Gas-phase ion chemistry

Transition state theory is the theory of the rates of elementary reactions which assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated complexes.[3]

RRKM theory is used to compute simple estimates of the unimolecular ion decomposition reaction rates from a few characteristics of the potential energy surface.

Associative ionization is a gas phase reaction in which two atoms or molecules interact to form a single product ion.[4]

${\displaystyle A^{*}+B\to AB^{+\bullet }+e^{-}}$

where species A with excess internal energy (indicated by the asterisk) interacts with B to form the ion AB⁺.

One or both of the interacting species may have excess internal energy.

Charge-exchange ionization (also called charge-transfer ionization) is a gas phase reaction between an ion and a neutral species

${\displaystyle A^{+}+B\to A+B^{+}}$

in which the charge of the ion is transferred to the neutral.[5]

In chemical ionization, ions are produced through the reaction of ions of a reagent gas with other species.[6] Some common reagent gases include: methane, ammonia, and isobutane.

Chemi-ionization can be represented by

${\displaystyle G^{*}+M\to M^{+\bullet }+e^{-}+G}$

where G is the excited state species (indicated by the superscripted asterisk), and M is the species that is ionized by the loss of an electron to form the radical cation (indicated by the superscripted "plus-dot").

Penning ionization refers to the interaction between a gas-phase excited-state atom or molecule G^* and a target molecule M resulting in the formation of a radical molecular cation M^+., an electron e⁻, and a neutral gas molecule G:[7]

${\displaystyle G^{*}+M\to M^{+\bullet }+e^{-}+G}$

Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom or molecule. Associative Penning ionization can also occur:

${\displaystyle G^{*}+M\to MG^{+\bullet }+e^{-}}$

CID (also called collisionally activated dissociation - CAD) is a method used to fragment molecular ions in the gas phase.[8][9] The molecular ions collide with neutral gas molecules such as helium, nitrogen or argon. In the collision some of the kinetic energy is converted into internal energy which results in fragmentation.

Charge remote fragmentation is a type of covalent bond breaking that occurs in a gas phase ion in which the cleaved bond is not adjacent to the location of the charge.[10][11]