Bond dissociation energy experimental methods

Generally, it is not possible to measure, by direct calorimetric methods, the strength of a bond formed when two radicals or atoms are joined together. It is more convenient to measure the bond dissociation energy. Experimental methods for determining bond dissociation energies in diatomic and polyatomic molecules have been described in detail elsewhere. The two methods which... 

The ntility of the experimental methods are illnstrated in this chapter by considering their applications to the stndy of reactive molecules, including radicals, car-benes and diradicals, carbynes and triradicals, and even transition states. These are provided in Section 5.4, which inclndes resnlts for representative bond dissociation energies and an extensive list of thermochemical results for carbenes, diradicals, carbynes, and triradicals. Section 5.5 provides a comparison and assessment of the resnlts obtained for selected carbenes and diradicals, whereas spectroscopic considerations are addressed in Section 5.6. 

In this section we deal with the first of the physical effects which impinge on reactivity — the influences which heats of reaction and bond dissociation energies have on the course of chemical reactions. Both heats of reaction and bond dissociation energies are enthalpy values that are experimentally determined by thermochemical methods, in the first case usually by direct calorimetric methods, in the second by more indirect techniques 22). 

Pensak and McKinney (28) [PM], using this method, have recently reported a systematic study of first-row transition metal carbonyl complexes for which experimental bond distances and angles were reliably reproduced, along with key bond dissociation energies. 

Table 6.9 Comparison of experimental C-H bond dissociation energies at 0 K (kJ/mol) with those calculated with wavefunction-based electronic structure methods.

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]. 

In order to test the point-charge method experimentally measured dissociation energy and interatomic distance are required for each chemical bond. Dissociation energies for most homonuclear diatomic molecules have been measured spectroscopically and/or thermochemically. Interatomic distances for a large number of these are also known. However, for a large number of, especially metallic diatomic molecules, equilibrium interatomic distances have not been measured spectroscopically. In order to include these elements in the sample it is noted that for those metals with measured re, it is found to be related, on average, to 5, the distance of closest approach in the metal, by re = 0.78(5. On this assumption reference values of interatomic distance (d) become available for virtually all elements, as shown in the data appendix. In some special cases well-characterized dimetal bond lengths have also been taken into account for final assessment of interatomic distance. 

Feng, Y. Liu, L. Wang, J.-T. Huang, H. Guo, Q.-X. Assessment of experimental bond dissociation energies using composite ab initio methods and evaluation of the performances of density functional methods in the calculation of bond dissociation energies, J. Chem. Inf. Comput. Set. 2003,43, 2005-2013. 

Table 1 summarizes the trends in the structures and bond dissociation energies of hydrides (MH , n = 1, 2 and 3 for M = As, Sb and Bi) calculated with the CASSCF/CI method together with the available experimental data. The spectroscopic properties and potential energy curves for monohydrides (MH) have been fully summarized by Balasubramanian. Even for simple hydrides, there are only few experimental data for comparison. As Table 1 shows, however, good agreement is seen between the calculated and limited experimental values. This will permit the periodic trends to be discussed on the basis of the calculated values. 

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