombination Reactions." JOURNAL OF CHEMICAL EDUCATION 60, no. 1 (1983): 53-56.

A treatise on the mechanisms of molecule formation from molecular collisions.

Discusses both radiative association and collision-stabilized association processes.

Carbo, R., and Ginebreda, A. "Interstellar Chemistry." JOURNAL OF CHEMICAL EDUCATION 62 (October, 1985): 832-836. A discussion of the role of molecular collisions and reaction dynamics in the formation of molecules in the interstellar medium.

Gruebele, Martin, and Ahmed H. Zewail. "Ultrafast Reaction Dynamics." PHYSICS TODAY 43 (May, 1990): 24-33. A clear description of the wave of chemical experimentation directed at a complete description of reactive collisions during the collision event itself.

Kovalenko, Laurie J., and S. R. Leone. "Innovative Laser Techniques in Chemical Kinetics." JOURNAL OF CHEMICAL EDUCATION 65 (August, 1988): 681-687.

A thorough survey of the uses of modern laser techniques for the study of molecular collision phenomena, both reactive and inelastic.

Lee, Yuan T. "Molecular Beam Studies of Elementary Chemical Processes." SCIENCE 236 (May 15, 1987): 793-798. A condensed version of the author's Nobel Prize lecture concerning the use of molecular beam technology in elucidating molecular reaction dynamics.

Levine, R. D., and R. B. Bernstein. MOLECULAR REACTION DYNAMICS. New York: Oxford University Press, 1974. This superbly written book, the quintessential treatise on molecular-reaction dynamics, provides the student with the bridge between a simple elementary understanding of chemistry and physics, and modern thought and experiments as they are practiced in the field. Perhaps moderately advanced, but not to be missed by the serious student who wishes to continue study.

Polanyi, John C. "Some Concepts in Reaction Dynamics." SCIENCE 236 (May 8, 1987): 680-690. A condensed version of the author's Nobel Prize address detailing the investigation of reaction mechanisms and molecular dynamics through the study of energy release in reaction products, the role of reagent energy, and what is expected from future direct studies of transition states.

Chemical Reactions and Collisions

Electrons and Atoms

Interstellar Clouds and the Interstellar Medium

Quantum Mechanics of Molecules