Relation of Bond Energies to Other Molecular Properties

Relation of Bond Energies to Other Molecular Properties 

Intuitively, one would expect a volume contraction on forming a strongly bonded compoimd from the elements. Indeed, Richards (190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume — 2 atomic volume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures (151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

A more direct link with molecular volumes holds for alkali halides, because the lattice energy (U) is inversely proportional to interatomic distance or the cube root of molecular volume (MV). The latter has been approximated by a logarithmic function which gives a superior data fit. Plots of against log(MV) are linear for alkali halides (37a). Presumably, U and A/f can be equated because A/f (M(g)) is a constant in a series, and AHf (halidec ) is approximately constant when the anion is referred to the dihalogen as the standard state. 

Extension of the method to nonisostructural metal halides, some of which yield erroneous values via Born-Haber cycles, is shown in Fig. 1. All curves are nonlinear with the bow increasing in the expected order T1(I) Pb(II) Bi(III) Ag(I). For the first transition metal dihalides, however, straight lines can be drawn within the limits of enthalpy errors except for Zn(II) or Mn(II) salts. Thus heats of formation of the fluorides can be extrapolated linearly from the other three halides to a first approximation. 

Vague statements about relations between Ke and D even for mono- 

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D. Relation of Bond Energies to Other Molecular Properties. 27... 

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