Bond dissociation energy periodic table trend

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. 

Frigden [429] has addressed the underlying causes of the differing hydrohalide acidities in terms of the physical properties of the compounds. As shown in Table 7.2, several properties such as the trend to lower bond dissociation energies (BDEs) and smaller differences in X-H electronegativi-ties fAri ) as one proceeds down a group in the periodic table are consistent... 

These trends are consistent with observations made to characterize the chain growth of surface carbon that was deposited by methane decomposition. In a row of the periodic table, the selectivity to hydrocarbon formation was foimd to increase from right to left for example, palladium shows a lower selectivity than ruthenium 111,112). Metals such as platinum and iridium are characterized by higher selectivities for chain growth initiated from "Cl" species than other metals because of their relatively high M—C bond energies. However, platinum and iridium are unsuitable as Fischer-Tropsch catalysts because the dissociation of CO is too slow. 

In Fig. 3.20 we show that the change in metal affects the adsorption energy of adatoms much more so than that of adsorbed molecules. The thermodynamics for dissociative adsorption therefore become more imfavorable with increasing d-electron valence-bond occupation of the metal atoms in a row of the periodic table. This trend is a general result that is likely observed for all dissociation reactions. 

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