Bond and Group Dipole Moments

BOND AND GROUP DIPOLE MOMENTS Table 4.12 Bond Dipole Moments... 

All values for bond and group dipole moments in Tables 4.12 and 4.13 were obtained in benzene solutions. 

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

Fig. 3.34 Proposed structures of [(CH3CONHC5H4)Fe(C5H4CONHCH3)] and [(CH3CONHC5H4) Fe(C5H4CONHC5H4)Fe(C5H4CONHCH3)] in the solid state (a) and in dichloromethane (b). The dotted lines and arrows indicate hydrogen bonds and the dipole moments of amide groups, respectively. The values of v(NH) are shown near the corresponding amide NH in cm unit. The v(NH) band of the bridging amide group is involved in the broad signal at ca. 3,300 cm ...

One center has been attributed to the central sulfur atom because it is impossible to assign it either to the S-0 bond or the methyl groups, as can be seen in Table I and Fig. 1. As expected, the main contribution to the total dipole moment of DMSO stems from the S-0 bond dipole, see third row of Table I. The methyl groups contribute only in a small way. Whereas the S-0 bond dipole increases, the methyl group dipole moments decrease upon solvation which is in accordance with the observed red-shift and blue-shift from many experiments. The increase of the DMSO dipole moment upon solvation is about 81%. This increase can only partly be explained by the 23% increase of the S-0 bond dipole. A much larger share is due to the reduction of about 35% of the methyl group. 

Dipole moments of PMA are calculated with the RIS scheme with two states per bond. When the dipole moment of the ester group lateral to the chain is placed in the orientation deduced by semiempirical methods, calculated values of the dipole moment are in excellent agreement with experimental results without any kind of adjustement of the energy parameters obtained by Flory and co-workers (A 0751. 

Since a molecule with a center of symmetry, such as one belonging to point groups D h (n even). Cntl (n even). Dtlll (n odd), O, and //, cannot have a dipole moment, no matter how polar the individual bonds (Chapter 3), dipole moments have proved to be useful in distinguishing between two structures Much of the classic chemistry of square planar coordination compounds of the type MA2B2 was elucidated on the basis of cis isomers having dipole moments and traps isomers having none (see Chapter 12). ... 

If pyran-4-one is a 677-electron aromatic system, it must adopt the betaine structure (212 X = 0), which would exhibit a large dipole moment. The observed moment for (18) is 3.7 D. A dipole moment of 1.9 D is obtained by vector summation of bond and group moments. Clearly, if mesomeric effects were absent, the difference between the calculated and observed moments would be very small. The magnitude of the observed dipole moment suggests there is considerable it- electron delocalization in pyran-4-one. However, the calculated value for the betaine structure (212 X = 0) is 21 D (73MI22203) and thus it appears that the extent of delocalization is, after all, quite small. 

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