Thermochemical data ionization potentials

The original paper defining the Gaussian-2 method by Curtiss, Raghavachari, Trucks and Pople tested the method s effectiveness by comparing its results to experimental thermochemical data for a set of 125 calculations 55 atomization energies, 38 ionization potentials, 25 electron affinities and 7 proton affinities. All compounds included only first and second-row heavy atoms. The specific calculations chosen were selected because of the availability of high accuracy experimental values for these thermochemical quantities. 

The properties of the hydrogen molecule and molecule-ion which are the most accurately determined and which have also been the subject of theoretical investigation are ionization potentials, heats of dissociation, frequencies of nuclear oscillation, and moments of inertia. The experimental values of all of these quantities are usually obtained from spectroscopic data substantiation is in some cases provided by other experiments, such as thermochemical measurements, specific heats, etc. A review of the experimental values and comparison with some theoretical... 

In the gas phase, homolytic bond dissociation enthalpies (D//) relate the thermochemical properties of molecules to those of radicals while ionization potentials (IP) and electron affinities (EA) tie the thermochemistry of neutral species to those of their corresponding ions. For example, Scheme 2.1 represents the relationships between RsSiH and its related radicals, ions, and radical ions. This representation does not define thermodynamic cycles (the H fragment is not explicitly considered) but it is rather a thermochemical mnemonic that affords a simple way of establishing the experimental data required to obtain a chosen thermochemical property. 

Appendix 1 presents numerous reference tables containing most important data on the solubility of inorganic compounds in water, the density, dissociation constants, solubility products, ionization potentials of various atoms, etc., as well as thermochemical constants because many laws of inorganic chemistry cannot be explained without these quantities. 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

The indirect method was put forward by Stevenson in it the appearance potentials of the same ion produced from two different but related molecules are measured, and their difference is combined with thermochemical data to give the required dissociation energ) It is thus possible to calculate bond dissociation energies from appearance potentials and thermochemical data without a knowledge of ionization potentials. The essence of the method is to produce as the non-ionized partner in a dissociative ionization process of the type... 

The new mechanism is supported by the elucidation of the thermochemistry of the various steps in terms of the ionization potential (/) of the ferrous ion in aqueous solution, the electron affinities (E) of the radicals HO and H02 plus the heats of solvation (/S) of the corresponding ions OH- and OaH in water, the various 0 H and O 0 bond strengths (Z>) and other thermal quantities such as heats of evaporation (X) and heats of solvation (S) (see 83,84,87,88,89). All the steps with the exception of (i ) and (0) are exothermic, and being electron transfer reactions or a simple bond breaking reaction (step 2) are to be expected to proceed extremely rapidly as is required by the kinetic mechanism. Steps (i ) and (0) are endothermic to the extent of 28 and 5 kcal., respectively, and these values are in accord with their activation energies which are 28 + 8 and 9.4 kcal. As with the kinetic evidence this thermochemical evidence is indirect support for the radical mechanism. An important point which will be elaborated later is that thermal data of this kind provide criteria for rejecting certain steps when the endothermicity becomes impossibly large for the reaction to proceed at a sensible rate. 

The thesis begins with Section 2, where a brief history about the explicitly correlated approaches is presented. This is followed by Section 3 with general remarks about standard and explicitly correlated coupled-cluster theories. In Section 4, the details about the CCSD(F12) model relevant to the implementation in TuRBOMOLE are presented. The usefulness of the developed tool is illustrated with the application to the problems that are of interest to general chemistry. A very accurate determination of the reactions barrier heights of two CH3+CH4 reactions has been carried out (Section 5) and the atomization energies of 106 medium-size and small molecules were computed and compared with available experimental thermochemical data (Section 6). The ionization potentials and electron affinities of the atoms H, C, N, O and F were obtained and an agreement with the experimental values of the order of a fraction of a meV was reached (Section 7). Within all applications, the CCSD(F12) calculation was only a part of the whole computational procedure. The contributions from various levels of theory were taken into account to provide the final result, that could be successfully compared to the experiment. 

Alternatively, Bratsch and Lagowski (1985a, b, 1986) proposed an ionic model to calculate the thermodynamics of hydration AGj, A/fJ and ASj using standard thermochemical cycles. This model is based on the knowledge of the values of quantities such as the enthalpy of formation of the monoatomic gas [A/f (M )], the ionization potential sum for the oxidation state under consideration and the crystal ionic radius of the metal ion. This approach, however, is difficult to apply for the actinides since the ionization potentials are, for the most part, unavailable. To overcome this problem, the authors back-calculated an internally consistent set of thermochemical ionization potentials from selected thermodynamic data (Bratsch and Lagowski 1986). The general set of equations developed are ... 

Determined from ionization potentials and thermochemical data, in CRC Handbook of Chemistry and Physics, 68th Edition, edited by R.C. West (CRC Press, Boca Raton, Florida, 1987). 

A number of efforts have been made to calculate ionization-potential sums from thermochemical data and appropriate Born-Haber cycles. When an isostructural set of compounds is used, and covalence/repulsion corrections are made from a systematic lanthanide-actinide comparison, such sums can be quite reliable, as has been repeatedly demonstrated for the trivalent lanthanides [88]. For example, Morss [89] was able to estimate the sum of the first three ionization energies (/i +I2 + I3) for Pu as... 

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