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 mecha ...

, 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 to be contrasted with chemical thermodynamics, which deals with the direction in wh ...

, 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 provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in ...

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

rotational Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

, 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 Crossed molecular beam experiments are chemical experiments where two beams of atoms or molecules are collided together to study the dynamics of the chemical reaction, and can detect individual reactive collisions. Technique In a crossed molec ...

and infrared chemiluminescence experiments, both recognized by the 1986

Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...

awarded to

Dudley Herschbach Dudley Robert Herschbach (born June 18, 1932) is an American chemist at Harvard University. He won the 1986 Nobel Prize in Chemistry jointly with Yuan T. Lee and John C. Polanyi "for their contributions concerning the dynamics of chemical eleme ...

Yuan T. Lee Yuan Tseh Lee (; born 19 November 1936) is a Taiwanese chemist and a Professor Emeritus at the University of California, Berkeley. He was the first Taiwanese Nobel Prize laureate who, along with the Hungarian-Canadian John C. Polanyi and America ...

, 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 ( ar, أحمد حسن زويل, ; 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 ...

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

potential energy surface A potential energy surface (PES) describes the energy of a system, especially a collection of atoms, in terms of certain parameters, normally the positions of the atoms. The surface might define the energy as a function of one or more coordinates; ...

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 traj ...

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

quantum tunnelling Quantum tunnelling, also known as tunneling ( US) is a quantum mechanical phenomenon whereby a wavefunction can propagate through a potential barrier. The transmission through the barrier can be finite and depends exponentially on the barrier h ...

through the

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...

barrier, especially for the movement of hydrogen atoms.

References

Further reading