Reaction dynamics is a field within

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mech ...

, studying why chemical reactions occur, how to predict their behavior, and how to control them. It is closely related to

chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics, which deals with the direction in which a ...

, but is concerned with individual chemical events on atomic length scales and over very brief time periods. It considers ''state-to-state kinetics'' between reactant and product molecules in specific

quantum state In quantum physics, a quantum state is a mathematical entity that embodies the knowledge of a quantum system. Quantum mechanics specifies the construction, evolution, and measurement of a quantum state. The result is a prediction for the system ...

s, and how energy is distributed between translational, vibrational,

rotational Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersec ...

, and electronic modes. Laidler, K.J. ''Chemical Kinetics'' (3rd ed., Harper & Row 1987), Chap.12 Experimental methods of reaction dynamics probe the chemical physics associated with molecular collisions. They include

crossed molecular beam In analytical chemistry, crossed molecular beam experiments involve two beams of atoms or molecules which are collided together to study the dynamics of the chemical reaction, and can detect individual reactive collisions. Technique In a cros ...

and infrared chemiluminescence experiments, both recognized by the 1986

Nobel Prize in Chemistry The Nobel Prize in Chemistry () is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outst ...

awarded to Dudley Herschbach,

Yuan T. Lee Yuan Tseh Lee (; born 19 November 1936) is a Taiwanese chemist who was awarded the Nobel Prize in Chemistry in 1986 for his contributions to the development of reaction dynamics. Lee is a professor emeritus at the University of California, Be ...

, and John C. Polanyi "for their contributions concerning the dynamics of chemical elementary processes", In the crossed beam method used by Herschbach and Lee, narrow beams of reactant molecules in selected quantum states are allowed to react in order to determine the reaction probability as a function of such variables as the translational, vibrational and rotational energy of the reactant molecules and their angle of approach. In contrast the method of Polanyi measures vibrational energy of the products by detecting the infrared chemiluminescence emitted by vibrationally excited molecules, in some cases for reactants in defined energy states. Spectroscopic observation of reaction dynamics on the shortest time scales is known as

femtochemistry Femtochemistry is the area of physical chemistry that studies chemical reactions on extremely short timescales (approximately 10−15 seconds or one femtosecond, hence the name) in order to study the very act of atoms within molecules (reactants ...

, since the typical times studied are of the order of 1 ''femto''second = 10⁻¹⁵ s. This subject has been recognized by the award of the 1999 Nobel Prize in Chemistry to

Ahmed Zewail Ahmed Hassan Zewail (February 26, 1946 – August 2, 2016) was an Egyptian-American chemist, known as the "father of femtochemistry". He was awarded the 1999 Nobel Prize in Chemistry for his work on femtochemistry and became the first Egyptian a ...

. In addition, theoretical studies of reaction dynamics involve calculating the

potential energy surface A potential energy surface (PES) or energy landscape describes the energy of a Physical system, system, especially a collection of atoms, in terms of certain Parameter, parameters, normally the positions of the atoms. The Surface (mathematics), ...

for a reaction as a function of nuclear positions, and then calculating the

trajectory A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete tra ...

of a point on this surface representing the state of the system. A correction can be applied to include the effect of

quantum tunnelling In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a quantum mechanical phenomenon in which an object such as an electron or atom passes through a potential energy barrier that, according to classical mechanics, shoul ...

through the

activation energy In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...

barrier, especially for the movement of hydrogen atoms.

References

Further reading