Dimer gaseous halides

Thermal Dissociation of Metal Dimer Thermal Dissociation of Metal Carbide Thermal Dissociation of Gaseous Halide Thermal Dissociation of Gaseous Oxide Reduction of Metal Oxide... 

Vilcu and Misdolea [143] applied the significant structure theory and considered dimeric gaseous species too, obtaining acceptable values of the calculated constant volume heat capacity, but not of the constant pressure values. Contrary to these results, the constant pressure heat capacity calculated by Lu et al. [142] agreed with the experimental data, but the estimated critical temperatures were only 49-76 % of the values in Table 3.3, with opposite trends regarding the sizes of the anions. This was remedied by Cheng et al. [144] who applied a version of the significant structure theory to the estimation of thermophysical quantities for the same alkali metal halides as studied by Lu et al. [142] with acceptable success. 

As a final note in this section we mention a recent attempt to establish some correlation between gaseous molecular structures and their source crystal structures of highly ionic substances [76], For some metal halides both monomers and dimers occur in the vapor phase, while for others only the monomer is present in appreciable amounts. Systematic comparisons reveal that dimers are not detected in the vapor if the dimeric molecule cannot be recognized as a unit in the crystal structure. On the other hand, if the presence of dimeric molecules is discernible already in the crystal, it will be observed in the vapor only if its heat of vaporization does not exceed that of the monomer by more than, say, 10 kcal/mol. 

Milne and Cubicciotti(14) have applied ionic models to calculate the heats of dimerization for gaseous alkali halide dimer molecules, and the results are as follows ... 

Infrared absorption spectra have been measured for NaF and KF isolated in solid argon. The fundamental modes of these species and the B3u, B2u, and Blu modes of (NaF)2 and (KF)2 are assigned assuming a planar rhomboid structure of D2h symmetry for the dimers.171 Surface ionization of lithium and its halides has been studied by a double-filament technique. At relatively high temperatures the temperature dependence of the lithium surface ionization current from all molecules studied was identical with that from lithium. Incomplete dissociation of LiCl can account for Li ionization threshold temperatures well above that for surface ionization of Li atoms. Dissociation energies of 4.8 0.1 eV for LiCl(g) and 4.3 0.1 eV for LiBr(g) were obtained.172 The equilibrium ionic forms in salt vapours have been studied. The concentration of M+ and X ions in the saturated vapours of MX (M = Na or K, X = halogen) at 850 °C is 107—1010 ions cm-3 and the concentration of M2X+ and M2X ions is 109—1011 ions cm-3, so that the vapours exhibit measurable electric conductivity. The equilibrium constants for the gaseous reactions are as follows ... 

In addition to monomers of MX2 being present in the vapour state, there is evidence that magnesium and calcium halides form dimers. Electron diffraction data are consistent with the presence of <5% Ca2X4 for calcium halides, while data at 1065 K for magnesium bromide indicate that 12% of the gaseous sample is composed of Mg2Br4. 

The alkali metal halides, MX, fonn solids at room temperature. The best known member of the family, NaCl, melts at 801°, the vapor pressure reaches 1 torr at 865°, and the melt boils at 1413°. The dominant species in gaseous alkali metal halides is the monomeric formula unit, MX(g), but smaller amounts of dimers (M2X2), trimers (M3X3) and tetramers (M4X4) have also been detected by mass spectroscopy. Information about the monomeric molecules obtained by spectroscopic studies at high temperatures is collected in Table 5.1 [1]. 

Table 5.2. Coordination numbers, C, Madelung constants, M, and calculated bond distance ratios, R R, for gaseous, monomeric alkali metal halides MX, gaseous square dimers M2X2, for cubic tetramers M4X4 and for MX crystals with rock-salt structures.

Since the dissociation energies of many metal atom dimers M2 were unknown, Pauling adopted a less rigorous approach in order to determine the electronegativity constants of metals. Consider the energy of formation of a gaseous metal halide Ml from the gaseous reactants ... 

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