Gas-phase thermochemical data

Quantum chemistry has reached the stage where it is possible to calculate gas-phase thermochemical data to a precision of a few kJ/mol. Although computationally expensive, such calculations can be carried out routinely for a broad range of molecules containing first-row atoms. 

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. 

Gas-phase thermochemical data were obtained from the GRI mechanism [55]. 

Scheme 1 Preparation of 7-oxabicyclo[2.2.1]heptane and gas phase thermochemical data...

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],... 

In practice, one usually defines a training set of molecules and associated experimental properties and fits the relevant data with an assumed force field.The next step is to test the results on molecules and data outside the training set. Experimental data that depend on the energy surface and that may be used to determine the parameters of the intramolecular interactions consist mainly of gas-phase structural data derived from microwave spectra or electron diffraction patterns, crystal structures derived from X-ray and inelastic neutron scattering measurements, and vibrational frequencies obtained from infrared and Raman spectra. Torsional barriers are derived from NMR band shapes and relaxation times, whereas conformational energies are determined from spectroscopic and thermochemical data. Nonbonded parameters are determined mainly from... 

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

The thermochemical data for the sulfoxides, sulfones, sulfites and sulfates, derived from calorimetric measurements, are given in Tables 1-5. All entries in the tables were checked by examination of the original sources. Where available, data are given for the gas phase and either the liquid (lq) or solid (c) phase. Preference was given to gas and liquid phase data. 

The situation is somewhat better for the gas-phase chemistry of isolated transition-metal ions or complexes, and this area of research has received a lot of attention in the past. On the experimental side, comprehensive mass-spectrometric techniques allow for an explicit measurement of thermochemical and kinetic parameters of reactants, intermediates, and products occurring along the reaction pathways. These data can be obtained without the influence of ligands, counter ions, solvents etc. which would be a highly complicated enter-... 

Even when solvation energies for the ions are available problems of interpretation remain. The factors which cause given observed solvation energy differences may not be obvious. Thermochemical data obtained from the determination of sequential gas-phase ion-solvent molecule equilibria can be extremely useful for this purpose. 

It is not an exaggeration to say that electrospray has introduced a new era, not only for the analytical mass spectroscopist, but also for the more physically oriented researcher interested in physical measurements involving the above ions, which are of such great importance in condensed-phase ion chemistry. In particular, gas-phase ions produced by electrospray allow, for the first time, thermochemical measurements involving ions of biochemical significance such as protonated peptides, deprotonated nucleotides, and metal ion complexes with peptides and proteins. It is to be expected that such data will be of importance in the development of theoretical modeling of the state of these systems in the condensed phase.34,35... 

We have discussed above some of the applications of gas-phase ion thermochemical data to ionic reactions in solution. However the new analytical ion-transfer from solution to the gas-phase techniques have also created an application for these data in the new analytical mass spectrometry. In fact, much of the background knowledge required for this new analytical mass spectrometry, and particularly MALDI and electrospray, is the gas-phase ion chemistry developed for applications... 

Proton Transfer and Electron Transfer Equilibria. The experimental determination used for the data discussed in the above subsections of Section IV.B were obtained from ion-molecule association (clustering) equilibria, for example equation 9. A vast amount of thermochemical data such as gas-phase acidities and basicities have been obtained by conventional gas-phase techniques from proton transfer equilibria,3,7-12-87d 87g while electron affinities88 and ionization energies89 have been obtained from electron transfer equilibria. 

For the gas-phase, second-order reaction C2H4 + C4H6 - CgHio (or A + B - C) carried out adiabatically in a 2-liter experimental CSTR at steady-state, what should the temperature (T/K) be to achieve 40% conversion, if the (total) pressure (P) is 1.2 bar (assume constant), the feed rate (q0) is 20 cm3 s-1, and.. the reactants are equimolar in the feed. The Arrhenius parameters are EA = 115,000 J mol-1 and A =3.0x 107L mol-1s-1 (Rowley and Steiner, 1951 see Example 4-8). Thermochemical data are as follows (from Stull et al., 1969) ... 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

Kinetic studies in solution and in the gas phase have been playing an increasingly important role as a source of thermochemical data (see examples in chapter 15). Here we discuss how to relate thermochemical and kinetic information by approaching the subject as we did in the previous chapter by highlighting important practical issues and reducing to a minimum the description of theoretical models. In other words, the present chapter also relies on the material usually covered at the undergraduate level [1]. Further details can be found in more specialized books [55-59],... 

The experimental methods designed to investigate the energetics of gas-phase ions have been another important source of thermochemical data, particularly throughout the past two or three decades [9,10]. In this chapter, we discuss the main quantities that are measured experimentally and lead to reaction enthalpy values. 

This useful and simple-to-use software package relies on Benson s group additivity scheme [47] to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics [32] and JANAF Tables [22]. The first version of... 

Historically, some of those approaches have been developed with a considerable degree of independence, leading to a proliferation of thermochemical concepts and conventions that may be difficult to grasp. Moreover, the past decades have witnessed the development of new experimental methods, in solution and in the gas phase, that have allowed the thermochemical study of neutral and ionic molecular species not amenable to the classic calorimetric and noncalorimetric techniques. Thus, even the expert reader (e.g., someone who works on thermochemistry or chemical kinetics) is often challenged by the variety of new and sophisticated methods that have enriched the literature. For example, it is not uncommon for a calorimetrist to have no idea about the reliability of mass spectrometry data quoted from a paper many gas-phase kineticists ignore the impact that photoacoustic calorimetry results may have in their own field most experimentalists are notoriously unaware of the importance of computational chemistry computational chemists often compare their results with less reliable experimental values and the consistency of thermochemical data is a frequently ignored issue and responsible for many inaccuracies in literature values. 

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. 

Thermochemical data are also available from the Internet. Some examples are the NIST Chemical Kinetics Model Database (http //kinetics.nist. gov/CKMech/), the Third Millennium Ideal Gas and Condensed Phase Thermochemical Database for Combustion (A. Burcat and B. Ruscic, ftp //ftp. technion.ac.il/pub/supported/aetdd/thermodynamics/), and the Sandia National Laboratory high-temperature thermodynamic database (http //www.ca.sandia. gov/HiTempThermo/). 

The NIST Chemical Kinetics Model Database web site (http //kinetics.nist. gov/CKMech/) is a good resource for chemical kinetic models, thermochemical property data, and elementary rate coefficients. The book Gas-Phase Combustion Chemistry edited by W. C. Gardiner, Ir. (Springer-Verlag, NY, 1999) also lists many detailed mechanisms for different fuels that are available in technical papers and from the Internet. 

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