Thermochemistry bond dissociation energies

The application of newer methods to studies of gas phase organometallic reactions will lead to the development of routine techniques for determination of the thermochemistry of organometallic species. The examples discussed above demonstrate that an analysis of kinetic energy release distributions for exothermic reactions yields accurate metal ligand bond dissociation energies. This can be extended to include neutrals as well as ions. For example, reaction 15 has been used to determine accurate bond dissociation energies for Co-H and C0-CH3 (57). 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

The continuous development and implementation of molecular orbital theory ab initio methods have enlarged the applications to this area too. Indeed, the impact of theoretical calculations in thermochemistry is substantial. Experimental groups often use calculations as a supplement to the interpretation of their results. In this section we will mention a few recent and representative studies that are directly associated with the bond dissociation energies of silanes. Early theoretical investigations of the Si—H bond strength in silanes have been summarized [13]. 

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. 

Table I. Recommended Values for Bond Dissociation Energies and Radical Thermochemistry in Aromatic Oxidation...

The Met+—L bond dissociation energies were measured for some Zn-containing gas phase cations using different methods ". Armentrout reviewed guided ion beam studies of transition metal-ligand thermochemistry and reported the following... 

Bond dissociation energies (BDEs) are at the heart of organic thermochemistry and many mechanistic considerations, so it is imperative to be able to compute and predict these quantities accurately. The problem is that, upon lengthening a bond in the early stages of cleaving... 

An important aspect of fluorine thermochemistry is the effect of increasing a-fluorine substitution on the C—F bond dissociation energy (BDE). This geminal stabilization is illustrated by the C—F BDE in the fluorinated methanes—the CHF series with associated C—F and C—H BDEs given below (for radical heats of formation used see Table 10, for halon atom heats of formation see Table 8) ... 

Keywords. Bond dissociation energies, Collision-induced dissociation, Guided ion beams, Reaction barriers, Thermochemistry... 

The dynamics of hydrogen abstraction reactions promoted by F, O, OH and OD with monogermane have been studied as a function of the vibrational and rotational state by infrared chemiluminescence While this technique provides enormous insight on the energy disposal in a reaction, it also led to a value of 326 4 kJmoP for the H3Ge—H bond dissociation energy at 0 K. This value is somewhat lower than the value of 346 10 kJmoP obtained from a kinetic study of reaction 44 and its thermochemistry . ... 

The isoelectronic principle, by which the chemistry of many nitrogen compounds can be related to that of their hydrocarbon counterparts can be useful if used with caution, e.g. account must be taken of the differences in the thermochemistry of the reactions involved s . A comparison of the bond dissociation energies in such isoelectronic sequences is instructive in this regard (Table 27) . 

Moreover, it can also be used for rationalizing the thermochemistry of free-radical reactions. At first, it is easy to show that the R—X bond dissociation energy of a given molecule may be written as ... 

Fig. 9. Potential energy diagram for breaking chemical bonds in an energetic molecule. The specific coordinate R shown here is identified as the reaction coordinate. In ascending energy these levels are the electronic ground state, a bound excited state and a dissociative excited state. Thermal cleavage of a bond in the electronic ground state requires a minimum energy Dq. In bound electronic states the bond dissociation energy Do is usually smaller than Do, so thermochemistry often has a lower barrier electronic excited states. Chemical bonds can also be broken by electronic excitation to predissociative or dissociative electronic states.

The data listed in Table 9 for I atom attack on alkanes have largely been derived from studies of Benson, Golden and co-workers who have also reviewed the data for these systems with particular reference to their application to thermochemistry and the determination of bond dissociation energies [98]. 

Thermochemistry is not only concerned with the enthalpies of formation of compoimds. Some energies of reaction types, such as ionization potentials, electron affinities, oxidation or reduction potentials, bond dissociation energies and energies associated with acid-base processes are also studied by thermochemical means. 

The NIST Chemistry WebBook contains thermochemical data, reaction thermochemistry data, spectra, spectroscopic data, ion energetics data, thermophysical property data [150]. Several compilations of enthalpies of vaporization and sublimation measured by different means can be foxmd in the literature [3,151-154]. A compilation of bond dissociation energies of organic compounds has recently been published by Luo [155]. 

Studies of coordination chemistry in the gas phase include spectroscopic and structural characterizations, similar to information gleaned in condensed phase work. Somewhat uniquely, the gas phase also allows the characterization of unsaturated coordination complexes and the measurement of the thermochemistry of metal ligand bonds. This latter topic will be the focus of the present article. In particular, trends in the thermochemistry will be discussed, including comparisons among the bond dissociation energies (BDEs) for a variety of metals, ligands, and numbers of coordinated ligands. Most of the work discussed here involves the +1 oxidation state of all metals, an unusual oxidation state for many metals in the condensed phase except for... 

The thermochemistry of boron alkoxides was studied by Chamley et al., which led to an estimate of the average B-O bond dissociation energies for boron alkoxides in the range of 110 5kcalmol these results are consistent with the order B-F > B-N > B-O > B-Cl, reported earlier by Sidgwick." In another study " the reported B-O bond dissociation energy D(B-OR) of B(OMe)3, B(OEt>3, B(OPr">3, were 118.0, 117.7, 119.0 2kcalmor, respectively. 

Much of the available thermodynamic data, snch as bond dissociation energies, gas-phase acidities and basicities, and heats of formation for ions and neutral target species, has been determined nsing TCID [37, 38]. These data inclnde a variety of metals (alkalis, magnesitrm, altrmintrm, and first and second row transition metals), and many types of target molectrles. For instance, Armentront [39] stndied an abso-Inte cation affinity scale, thermochemistry of alkali-metal cation interactions with histidine,and host-gnest interactions of crown ethers with alkali ions nsing TCID. 

PROBLEM 11.26 Analyze the thermochemistry of the two propagation steps for the photochlorination of ethane. The bond dissociation energies of the chlorine-chlorine and carbon-chlorine bonds of ethyl chloride and the carbon-hydrogen bond of ethane are 59, 84.8, and 101.1 kcal/mol, respectively. 

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