Vector addition of dipole moments

The measured dipole moments for (258-261) (Figure 3) (70AK(32)217, 75JHC615) agree closely with predicted values (75JHC615) based on vector addition of dipole moments for reference constituent compounds (benzo[6 ]thiophene, quinoline, isoquinoline). 

The vector addition of dipole moments is only permissible if the weights of the ionic states in the various bonds does not alter significantly in the various compounds, such as would be produced if molecular resonance occurred in these cases only is it possible to assign definite values for the contributions of the various valence bond structures. The data for the bonds OH, OR, NH, NR and SH are given in Table... 

Vector Addition of Dipole Moments in Some Disubstituted... 

The dipole moment of 1,10-phenanthroline was first measured by Fielding and Le Fevre,18 who obtained a moment of 4.1 D, close to the value of 3.8 D expected for the hypothetical m-2,2 -bipyridyl. Since then the dipole moments of the phenanthrolines have been studied by other groups.16,17,64 The experimental values are recorded in Table I. They were found to be in good agreement with values computed by the vector addition of the moments of the individual pyridine nuclei. The molecular polarizability of 1,10-phenanthroline has been further studied and its molar Kerr constant determined.65... 

The dipole moments of the two conformers 146 and 147 of 1-rm-butyl-piperidine-4-spiro-4 (l, 3 -dioxolan) were calculated by vector addition of the moments of 1-fert-butylpiperidine and 4,4-dimethyl-1,3-dioxolan, making reasonable assumptions regarding geometries. This gave 1.15 and 1.93 D as the moments of the two conformers. The observed moment of 1.47 D gave AG° of 0.37 kcal mol-1 in favor of the axial oxygen conformer 146 in cyclohexane solution. AG%2- of 0.26 kcal mol-1 was estimated from low-temperature H-NMR studies.121 The differences between the piperidine and... 

Experimentally, the molecular dipole can be measured. Individual bond dipoles cannot be measured but they have been inferred from experimental data for a variety of compounds. Estimates of the molecular dipole can be made by vector addition of individual moments. Such estimates (calculations) are indicative but may differ significantly from the measured values. The latter are recorded in Table 4.10. 

Dipole moment measurements have been used for structure determination and for discussion of the probable conformation of two possible isomeric l-(a-aroyloxyary-lideneamino)- ,2,3-triazoles (25) formed by oxidation of bis-aroylhydrazones of a-dicarbonyl compounds <77JCS(P2)1779,79JHC571). The probable conformation of these compounds was determined after theoretical calculation of the dipole moments by vector addition of the moments of all polar groups, the results being very similar to those of X-ray measurements. 

A most widely used method for the prediction of molecular dipole moments is the empirical vector addition of bond moments (45). It involves the concept of point dipoles situated in the directions of the individual bonds artd assumes that the magnitudes of the dipoles are transferable from one molecule to another. For allenes a somewhat modified approach has been established which is of relevance in connection with the discussion of influences of electrostatic field effects on spectroscopic properties of allenes (24 25) (Section III.D). This model uses fixed origines and Hxed orientations for all the different (point dipole) bond moments in substituted allenes. As the origins of the point dipoles the positions of the hydrogen atoms in allene (11) have been used (24). The directions of the dipole moments are assumed to make an angle of 40 with the C==C=C axis (Fig. 6) which represents an average value of experimental directions of dipole moments in differently substituted allenes (24). 

Dipole Moments While dipole moments are not generally useful with organic unknowns, occasionally they provide the best means of distinguishing among a few possible structures which would have differing dipole moments. Approximate moments can be calculated by vector addition of bond moments from the literature.K For example, rra is-l,4-dibro-... 

In all cases studied, the a-dipole moment component is increased beyond the value calculated from vector addition of the free monomer moments by 0.1-1 Debye. The large dipole moments and polarizabilities of the monomer substituents for the molecules in Table 4 suggest that these enhancements are electrostatically induced ... 

Table 4. Electric dipole moment components and the enhancement, Ap of the dimer p, over vector addition of free monomer moments.

Dichlorobenzene. Compare your experimental results with the values computed from a vector addition of carbon-chlorine bond moments obtained from the dipole moment of monochlorobenzene (5.17 X 10 ° C m, or 1.55 D). If they do not agree, suggest possible physical reasons for the disagreement. 

The dipole moments of substituted phenyl azides were calculated by vector addition of the group moments, assuming that the substituted groups are colinear and lie in the plane of the ring . Significant discrepancies from experimental values are displayed by ortho derivatives, where mutual induction between the close polar groups should occur ortho effect). In nitro derivatives a more extended type of interaction was postulated to account for the discrepancy shown by the para compound . ... 

Favini applied the PPP method to vinyl azide. Here the delocalized tt system consists of 6 electrons and 5 atomic 2p orbitals. The calculated data include charge densities, bond orders and dipole moments (obtained by vector addition of the a and tt moments). Cis and trans isomers were considered... 

FIGURE 3.22 The total dipole moment of a molecule is obtained by vector addition of its bond dipoles. This operation is performed by adding the arrows when they lie pointing in the same direction, and subtracting the arrows if they lie pointing in different directions, (a) CO2. (b) OCS. (c) H2O. (d) CCI4. 

The dipole moments of the linear dimers II and III were found to be larger than the values obtained by simple vector addition of the monomer moments. Similar conclusions were obtained for dimers of N-methyl acetamide 91). 

Use VSEPR theory to determine the geometry of the molecule. Assign bond dipoles to any polar bonds. Predict the overall dipole moment by estimating the result of vector addition of the bond dipoles. 

The molecular dipole moment can be estimated by vector addition of individual bond moments if the bond angles are known. The possession of a dipole moment permits direct interaction with electric fields or interaction with the electric component of radiation. 

The use of dipole moments p [296, 316] in QSAR studies was proposed by Lien [316 — 318] group dipole moments of 311 aromatic [317] and 214 aliphatic substituents [318] are tabulated in literature. A certain problem of the use of dipole moments is their directionality. Vector addition has to be used to estimate the dipole moment of compounds having more than one site of substitution. 

Dipole moments of diatomic molecules can be calculated directly. In more complex molecules, vector addition of the individual bond dipole moments gives the net molecular dipole moment. However, it is usually not possible to calculate molecular dipoles directly from bond dipoles. Table 3.8 shows experimental and calculated dipole moments of chloro-methanes. The values calculated from vectors use C—H and C—Cl bond dipole moments of 1.3 X 10 ° and 4.9 X 10 C m, respectively, and tetrahedral bond angles. Clearly, calculating dipole moments is more complex than simply adding the vectors for individual bond moments. However, for many purposes, a qualitative approach is sufficient. 

Thus in a molecule containing more than one polar bond, the dipole moment of the molecule is given by vectoral addition of the dipole moments of the individual bonds. The dipole moment is zero in a symmetrical molecule. 

Each of the CO2 and HiO molecules contains two identical atoms bonded to a central atom and two polar bonds. However, only one of these molecules is polar. To understand why, think of each individual bond dipole as a vector. The overall dipole moment of the molecule is determined by vector addition of the individual bond dipoles. 

In addition, we can reduce the number of basis vectors that need to be used by noting that the A2 vibration is expected to give a change of dipole moment in the Z-direction (from the character table), so projection of m or v basis vectors can be neglected. The projection using the Hi atom z vector in the Cg subgroup is given in Table 6.17 and simply... 

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