Refractivity and Atomic or Molecular Dimensions

Of side interest are applications now being made (Guy and Harrand, 1952 Pople and Schofield, 1957 Sundbom, 1958) of Kirkwood s variation method to atoms and ions with partly filled d-shells (transition elements). For these theory predicts refractions considerably larger than those for closed shell atoms or ions. Indirect measurements are possible through the field emission microscope (Drechsler and Muller, 1952 Drechsler and Henkel, 1954 Drechsler and Liepack, 1956). 

The mean ratio 1 2-3 was taken for computing the refractions of double bonds which had not been—or could not be—investigated, e.g. C=C1. Subtraction of ( benzene — i c—h) from the B observed for C6H5—X gave Bobs for the bond Car—X then the percentage double-bond character was obtained as 

The pd values of Table 15, whatever their errors, run roughly as expected for substituents having +M or —M effects in aromatic molecules. With +M groups the stabilities of dipolar structures, 

3-diene chain, shown in Table 17 against E21 refn. = 1 0 cm3, produces no exaltation in furan, thiophen, pyrrole, cyclopentadiene, or benzene— actually the first four of these molecules display depressions of re-fractivity, i.e. they are less deformable than would be expected if their component units behaved without conjugation and merely additively. 

Among isomers with different shapes the rule may not hold  

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