问题：堦揰离巕问题揑塸暥夝说
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Ionization Energy
The ionization energy, sometimes called (less correctly) the ionization potential (usually designated by IE, IP, or, in the older literature, I), is the energy required to remove an electron from a molecule or atom:

M 仺 M+ + e− 儮Hrxn= IEa

Ionization energies are characterized as adiabatic or vertical values.

Adiabatic Ionization Energy
The adiabatic ionization energy is the lowest energy required to effect the removal of an electron from a molecule or atom, and corresponds to the transition from the lowest electronic, vibrational and rotational level of the isolated molecule to the lowest electronic, vibrational and rotational level of the isolated ion. Adiabatic ionization energy data can be used to obtain values for the enthalpy of formation of the ion, M+.

Vertical Ionization Energy
The vertical ionization energy is the energy change corresponding to an ionization reaction leading to formation of the ion in a configuration which is the same as that of the equilibrium geometry of the ground state neutral molecule. According to the Franck-Condon principle, when a molecule is ionized by photoionization or by interaction with energetic electrons, the highest probability configuration of the resulting ion will be the configuration of the precursor neutral molecule. When the equilibrium geometry of the ion is very similar to that of the neutral precursor molecule, the vertical and adiabatic ionization energies will be the same, or nearly so. The vertical ionization energy must always be greater than or equal to the adiabatic ionization energy.

Appearance Energy
Since ionization energies are often determined in experiments in which the ionizing photon or electron energy is varied until the appearance of an ion is observed ("threshold measurements"), ionization energies have sometimes been called appearance energies. However, in general usage this term has come to have a more specific meaning. As used here, and in most of the technical literature, the term appearance energy, sometimes (less correctly) called the appearance potential, refers to the minimum energy required to form a particular fragment ion from a precursor neutral molecule:

AB 仺 A+ + B + e− 儮 Hrxn = AP

For more information see the description of the presentation of appearance energy data, the discussion of the use of appearance energy data to derive enthalpies of formation of fragment ions, as well as the description of some of the complicating factors involved in the interpretation of appearance energy data.

Proton Affinity and Gas Phase Basicity
Stable cations formed in the gas phase also include protonated neutral molecules, generated in proton transfer reactions. Formally, the relationship between the enthalpy of formation of MH+ and its neutral counterpart, M, is defined in terms of a quantity called the proton affinity, PA. This the negative of the enthalpy change of the hypothetical protonation reaction:

M + H+ 仺 MH+ 儮 Hrxn = −PA

儮fH亱(MH+) = 儮fH亱(M) + 儮fH亱(H+) − PA

The term proton affinity, as universally used, is a quantity defined at a finite temperature, usually 298 K, and is therefore not strictly analogous to the adiabatic ionization energy or electron affinity, both of which are the 0 K enthalpy changes of the corresponding reactions.

At 298 K, the enthalpy of formation of the proton, using the Ion Convention (or "stationary electron" convention), is 365.7 kcal/mol, 1530.0 kJ/mol.

The Gibbs energy change associated with the protonation reaction is called the gas phase basicity, GB, of molecule M. Most available data on gas phase basicities and proton affinities has been obtained from experiments in which the equilibrium constants of proton transfer reactions are determined. See the discussion of derivation of thermochemical data from ion/molecule equilibrium constants for relevant equations and details.

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