This is a compilation of ionization and appearance potentials of positive ions published from 1955 through June 1966. The compilation lists the ion formed, the parent species from which it was formed, the other products of the process, the threshold energy for the formation of this ion, and the method by which this data was obtained. Where feasible, the heat of formation at 298 K of the positive ion has been computed for each entry using auxiliary thermochemical data. From these computed values “best” values have been chosen.

Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions

On the Structure of CH2 in its Triplet Ground State

The appearance energy of the CH{sub 2}{sup +} fragment from CH{sub 2}CO has been carefully remeasured and fitted by a model curve, producing EA{sub 0}(CH{sub 2}{sup +}/CH{sub 2}CO) = 13743 {+-} 0005 eV This value can be sequentially propagated through selected thermochemical cycles to yield individual values for El(CH{sub 2}), D{sub 0}(H-CH{sub 2}), {Delta}H{sup o}{sub f 0}(CH{sub 2}CO) A set of values with a statistically larger weight is produced by analyzing a local thermochemical network, which combines the present measurement with thirteen other experimental determinations from the literature and encompasses the enthalpies of formation of CH{sub 3}, CH{sub 3}{sup +}, CH{sub 2}, CH{sub 2}{sup +}, and CH{sub 2}CO The recommended simultaneously adjusted thermochemical values are: {Delta}H{sup o}{sub f 0}(CH{sub 3}) = 3586 {+-} 007 kcal/mol (3505 {+-} 007 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 3}{sup +}) = 26273 {+-} 006 kcal/mol (26183 {+-} 006 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}) = 9318 {+-} 020 kcal/mol (9331 {+-} 020 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}{sup +}) = 33292 {+-} 019 kcal/mol (33304 {+-} 019 kcal/mol at 298 K), {Delta}H{sup o}{sub f 0}(CH{sub 2}CO) = -1110 {+-} 021 kcal/mol (-1185 {+-} 021 kcal/mol atmore » 298 K), as well as D{sub 0}(H{sub 3}C-H) = 10342 {+-} 003 kcal/mol (10499 {+-} 003 kcal/mol at 298 K), D{sub 0}(H{sub 2}C-H) = 10895 {+-} 020 kcal/mol (11035 {+-} 020 kcal/mol at 298 K), D{sub 0}(H{sub 2}C{double_bond}CO) = 7708 {+-} 002 kcal/mol (7873 {+-} 002 kcal/mol at 298 K), El(CH{sub 3}) = 93830 {+-} 00005 eV, and El(CH{sub 2}) = 103962 {+-} 00036 eV These values are in excellent agreement with current and several previous experimental measurements The recommended enthalpy of formation of CH{sub 2} implies that the reaction of singlet methylene with water is essentially thermoneutral (to within {+-}02 kcal/mol) at 0 and 298 K, and slightly endothermic (05 {+-} 02 kcal/mol) at 1000 K« less

Ionization Energy of Methylene Revisited: Improved Values for the Enthalpy of Formation of CH2 and the Bond Dissociation Energy of CH3 via Simultaneous Solution of the Local Thermochemical Network

The C+ + H2 ion‐molecule reaction has been studied by several experimental groups and appears likely to become the focal point of much experimental and theoretical activity. Ab initio self‐consistent‐field and configuration interaction calculations have accordingly been carried out for this system. A double zeta basis set of contracted Gaussian functions was employed and as many as 648 configurations included. For isosceles triangle configurations (C2V point group) the 2A1, 2B1, and 2B2 potential surfaces were considered, while for linear geometries (C ∞V) the 2Σ+ and 2Π surfaces were studied. For general (Cs) geometry, the lowest 2A′ potential surface was considered. Properties reported include minimum energy paths and energy profiles for the various processes considered. The intuitive correlation diagram of Mahan and Sloane is given qualitative reliability. Pathways to CH2+ complex formation are shown to depend crucially on the Cs potential surface.

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Potential energy surfaces related to the ion‐molecule reaction C+ + H2

Reactions of rare‐gas ions with N2O, C2H2, C2H4, and D2 are studied, and cross sections are given for primary‐ion energies below 120 eV. These cross sections are compared for qualitative differences. Most of the reactions have rather sharp onsets, and a semiempirical formula is fitted to the data near the apparent threshold energy. The results of this investigation show that useful information about bond energies can be obtained from ion‐molecule reactions. The data are consistent with D(H2C=CH2)=8.11 +0.10−0.18 eV and thus with D(CH–H)=3.99 +0.05−0.09 eV, although smaller values of D(H2C=CH2), and thus larger values of D(CH–H), cannot be entirely excluded. The results indicate that ion‐impact methods may possess inherent advantages over electron‐impact methods in certain cases; for example, the present data show that the electron‐impact appearance potential of C2H3+ from C2H4 is certainly 0.3 eV and probably 0.6 eV larger than the true threshold energy. The threshold behavior of an endothermic charge‐tra...

G. Herzberg

Papers

The ionization potential of ch2