Electronic and Ionic Impact Phenomena

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Therefore, in standard thermodynamics data compilations [ 6 ], cited values for enthalpies of formation of ions at temperatures other than 0K differ from those given here. The collection of gas phase ion energetics data presented in the WebBook traces its origin to a series of publications concerned with this subject area, beginning with a table of ionization energies and evaluated enthalpies of formation of ions included in the book "Electron Impact Phenomena and the Properties of Gaseous Ions" by F.

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Field and J. Franklin [ 1 ]. In , H. Rosenstock, K. Draxl, B. Steiner, and J.

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Herron published an update, "Energetics of Gaseous Ions," [ 3 ] which included a complete re-evaluation of the data, and for the first time, a table of electron affinity data. In , an extensive compilation of ionization potential and appearance potential data, "Ionization Potential and Appearance Potential Measurements, " [ 4 ], presented unevaluated measurements which had appeared in the literature from the cut-off date of the previous collection [ 3 ] up to mid In , an evaluation or re-evaluation of the data from the collective database presented in all these earlier publications [ 1 , 2 , 3 , 4 ], was published in "Gas-phase Ion and Neutral Thermochemistry" [ 7 ] commonly referred to as "the GIANT Tables".

The publication also included more recent data and evaluated proton affinity values, as well as a comprehensive table of data on negative ions, including both electron affinity and gas phase acidity values. The data on proton affinities were taken from a publication [ 8 ] in which the entire thermochemical scale of gas phase basicities and proton affinities had been evaluated.

The publication of evaluated data on ion thermochemistry [ 9 ] was made available by the National Institute of Standards and Technology's Standard Reference Data program as a searchable computer database available on diskettes — in fact, as two jointly-distributed databases, one presenting the data relevant to positive ions [ 9a ] and the other, data on negative ions [ 9b ].

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In the initial release, the negative ion database [ 9b ] displayed the entire corpus of data from which the evaluations were drawn, but the original version of the electronic database on positive ions [ 9a ] presented only evaluated values for ionization energies, proton affinities, and enthalpies of formation of positive ions in the gas phase. The and updates [ 5 ] to the electronic database added more recent data, and, in the case of the positive ion collection [ 5a ], for the first time displayed some of the original data e.

After new experimental determinations had made necessary a re-evaluation of the entire proton affinity scale as presented in the publication [ 8 ] data derived from equilibrium constant determinations are interdependent and must be evaluated collectively , proton affinity data which had appeared in the original electronic database [ 9a ] were removed from the updated version [ 5 ] to prevent dissemination of out-of-date information. The re-evaluation of the proton affinity scale has now been completed [ 10 ], and the evaluated proton affinity and gas basicity data are included in the WebBook.

Since most of these precursor publications are still in widespread use even the woefully out-of-date compilation [ 2 ] continues to be cited , it is worthwhile to summarize the contents and coverage of the various publications, and compare these to the contents and coverage of the WebBook:. The ion energetics database at the present time has as its focus numeric data concerned with the energies for formation of particular positive and negative ions in the gas phase, as well as with the energetics of certain reactions, namely protonation and deprotonation, of those ions.

That is, the database includes values for ionization energies, appearance energies, electron affinities, gas phase acidities and basicities, as well as proton affinities. In addition, the database contains data on the energies associated with the clustering of neutral molecules to anions. Some information on ionization energies of small cluster ions with not more than three or four ligand molecules , especially in inorganic systems, is included.

The database does not at the present time include thermochemical data derived from collisionally-activated dissociation experiments of positive ions, although such results for negative ions are included. In recent years, numerous publications giving quantum mechanical calculations of very high accuracy on the thermochemical properties of ions, especially small ions, have appeared. The present work includes only data derived from experimental determinations, although some calculational results have been used in carrying out evaluations, especially of the proton affinity data.

A particular explanation about the coverage of ionization energy data is in order. In most of the precursor publications [ 1 , 2 , 3 , 4 , 7 ] the primary focus was the thermochemistry of ions in the gas phase. That is, the goal of collecting ionization energies, appearance energies, electron affinities, and so forth was the derivation of enthalpies of formation of ions.

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Because of this emphasis, the coverage of data was restricted to information directly relevant to deriving ionic enthalpies of formation. This limited focus had particular implications for the coverage of data on ionization energies, since it necessarily meant that only data on the lowest ionization energies was included; ionization energies leading to the formation of excited ions, or multiply charged ions were excluded except for atoms and diatomic molecules in the publication [ 3 ].

Furthermore, coverage was restricted to adiabatic ionization energy data except in cases where publications gave only vertical ionization energy values. In abstracting data from the more recent literature since , we have attempted to include both adiabatic and vertical values where both are available, but have not gone back to the thousands of earlier papers to re-abstract data on vertical ionization energies or upper ionization energies that were not originally included.

The positive ion database ionization energies , appearance energies , proton affinities and negative ion database electron affinities , acidities were developed and are maintained separately. As a result, certain inconsistencies exist at the present time in the presentation of the two types of data in the Web Book. For example, the primary sort in the anion database depends on the identity of the anion, while the positive ion database has always been organized around the identity of the precursor neutral molecule.

For this reason, until further work has been accomplished, users of the WebBook will occasionally see an apparently illogical list of "hits" when data for a particular chemical species has been requested. It is intended that such inconsistencies be cleaned up up in the next version of the WebBook. Retrieval of data listed under M will also lead to a table giving the proton affinity and gas phase basicity of molecule M, if they are available. Unlike gas phase basicity data, data on gas phase acidities can be retrieved only if you started the search by requesting data on "reactions" in addition to "ion energetics" on the opening screen.

The formal organization of the positive and negative ion data based on neutral precursor positive ions or on the ion also has implications for carrying out a search for data based on registry number. For positive ion data, searches should be conducted based on the registry number of the neutral species; for anion data searches should be conducted on the registry number of the anion.

Since the database contains registry number data for a limited portion of the anions in the database, a chemical formula search will often be a better choice for finding a specific anion. Appearance energy data are presented in two ways. First, by calling up all data for a particular molecule, you access a listing of all ionization energy determinations, and also the appearance energies that have been determined for fragmentation processes of that parent ion.

By clicking on this text, you will access a list of all appearance energies determined for formation of fragment ions of that particular formula from other larger molecular species. It should be understood that in the great majority of cases, the structure of the fragment ion is not specified, and such data should be used mainly as auxiliary information, or as a guide to the original literature. However, in some cases where the original authors have carried out a sufficient analysis to be able to specify the structure of the fragment ion, this structure is indicated. In other cases, the structure may be obvious, and such an indication is not necessary.

A word of caution is in order: again, in earlier versions of the database, such specifications were not included, and we have not yet gone back to fill in this missing information. Units used for the display of information are dictated by the current practices for reporting data of a particular kind. For example, ionization energy and electron affinity values are usually reported in electron volts, and that is the unit used here for these data.

The enthalpy of formation of any chemical species is defined as the difference between the enthalpy of the compound and the sum of the enthalpies of the elements of which it is composed. The "Ion Convention" is used here.

There is considerable confusion and misunderstanding of the basic assumptions and treatment of the thermochemistry of the electron in the two approaches. In fact, the so-called "electron" and "ion" conventions are really names assigned by the ion chemistry community to the two different conventions used by thermodynamicists for handling the integrated heat capacity of elements.

The more widely-used convention corresponding to the "electron convention" defines the enthalpies of formation of elements in their standard states to be zero at all temperatures; in this case, the integrated heat capacity of the element must be accommodated elsewhere in any thermodynamic equation, as shown in the more detailed discussion below — and in the particular case in which we are interested ionization end up as an increment in the derived enthalpy of formation of the ion.

There is also a community of scientists in Europe SGTE who define the enthalpy of formation of elements at temperatures above absolute zero to be equal to the integrated heat capacity of the species; this corresponds to the treatment used for the electron in the so-called "ion convention". As will be derived below, when this treatment is followed, the integrated heat capacity term cancels out in the expression defining the enthalpy of formation of the ion.

Of course, enthalpies of reaction are identical when using the two conventions provided the same value is chosen for the integrated heat capacity term in question ; the value of the integrated heat capacity merely appears on opposite sides of the equation, with opposite signs. Further confusion exists because in the past, users of the two conventions have cited different values for the integrated heat capacity of the electron.

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  • A correct treatment of the heat capacity of an electron gas uses Fermi-Dirac, rather than Boltzmann, statistics [ 12 ]. A publication [ 12 ] arrives at a value of 3. Values for enthalpies of formation under the Electron Convention are higher more positive for positive ions and lower less positive for negative ions than the corresponding values expressed in the Ion Convention used here. Problems arise when users unknowingly mix inconsistent values for enthalpies of formation in the same equation. The Table lists several commonly-used compilations and shows which convention is used in each.

    When sufficient information is available. See discussion.

    Indicates treatment of integrated heat capacity terms for molecules and ions. The following more detailed discussion of these issues is intended to present the question of how the electron is treated in a thermochemical equation in a tutorial manner, in the hope that some of the confusion will be dispelled. This discussion is also intended to justify the choice of the usual mass spectrometrists' convention for use in these tables.

    The relationships between the various quantities that must be considered are shown in the thermochemical cycles:. This discussion will be concerned with the standard temperature, K, but the arguments can obviously be extended to any other temperature. In both conventions, at 0 K the enthalpy of formation of the electron is zero and the enthalpies of formation of the ions are exactly equal to the 0 K enthalpy of formation of the molecule M plus the energy difference between M and the corresponding ion:.

    The enthalpy changes of reaction at K are related to the 0 K ionization energy and electron affinity through the relationships:. In the present discussion, merely for the sake of focusing our attention on the treatment of C , the integrated heat capacity of the electron, let us temporarily make this assumption in order to simplify the equations. In the Electron Convention , the electron is treated like a standard chemical element.

    Therefore, using the normal procedure for treating the thermochemistry of an element, its enthalpy of formation is constrained to be zero at all temperatures, but the integrated heat capacity is not taken to be zero. Therefore, the expressions for the enthalpies of formation of positive and negative ions reduce to:.

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    Thus, for the purposes of deriving enthalpies of formation of ions from ionization energy or electron affinity data, it does not matter what value is chosen for the integrated heat capacity of the electron, C , since it does not appear in the final expression for the enthalpy of formation. However, the expressions relating the enthalpies of reaction at finite temperatures to the zero degree quantities, IE a and EA, do explicitly include the integrated heat capacity of the electron:. As mentioned above , it is recommended that the value for the integrated heat capacity term for the electron derived using Fermi-Dirac statistics 3.

    In preparing the current edition of this data collection for distribution via the WebBook, the possibility was considered of using the Electron Convention. However, the decision was made to retain the use of the Ion Convention. The reasons for this decision were:. That is, it appears that considerable confusion could result from an attempt to change the presentation of data here.

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    Therefore, in this database, the use of the "Ion Convention" is retained. It is recommended that in all literature dealing with thermochemistry of ions in the gas phase, clear signposts should be provided to indicate which convention is being used. This section provides brief descriptions of some of the factors relevant to the interpretation and evaluation of ionization energy and appearance energy data. More detailed discussions of the ionization process are available in many books and reviews, notably in the Introduction to "Energetics of Gaseous Ions" [ 3 ].

    Ionization of a molecule by photoionization or by energetic electrons sometimes called "electron impact" is governed by the Franck-Condon principle, which states that the most probable ionizing transition will be that in which the positions and momenta of the nuclei are unchanged. Thus, when the equilibrium geometries of an ion and its corresponding neutral species are closely similar, the energy dependence of the onset of ionization will be a sharp step function leading to the ion vibrational ground state.

    These situations are illustrated for hypothetical diatomic species in Fig. In evaluating ionization energy data, the shapes of photoelectron bands are useful indicators as to which of the situations pictured in Fig. A sharp onset indicates that the equilibrium geometries of ion and neutral are quite similar, and that photoionization or electron impact determinations of the ionization threshold are likely to be free of complications.

    When an ionization process proceeds according to the second situation pictured in the figure, the onset of the photoelectron band is observed approximately at the adiabatic ionization energy; adiabatic ionization energies derived from observation of the onsets of photoelectron bands are usually in excellent agreement with adiabatic ionization energies obtained from optical spectroscopy analyses of Rydberg series or from the most reliable threshold determinations.

    When the equilibrium geometry of the ion is very different from that of the corresponding neutral molecule and the lowest vibrational level is not populated in ionization by photon absorption or electron ionization, it has been shown that values for the adiabatic ionization energies can be obtained by determining the equilibrium constant for charge transfer to another molecule of known ionization energy:.

    In such determinations, the ions are at thermal equilibrium with their surroundings, and one measures the thermochemical properties of the ions in their equilibrium geometries.

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