Group moments

All bonds between equal atoms are given zero values. Because of their symmetry, methane and ethane molecules are nonpolar. The principle of bond moments thus requires that the CH3 group moment equal one H—C moment. Hence the substitution of any aliphatic H by CH3 does not alter the dipole moment, and all saturated hydrocarbons have zero moments as long as the tetrahedral angles are maintained. 

The group moment always includes the C—X bond. When the group is attached to an aromatic system, the moment contains the contributions through resonance of those polar structures postulated as arising through charge shifts around the ring. 

The comparison of the experimental mean values with the theoretically calculated ones for individual tautomers (Section 4.04.1.5.1) (76AHC(S1)1) or conformers (Section 4.04.1.4.3) has been used in the literature to determine equilibrium constants. Thus, the experimental value for l,l -thiocarbonylbis(pyrazole) (40) is 3.19 D and the vector sums of the simple group moments after addition of the extra mesomeric moments are shown in Figure 8. From these values Carlsson and Sandstrom (6SACS1655) concluded that conformation (40b) exerts the largest influence. 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

To a first approximation143) the dipole moment of cyclopropenone can be considered as an additive combination of the moments of structures a and b, which were estimated from group moments... 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

Disparity between the observed moment and that calculated by vector addition of the individual group moments may indicate that the actual structure is not truly represented by the simple molecular diagram. There are several instances of the application of this technique to structural investigations in the field of O- heterocyclic chemistry, not the least of which is the question of the aromaticity of pyranones and their benzologues. 

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. 

The group moment for an epoxide function has been taken to be 1,3 debye for calculating the expected dipole momonts of 4-bromo-2.3-epoxycyolopentanol and 5-bromo-2,3-epoxycyclopentarwl respectively >6 The dipole moments predicted in this fashion are in aooord With experimental values. 

The dipole moment of 4,5-diphenyl isosydnone (146, R1 = R2 = Ph) is 7.82 D (benzene), and comparison with the dipole moments of other diaryl isosydnones has given a value of 7.3 D for the isosydnone group moment.19 This and the spectroscopic properties of isosydnones are in full accord with their meso-ionic formulation (146).19... 

The Cl2C=CH group moment was not further resolved and no mesomeric moment was calculated. Ethenethiol has sp conformation in the gas phase as determined by MW and IR spectroscopies at this opportunity the dipole moment was also measured87. 

Another example from the chemistry of hydrazones (45) represents a complex problem132. The group moment of the heterocycle was taken from theoretical calculations conformation on (0)C—O is evident. One has to determine conformations on N—N and (N)C—O bonds, but the most important problem is the configuration. The final result was a form near to 45 with a torsional angle previous examples, we preferred to express the conformations at this position as an equilibrium of two planar forms or a formally free rotation all these interpretations are indistinguishable within the framework of dipole moment theory. In any case, this example is at the limits of possibilities of this approach. 

O. 3 Debye. The molecular moment is the vector sum of two group moments, each approximately 3.3 Debye. Figure 30 shows the field dependencies at different temperatures. The results were described as log/r < pE. Figure 31 shows the temperature dependence of the zero-field mobility. Figure 32 shows... 

The directionality of group moments is usually clear from chemical evidence or by a study of the resultant dipole moment of a compound following the introduction of other polar substituents. 

The situation is much more complicated in solids because the intermolecular effects can no longer be ignored, i.e. the approximation EM = 0 inherent in the simple formula for the local field (2.29) is not generally true. Consequently, although we can predict the molecular dipole moment from known group moments, it is not possible to calculate the molar polarisation and thereby the relative permittivity, without further elaboration of the dielectric model. In the case of a polymer there are further complications which arise from the flexibility of the long chains. 

II. If an H-atom of a hydrocarbon is replaced by another atom or radical, a moment is produced which is characteristic of the substituent in question ( group moment ). 

The fact that rule II only holds qualitatively is also shown by the difference of the moments produced (especially in the case of halogens) when the substituent is introduced into an aromatic or an aliphatic residue. This is shown in Table II, which gives the approximate values of the group moments of various substituents in aliphatic and in aromatic hydrocarbons. 

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