Thermochemical data proton affinities

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. 

By means of appropriate thermochemical cycles, it is possible to calculate proton affinities for species for which experimental values are not available. For example, using the procedure illustrated by the two foregoing examples, the proton affinities ofions such as HC03-(g) (1318 k J mol-1) and C032-(g) (2261 kj mol-1) have been evaluated. Studies of this type show that lattice energies are important in determining other chemical data and that the Kapustinskii equation is a very useful tool. 

The advent of methods for determining proton affinities by studying bimolecular reactions in the gas phase has provided a wide range of interesting thermochemical data. 

This section will use gas-phase thermochemical data from Appendices 6 for molecules and 7 for radicals. These data include ionization energy (IE), electron affinity (EA), proton affinity (PA), gas-phase basicity (GB) and gas-phase acidity. Definitions of these parameters are given in Table 1.5. Some values of gas-phase basicities are given in Table 1.6. 

The kinetic method [42,43] is a relative method for thermochemical data determination which is based on measurement of the rates of competitive dissociations of mass-selected cluster ions. This method was introduced by Cooks [44] for proton affinity determination. Later, an extension of this method was proposed by Fenselau [45]. 

The kinetic method provides an alternative to equilibrium measurements for the determination of gas-phase thermochemical properties. It has been applied more and more in thermochemical data determination mainly because of its ability to measure very small energy differences and its simplicity. Indeed, it can be executed easily on any tandem mass spectrometer. Furthermore, this method is sensitive and is applicable with impure compounds. Its applications are broad, covering thermochemical properties in the gas phase such as proton affinity [46], electron affinity [47], metal ion affinity [48], ionization energy [49], acidity [50] or basicity [51], In addition to the determination of thermochemical data, the kinetic method has also been applied in structural and chemical analysis such as chiral distinctions. This method is able to distinguish enantiomers and to measure precisely enantiomeric ratios [52],... 

Other thermochemical data (e.g., heat of formation and proton affinity). These thermochemical data are very helpful to atmospheric modeling to understand the complex chemical reactions. Thermochemical data can be obtained by high levels of calculations, e.g., CCSD(T), Gn series, CBS, QCI, etc. 

Afeefy, H.Y. Liebman, J.F. Stein, S.F. Neutral Thermochemical Data , and Hunter, E.P. Lias, S. G. , Proton Affinity Data , in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Mallard, W.G. Linstrom, P.G. Eds. February 2000, National Institute of Standards and Technology, Gaithersburg, MD, 20899 http //webbook.nist.gov. 

We present a brief review of G2 and G3 theories which are composite techniques for the accurate prediction of experimental thermochemical data for molecules. We discuss the components of G2 and G3 theories as well as approximate versions such as G2(MP2), G3(MP2) and G3(MP3). Additional methods such as extended G3 theory (G3X) as well as scaled G3 theory (G3S) are also discussed. The methods are assessed on the comprehensive G2/97 and G3/99 test sets of experimental energies (heats of formation, ionization energies, electron affinities and proton affinities) that we have assembled. The most accurate method, G3X, has a mean absolute deviation of 0.95 kcal/mol from experiment for the 376 energies in the G3/99 test set. Some illustrative applications of the methods to resolve experimental data for other systems are also discussed. 

It is difficult to determine these thermochemical parameters from experiment, because it is hard to monitor the precursor hydrocarbon radical and the formed peroxy radical. The experiment is further complicated by the presence of reactions to new products by the energized peroxy radicals which can prevent the monitoring of equilibrium. Experiments on ion methods using proton affinity or basicity, often with mass spectrometric analysis, are also utilized to determine enthalpies of formation of radicals. Our methods rely heavily on experimentally determined thermochemical data and we would like to point out that this data is very valuable to validate the computational methods. 

High-pressure mass spectrometry and pulsed high-pressure mass spectrometry have also been a source of thermochemical data for organometaiiic compounds (e.g., proton affinities, electron affinities, and bond dissociation enthalpies). The essential difference between both techniques is the fact that in the former the reactants are produced outside the collision cell. 

Thermochemical scales derived from equilibrium constant determinations are relative values, and absolute assignments for appropriate quantities require reliable calibration values of ionization energy, electron affinity, or proton affinity, for example. Moreover, because of the interrelationships among the thermochemical data for structurally similar molecules, the scale must be evaluated as a whole, not just for individual molecules, and must demonstrate internal consistency among the variables G, H, and S. 

Three important classes of gas-phase ion-molecule reaction schemes demonstrate the value of thermochemical data deduced from equilibrium systems. The first of these concerns the derivation of extensive scales of relative proton affinities, gas-phase acidities, and electron affinities. These results derive primarily from measurements of enthalpy changes for proton or electron transfer reactions ... 

At ion source pressures on the order of 0.5 Torr and ion source temperatures of approximately 373 K, the rate constants for electron attachment and proton abstraction suggest that there are an adequate number of collisions in the ion source to permit equilibria to be sufficiently established. This is a prerequisite in order to assume a Boltzmann distribution of internal energies of the anions (or cations). Thus thermochemical data, such as electron affinities and the proton affinities of anions, can be used to calculate the energetics of these reactions (75). The cluster adduct anion [M- -C]" (Reactions 7.39-7.41) have third-order rate constants. Where comparisons can be made, the magnitude of positive- and negative-mode third-order rate constants are similar (69,103). The clustering reactions are important in NlCl spectra for polar compounds in the presence of polar molecules such as water and alcohol. 

The evaluation of such a body of interrelated thermodynamic data involves first an evaluation of the thermochemical scales for internal consistency in the three parameters, AG° (at different temperatures), AH° and AS°. Final values assigned for the proton affinities and entropy changes must be consistent with what is known about the thermochemistry of M and MFI+. The lengths of segments of the scale linking different primary standards (compounds for which Eqn [2] can be used to derive an absolute proton affinity value) must of course match the known interval between the known proton affinity values. 

G2 theory has been applied to many molecular systems and has in most cases been quite successful. It has been used to predict bond dissociation energies, ionization energies, electron affinities, appearance energies, proton affinities, and enthalpies of formation. This section describes several typical examples of the use of G2 theory to obtain thermochemical data. The reader is referred to several recent reviews for specific references and more details. 

Silene 2, 1-fluorosilene 602 and 1,1-difluorosilene 603 have been examined by FT ion cyclotron resonance spectroscopy in order to estimate the jr-bond strength. Bracketing studies with various bases (B) and fluoride acceptors (A) gave values for the proton and fluoride affinity of the silenes284. The TT-bond energy was calculated by thermochemical cycles and found to increase with fluorine substitution. The data are given in Table 16. 

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