Mercury bond dissociation energies

The Arrhenius parameters and the thermochemical sum of the phenyl-carbon and phenyl-halogen bond dissociation energies are shown in Table 8. The extent of the diphenyl mercury decomposition was determined from the weight of mercury produced. It is the present author s opinion that in calculating the Arrhenius parameters for this compound Carter et al.81 gave too great a statistical... 

The mean bond dissociation energies (E ) given in Table 12 are based on thermochemical data at 25 C19. Unless previously discussed, the heat of formation of the metal alkyl used is that given by Long60. The higher values of E and D2 for dimethyl mercury are obtained when Long s recommended value for the heat... 

Unfortunately, there were two errors in Heicklen and Knight s analysis. Subsequent work66 has shown that their quantum yields were too low by a factor of two. Second, at the lowest pressures used (0.6 torr) some of the excited mercury will disappear by fluorescence, thus giving lower quantum yields than would otherwise have been obtained. Both of these errors would give too large a value for E0 it should be smaller than 2.2 kcal/mole. Consequently, the double bond dissociation energy in C3Fe should be between 112.7 and 114.9 kcal/mole. 

The homolytic thermal dissociation of R—M, previously used by Paneth in generating and studying free alkyl radicals, occurs with the more electronegative alkyls, such as those of mercury, lead, and the metalloids. Currently there is much interest in evaluating carbon-metal bond dissociation energies from such pyrolyscs (Section III.B). 

As discussed above, for the measurement of atomic absorbance free ground-state atoms must be present in the sample cell. With the exception of mercury, free atoms do not exist in a stable form at room temperature. The atomizer is the place where free atoms of the analyte are created. The number of free atoms depends on the dynamic equilibrium between the number created and the number lost, e.g., by compound formation and ionization. The bond dissociation energy of the corresponding monoxide Dq and the first ionization energy i (shown in Table 1) are among the factors affecting this equilibrium. 

The most extensive studies on the dissociation energies of metal-carbon bonds have been made on organomercury compounds, in particular on the dialkyls of mercury. The mean bond dissociation energies, Zl(Hg—R), are of the order 20-30 kcal/mole, implying that Hg—C is normally rather a weak bond, and that mercury alkyls should decompose easily on heating. Although this is the case, it has transpired that Hg—C bonds are more... 

The results for Hgj thus indicate that the use of RECPs is a reliable procedure. However, since mercury is not a transition metal, it is still not clear that the procedure will work for cases with open d shells. To see if this is the case, results from a recent study by Wittborn and Wahlgren [40] on the third-row transition metal hydrides are shown in Table XII. The main conclusion from these results is that the RECPs work excellently. The PCI-80 results using RECPs are almost indistinguishable from the results at the all-electron no-pair level. The deviation for the bond distance is at most 0.02 A, and the deviation for the dissociation energy is at most 2.8 kcal/mol and usually smaller. 

In summary, ionisation potentials, dissociation and cohesive energies for mercury clusters have been determined. The mass spectrum of negatively charged Hg clusters is reported. The influence of the transition from van der Waals (n < 13), to covalent (30 < n < 70) to metallic bonding (n > 100) is discussed. A cluster is defined to be metallic , if the ionisation potential behaves like that calculated for a metal sphere. The difference between the measured ionisation potential and that expected for a metallic cluster vanishes rather suddenly around n 100 Hg atoms per cluster. Two possible interpretations are discussed, a rapid decrease of the nearest-neighbour distance and/or the analogue of a Mott transition in a finite system. Electronic correlation effects are strong they make the experimentally observed transitions van der Waals/covalent and covalent/metallic more pronounced than calculated in an independent electron theory. 

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