Transition state

Transition state is a term used in chemistry. In the course of a chemical reaction is always through a stage of higher energy. For example, must first energy- consuming bond angles twisted and shortened or lengthened bond lengths. The state of maximum energy is called the transition state. Is this energy barrier overcome, the molecule reacts spontaneously on the product. Transition states are therefore of extremely short duration and can not be isolated. The energy needed for the reaction to reach the transition state is denoted as the activation energy.

Theoretically - chemical point of view corresponds to a reaction of a " walk " on the " mountains " of the potential energy of the nuclei ( which can be calculated by various methods, such as Hartree -Fock, MP2, coupled-cluster or DFT on an approximate solution of the Schrödinger equation) wherein each array of atoms is assigned a certain amount of energy in the molecule, that is, the so-called potential energy surface depends on 3n Cartesian coordinates for n atoms in the molecule. At the transition state, this energy is maximum on the reaction pathway, in global terms, however, there is a maximum in only one dimension ( corresponding to the pathway ), in all other dimensions, the transition state but a minimum on ( so he can be as " pass " in the said Mountains understand ). Mathematically, the transition state is a saddle point of first order on the potential energy surface, ie, the gradient vector vanishes and the Hessian matrix has a negative entry on (this corresponds to an imaginary vibrational frequency in the harmonic approximation ).