Diatomic metal ions, dissociation

Figure 4. Comparison of theoretical and experimental bond dissociation energies for first row diatomic metal hydride ions. Data from reference 27.

A second, shorter progression is observed in the resonance Raman spectrum of each ion, based on one quantum of the V2 (MX), A jg, stretching mode in each case (see Section 2.8). However, again it is the (MM) mode which acts as the progressionforming mode. Cross terms X12 have therefore been evaluated in these cases. The resonance Raman results on these ions, and on other metal-metal bonded species, are summarised in Table 9. More extensive tabulations of data on metal-metal stretching frequencies are available elsewhere (86). Metal-metal bond dissociation energies (Dq) for diatomic species can be estimated from the Birge-Sponer extrapolation. 

Chapter 1 deals with the kinetics of the dissociation of diatomic molecules and the recombination of atoms, and Chapters 2 and 3 with the reactions of atoms and radicals with molecules, abstraction (metathetical) processes and addition to double and triple bonds. Data for the reactions of metal atoms with a variety of inorganic, organic and metal organic compounds, derived from sodium flame and molecular beam techniques, are discussed in Chapter 4 and rapid substitution at labile metal ions in solution in Chapter 5. The theory of, and the experimental results for, ion-molecule reactions, i.e. chemical processes resulting from binary collisions of positive or negative ions with neutral molecules, are discussed in Chapter 6 and the reactions of solvated electrons in Chapter 7. 

The experiments with a beam of silver particles were conducted at room temperature. The energy of dissociation of diatomic molecules of silver is 1.78 eV, the heat of evaporation of silver molecules is 95 kcal/mol [46], and the heat of evaporation of an uniatomic silver is 64 kcal/mol. Mass-spectrometric studies [46] of silver vapour above a metallic silver showed that the ratio of number densities of ions Ag /Ag2 is equal to two. In other studies [47], a considerably larger value of this ratio was found. At 1037 - 1147 C molecular mass of silver particles in vapours was found to be 278 90 [46], i.e., an average number of atoms in a molecule of silver is 2.56. 

We assume that we have a solid metal M which reacts with a diatomic, gaseous nonmetal X2 (e.g., CI2, F2, 02). Similar cycles can be written for solid elements such as sulfur as the nonmetal. In either case, before we can connect U with AHf we must form gaseous ions of M and X. We need not only the relevant ionization potentials (IP) and electron affinities (EA), but also the heats of atomization of solid M and gaseous X2. These atomization energies are traditionally referred to as heats of sublimation AHsuh of M(s) and of dissociation AHd ss of X2. For NaCl itself, we have... 

A diatomic molecule has to be adsorbed parallel to the surface in order to dissociate. The more favourable adsorption complex for molecules like CO on a group 8-10 metal surface is that in which the molecular axis stands perpendicular to the surface. It has been demonstrated by ESDIAD (electron stimulated desorption, ion angular dependence) that the molecular axis vibrates with regard to the surface normal and that the amplitude of vibration increases with increasing temperature this is shown in Fig. 4.41. 

The dissociation energies of the homonuclear diatomic molecules, D (R2), were calculated from the mass spectrometrically measured ion currents and the thermal functions evaluated for the gaseous species. The error limits shown in table 1 are upper limits and include those due to experimental uncertainties and also those due to the uncertainties in calculated thermal functions. The Do(R2) values (last column, table 1) also show the double periodicity exhibited by the sublimation enthalpies. This double periodicity comes about because (1) in the thermodynamic calculations the of th rare-earth metals is needed,... 

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