Nitrotoluene, dipole moment

The dipole moment is a property of the molecule that results from charge separations like those discussed above. However, it is not possible to measure the dipole moment of an individual bond within a molecule we can measure only the total moment of the molecule, which is the vectorial sum of the individual bond moments.32 These individual moments are roughly the same from molecule to molecule,33 but this constancy is by no means universal. Thus, from the dipole moments of toluene and nitrobenzene (Figure 1.10)34 we should expect the moment of p-nitrotoluene to be about 4.36 D. The actual value 4.39 D is reasonable. However, the moment of p-cresol (1.57 D) is quite far from the predicted value of 1.11 D. In some cases, molecules may have substantial individual bond moments but no total moments at all because the individual moments are canceled out by the overall symmetry of the molecule. Some examples are CC14, tr[Pg.16]

The values for toluene, nitrobenzene, and p-nitrotoluene are from MacClellan, A.L., Tables of Experimental Dipole Moments, Vol. 1, W.H. Freeman, San Francisco, 1963 Vol. 2, Rahara Enterprises, El Cerrito, CA, 1974. The values for phenol and p-cresol were determined by Goode, E.V. Ibbitson, D.A. J. Chem. Soc. 1960, 4265. 

Theoretical calculation of a + n electronic structure of nitrobenzenes and nitrotoluenes were carried out by LCAO method and gave a good agreement with electronic spectra and dipole moments [22]. Later the CNDO and ab ittitio calculation for nitro-, dinitro- and trinitrobenzenes gave a very good agreement with spectral features and dipole moments [23]. 

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