Chemical dynamics

(physical chemistry) A branch of physical chemistry that seeks to explain time-dependent phenomena, such as energy transfer and chemical reactions, in terms of the detailed motion of the nuclei and electrons that constitute the system.

That branch of physical chemistry which seeks to explain time-dependent phenomena, such as energy transfer and chemical reaction, in terms of the detailed motion of the nuclei and electrons which constitute the system.

In principle, it is possible to prepare two reagents in specific quantum states and to determine the quantum-state distribution of the products. In practice, this is very difficult, and experiments have mostly been limited to preparing one reagent or to determining some aspect of the product distribution. This approach yields data concerning the detailed aspects of the dynamics.

Energy distribution

An important question regarding the dynamics of chemical reactions has to do with the product energy distribution in exothermic reactions. For example, because the hydrogen fluoride (HF) molecule is more strongly bound than the H₂ molecule, reaction (1)
1
releases a considerable amount of energy (more than 30 kcal/mol or 126 kJ/mol). The two possible paths for this energy release to follow are into translations, that is, with HF and H speeding away from each other, or into vibrational motion of HF.

In this case it is vibration, and this has rather dramatic consequences; the reaction creates a population inversion among the vibrational energy levels of HF—that is, the higher vibrational levels have more population than the lower levels—and the emission of infrared light from these excited vibrational levels can be made to form a chemical laser. A number of other reactions also give a population inversion among the vibrational energy levels, and can thus be used to make lasers.

Most effective energy

The rates of most chemical reactions are increased if they are given more energy. In macroscopic kinetics this corresponds to increasing the temperature, and most reactions are faster at higher temperatures. It seems reasonable, though, that some types of energy will be more effective in accelerating the reaction than others. For example, in reaction (2),
2
where potassium (K) reacts with hydrogen chloride (HCl) to form potassium chloride (KCl), studies have shown that if HCl is vibrationally excited (by using a laser), this reaction is found to proceed approximately 100 times faster, while the same amount of energy in translational kinetic energy has a smaller effect. Here, therefore, vibrational energy is much more effective than translational energy in accelerating the reaction.

For reaction (1), however, translational energy is more effective than vibrational energy in accelerating the reaction. The general rule of thumb is that vibrational energy is more effective for endothermic reactions (those for which the new molecule is less stable than the original molecule), while translational energy is most effective for exothermic reactions.

Lasers

Lasers are also important for probing the dynamics of chemical reactions. Because they are light sources with a very narrow wavelength, they are able to excite molecules to specific quantum states (and also to detect what states molecules are in), an example of which is reaction (2). For polyatomic molecules—that is, those with more than two atoms—there is the even more interesting question of how the rate of reaction depends on which vibration is excited.

For example, when the molecule allyl isocyanide, CH₂&dbnd;CHCH₂NC, is given sufficient vibrational energy, the isocyanide part (NC) will rearrange to the cyanide (CN) configuration. A laser can be used to excite a C-H bond vibrationally. An interesting question is whether the rate of the rearrangement process depends on which C-H bond is excited. Only with a laser is it possible to excite different C-H bonds and begin to answer such questions. This question of mode-specific chemistry—whether excitation of specific modes of a molecule causes specific chemistry to result—has been a subject of great interest. (For the example above, the reaction is fastest if the C-H bond closest to the NC group is excited.) Mode-specific chemistry would allow much greater control over the course of chemical reactions, and it would be possible to accelerate the rate of some reactions (or reactions at one part of a molecule) and not others.

Theoretical methods

The goal of chemical dynamics is to understand kinetic phenomena from the basic laws of molecular mechanics, and it is thus a field which sees close interplay between experimental and theoretical research. Many different theoretical models and methods have been useful in understanding and analyzing the phenomena described above. Probably the single most useful approach has been the calculation of classical trajectories. Assuming that the potential energy function or a reasonable approximation is known for the three atoms in reaction (1), for example, it is possible by use of electronic computers to calculate the classical motion of the three atoms. It is thus an easy matter to give the initial molecule more or less vibrational or translational energy, and then compute the probability of reaction. Similarly, the final molecule and atom can be studied to see where the energy appears, that is, as translation or as vibration.

It is thus a relatively straightforward matter theoretically to answer the questions and to see whether or not mode-specific excitation leads to significantly different chemistry than simply increasing the temperature under bulk conditions.

The most crucial step in carrying out these calculations is obtaining the potential energy surface—that is, the potential energy as a function of the positions of the atoms—for the system. The illustration shows a plot of the contours of the potential energy surface for reaction (1). Even without carrying out classical trajectory calculations, it is possible to deduce some of the dynamical features of this reaction; for example, the motion of the system first surmounts a small potential barrier, and then it slides down a steep hill, turning the corner at the bottom of the hill. It is evident that such motion will cause much of the energy released in going down the hill to appear in vibrational motion of HF.

Contour plot of the potential energy surface for the reaction F + H₂ = HF + H, with a typical reactive trajectory indicated.

This and other theoretical methods have interacted strongly with experimental research in helping to understand the dynamics of chemical reactions. See also Chemical kinetics; Inorganic photochemistry.