Bond dissociation energies multiple bonds

Table 3.6. Comparison of the results of various types of (local) density-functional calculations (LCAO-Za, LMTO-Aa, DV-Aa) with ah initio SCF Hartree-Fock and multiple-scattering Xa calculations (using overlapping spheres, OS) and experimental data (expt.) for equilibrium bond distance [i e(fl )] and bond dissociation energy (D in eV) in the nitrogen molecule (Nj)...

Fig. 19 Diagram of excitation energy (eV), predicted first bond dissociation energy (3.99 eV, lower arrow), and the sum of the first and the second bond dissociation (7.08 eV both arrows) of NiCp2. Spin multiplicity is given in parenthesis. In all calculations the ground state of the cyclopentadienyl radical is considered.

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

One use of the electron affinities of molecules is to predict the sensitivity and temperature dependence of the ECD to compounds that might be analyzed. Many environmental pollutants have different multiple substituents. Pesticides are highly chlorinated organic compounds. The chlorinated biphenyls, naphthalenes, and dioxanes are among the most toxic compounds. The temperature dependence of these compounds in the ECD is important, but has not been extensively studied. When the electron affinities and bond dissociation energies are known, the temperature dependence can be calculated from the kinetic model. This is done for the chlorinated biphenyls and naphthalenes, and the calculated temperature dependence is then compared with experiment. These calculations offer clues about the best conditions for analysis. 

None of the neutral second-row transition metal atoms react with methane at 300 K, but some of them (Rh and Pd) react with ethane and larger alkanes [15]. Ab initio calculations [15] indicate that Y, Zr, Nb, Ru and Rh atoms produce very stable insertion products H-M-CH3 in the reaction (1) with an activation barrier ( 0 4-20 kcal/mol), which is much lower than the C-H bond dissociation energy (103 kcal/mol). Some metals (Nb, Ru, Rh) have to change spin multiplicity [15] in the course of the reaction (1). 

Table 3.6 Average bond dissociation energies for a selecttion of single and multiple bonds...

Additional bond dissociation energies for C - C multiple bonds are given in Table 1.3. 

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