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BELL SYSTEM TECHNICAL JOURNAL 



Another point regarding molecular structure shown by such studies 

 is that it is not only the presence of polar groups in the molecule but 

 also their position which determines the electric moment of the mole- 

 cule. This is nicely illustrated by the dichlorobenzenes, of which 

 there are three isomers. As is shown in Fig. 3, ortho-dichlorobenzene, 

 having the two substituent groups in adjacent positions, is the most 

 asymmetrical of the three compounds, and consequently has the high- 



Fig. 3 — Ortho dichlorobenzene being the more asymmetrical has a higher electric 

 moment than the meta isomer; the para isomer which is symmetrical has zero electric 

 moment. 



est electric moment, /x = 2.3. The meta compound has about the 

 same moment as monochlorobenzene, /x = 1.55. The para compound, 

 however, is symmetrical and has zero electric moment because the 

 CI atoms are substituted on opposite sides of the benzene ring so that 

 their vector moments cancel. These values of electric moment are 

 reflected in the values of dielectric constant which are respectively 

 10, 5.5 and 2.8 for the three isomeric dichlorobenzenes. 



