Transition State Theory

Examples of Transition State Theory in the following topics:

• Transition State Theory

  • Transition state theory (TST) describes a hypothetical "transition state" that occurs in the space between the reactants and the products in a chemical reaction.
  • The species that is formed during the transition state is known as the activated complex.
  • According to transition state theory, between the state in which molecules exist as reactants and the state in which they exist as products, there is an intermediate state known as the transition state.
  • However, according to transition state theory, a successful collision will not necessarily lead to product formation, but only to the formation of the activated complex.
  • Transition state theory is most useful in the field of biochemistry, where it is often used to model reactions catalyzed by enzymes in the body.

• Physical Properties and Atomic Size

  • These can most easily occur when the metal is in a high oxidation state.
  • In each case the metals (Cr and Mn) have oxidation states of +6 or higher.
  • A metal-to ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is easily reduced.
  • The pattern of splitting of the d orbitals can be calculated using crystal field theory.
  • Ferromagnetism is the physical theory which explains how materials become magnets.

• Coloring Agents

  • This approach is described by the ligand field theory (LFT) and the molecular orbital theory (MO).
  • Ligand field theory, introduced in 1935 and built from molecular orbital theory, can handle a broader range of complexes.
  • Metal complexes often have spectacular colors caused by electronic transitions due to the absorption of light.
  • Most transitions that are related to colored metal complexes are either d–d transitions or charge transfer bands.
  • Conceptually, one can imagine the oxidation state of the metal increasing by one (losing on electron), while the oxidation state of the ligand decreases by one (becomes anionic).

• Crystal Field Theory

  • Crystal field theory states that d or f orbital degeneracy can be broken by the electric field produced by ligands, stabilizing the complex.
  • The Crystal Field Theory (CFT) is a model for the bonding interaction between transition metals and ligands.
  • CFT successfully accounts for some magnetic properties, colors, and hydration energies of transition metal complexes, but it does not attempt to describe bonding.
  • the metal's oxidation state (a higher oxidation state leads to a larger splitting)
  • Crystal field stabilization is applicable to the transition-metal complexes of all geometries.

• Transition Metals

  • The d-block elements are commonly known as transition metals or transition elements.
  • The formation of compounds in many oxidation states due to the relatively low reactivity of unpaired d electrons.
  • These can most easily occur when the metal is in a high oxidation state.
  • A metal-to-ligand charge transfer (MLCT) transition will be most likely when the metal is in a low oxidation state and the ligand is an easily reduced d-d transition.
  • This activity is attributed to their ability to adopt multiple oxidation states and to form complexes.

• Color

  • Most transitions that are related to colored metal complexes are either d–d transitions or charge band transfer.
  • The Laporte rule states that, if a molecule is centrosymmetric, transitions within a given set of p or d orbitals are forbidden.
  • Therefore, transitions are not pure d-d transitions.
  • These are most likely to occur when the metal is in a low oxidation state and the ligand is easily reduced.
  • These can most easily occur when the metal is in a high oxidation state.

• Solid to Gas Phase Transition

  • Sublimation is the phase transition from the solid to the gaseous phase, without passing through an intermediate liquid phase.
  • It is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point (the temperature and pressure at which all three phases coexist) in its phase diagram.
  • At a given temperature, most chemical compounds and elements can possess one of the three different states of matter at different pressures.
  • In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state.
  • But at temperatures below that of the triple point, a decrease in pressure will result in a phase transition directly from the solid to the gaseous.

• Heterogeneous Catalysis

  • Catalysts are chemical compounds that increase the rate of a reaction by lowering the activation energy required to reach the transition state.
  • The reason such catalysts are able to speed up a reaction has to do with collision theory.
  • Recall that according to collision theory, reactant molecules must collide with proper orientation.

• Planck's Quantum Theory

  • As a result of these observations, physicists articulated a set of theories now known as quantum mechanics.
  • Planck is considered the father of the Quantum Theory.
  • When an electric current is passed through a gas, some of the electrons in the gas molecules move from their ground energy state to an excited state that is further away from their nuclei.
  • This is because electrons release specific wavelengths of light when moving from an excited state to the ground state.
  • Each wavelength of light emitted (each colored line) corresponds to a transition of an electron from one energy level to another, releasing a quantum of light with defined energy (color).

• Molecularity

  • A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular.
  • If a single species makes up the transition state, the reaction would be called unimolecular.
  • The relatively improbable case of three independent species coming together in the transition state would be called termolecular.