Configuration, determination from dipole moments

DIPOLE MOMENTS AND MOLECULAR STRUCTURE In a number of cases the magnitude of the dipole moment may be used to determine the geometrical configuration of the atoms in a molecule. Thus the first proof of the triangular structure of water arose from dipole moment measurements. Calculation may also be made of the valency angles from dipole moment data, but the values obtained are not as accurate as the more direct methods employing spectra, x-ray and electron diffraction measurements. However, for complex molecules when the application of more exact methods is not possible, the analysis of the dipole moment data may give valuable information. Thus the moment of thianthrene is 1-57, D,... 

The structure of the dimers from mero-substituted derivatives was initially determined by comparison of the observed and calculated dipole moments. For a head-to-tail dimer the dipole moments resulting from the 9 and 9 substituents should cancel each other and the resultant dipole moment should be essentially zero. For a head-to-head arrangement the dipoles would be in the same direction and the resultant should be considerably greater than zero. The dimers produced upon irradiation of 9-chloro and 9-bromoanthracene solutions were observed to be 0.36 and 0.60 D, respectively. Since these values are much less than expected for a head-to-head arrangement for these derivatives (3.8 D), it was concluded that both of these dimers were formed in a head-to-tail configuration/30 ... 

French scientists (270) suggested that the configurations of stereoisomeric acyclic alkyl nitronates can be determined from the relative dipole moments which for trans- isomers of nitronates containing EWG at the a-C atom are substantially larger than those of the cis isomers (Chart 3.5). 

Mean-square dipole moments of polylthiodiethylene gycol), an alternating copolymer of ethylene oxide and ethylene sulfide, are determined from dielectric constant measurements on dilute solutions of the polymer in benzene. Since the configuration-dependent properties of one of the parent homopolymers, PES, are unknown, because of its insolubility in ordinary solvents, the results are preferably compared with those of POE chains. It is found that the dipole moments of polylthiodiethylene glycol) are somewhat larger than those of POE. 

The dipole moments of PSDET chains are determined as a function of temperature by means of dielectric constant measurements in dioxane. The experimental results are found to be in fair agreement with theoretical results based on an RIS model in which the required conformational energies are obtained from previous configurational analyses on polylethylene terephthalatei, polyfdiethylene terephthalate), and polylthiodiethylene glycol). 

However, it is necessary to determine precisely the orientation of the dipole moments, and to ensure that the observed CD is not the resultant from a mixture of con formers. Moreover, even with a correct choice of the dipoles, there are often shortcomings. For instance, studies of l,l -spirobisindanes have shown substantial discrepancies between the theoretical and experimental results for several products of known configuration 27). It has been shown that the signs of the bands stand in direct relation to the configuration of the spiro-atoms, which means that the optical activity is really governed by the coupling mechanism including other types of interaction. 

Modem structural chemistry differs from classical structural chemistry with respect to the detailed picture of molecules and crystals that it presents. By various physical methods, including the study of the structure of crystals by the diffraction of x-rays and of gas molecules by the diffraction of electron waves, the measurement of electric and magnetic dipole moments, the interpretation of band spectra, Raman spectra, microwave spectra, and nuclear magnetic resonance spectra, and the determination of entropy values, a great amount of information has been obtained about the atomic configurations of molecules and crystals and even their electronic structures a discussion of valence and the chemical bond now must take into account this information as well as the facts of chemistry. 

The C=N bond appears as most interesting from the point of view of dipole moments it has itself a considerable moment, gives rise to variable configurations and can be involved in many conjugated systems. Nevertheless, an isolated C=N has been dealt with relatively rarely. Configurational equilibrium of some azomethines was investigated91 mainly with the idea to compare the possibilities of dipole moments and of NMR spectroscopy. In the 4-chloro derivatives 19, the substituent enabled the equilibrium to be determined... 

In the more complex azomethines 21 the configuration on C=N was in fact known from NMR dipole moments served mainly to determine the ap conformation on the Car—1C(N) bond93. With the simple fluorinated azomethine 22 there are no stereochemical problems. The detailed geometry was determined94 by both electron diffraction (ED) and by microwave (MW), and the dipole moment was also measured but not interpreted even its direction is not known with certainty. 

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. 

In 1,2-diketones or their functional derivatives no conjugation is apparent in the dipole moment values, but these can serve for determining conformation on the central, formally single bond. In ( )- and (Z)-benzil monoximes (62) the configuration on C=N was known, and the ap conformation on N—O was assumed (and finally confirmed) from dipole... 

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