Stevensons Rule

On the other hand Wells (1949) provided strong evidence against the validity of the Schoemaker—Stevenson rule. Futhermore, if for the pairs VINi2+—X, VIFe2+—X and VICo2+—X the sums of Pauling s crystal and covalent radii are compared (Table 5), it is clear that the covalent distances are predicted to be shorter than the ionic distances. Roth (1967) in a comparison of the sums of ionic radii with observed... 

Pauling s revised Schomaker-Stevenson rule Rather, it is clear that steric repulsion alone is sufficient to explain why no methyl-bridged dimer forms the boron atom is too small to accommodate four methyl groups. In a hypothetical dimer (Fig. 3d) the eight Cb Q distances would be only about 2.70 A. In dimeric trimethylaluminum the corresponding distance is 3.23... 

A modified Schomaker-Stevenson rule for the prediction of single bond distances between p-block elements... 

Blom R, Haaland A (1985) A modification of the Schomaker-Stevenson rule for prediction of single bond distances. J Mol Struct 128 21-27... 

Table 7 Covalent bonding radii (in pm) for calculations of single bond distances in Lewis-valent compounds with the modified Schmnakcr-Stevenson rule (Eq. 8)...

The molecular structures of the fluorinated methanes have been accurately determined by rotational spectroscopy (Table 9). The C-H bmid distances all fall in the narrow range from 108.4 to 109.1 pm. The C-F bond distance in CH3F is indistinguishable from the value predicted by the modified Schomaker-Stevenson rule, 138.2 pm, but decreases monotonically with increasing number of F atoms to 131.5 pm in 03 4. 

This chapter is composed of three parts. The first covers principles of thermochemistry that are fundamental to the comprehension of the mechanisms involved in spectral interpretation electronegativity, chemical bonds, acidity and basicity, inductive and mesomeric effects, the Audier-Stevenson rule, and stability rules for radicals in the gas phase. 

The Audier-Stevenson rule allows an analyst to predict which side of the bond will keep the charge in case of bond cleavage. Consider an ionized molecule A-B+-. One must compare the ionization potentials of the radicals A" and B" to establish which of A or B will carry the charge in the case of the cleavage of the A-B bond. The charge will be carried by the entity whose ionization potential is the lowest. 

Figure 9.18 illustrates the Audier-Stevenson rule in the case of ionized acetone. Will the rupture of the CH3OC-CH3 bond lead to the CH3CO+ ion and to the radical CH3 (pathway a) or to the CH3+ ion and to the radical CH3CO (pathway b) The ionization potential of the radical CHjCO- (680 kJ/mol) is inferior to that of the radical CH3 (946 kJ/mol) so the latter will be ionized. 

FIGURE 9.18 Audier-Stevenson rule applied to ionized acetone. 

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