Dipole moment structure determination

K. A. Jensen Dipole moments to determine structures of Pt isomers... 

A simplified but much less accurate method determines only the basic features of a molecule and not its complete molecular structure [46]. Numerous dipole moments were determined by the dielectric absorption method in the microwave region [47-49]. 

Sutton [69] has reviewed the uses of dipole moments in determination of chemical structure and listed mesomeric moments for 25 different substituent groups. Mesomeric moment can be described as the anticipated contribution of a given group to the permanent dipole moment of the molecule to which it is attached, and was used by Sutton as a means of predicting the directive influences of substituents attached to aromatic rings. Tute [70] used mesomeric moments to estimate the dipole moments of 1 -phenoxymethyl-3,4-dihydroiso-quinolines (IV). Moments were resolved into two components along the plane of... 

An employment of effective dipoles determined from FIR bands has been proposed to enter the computation of static angular correlation factors, the Kirkwood g" factors. In the modified formula the integrated band intensity enters instead of the usual dipole moment value determined in vacuum (17). There, a dynamical property is being used to understand more closely the static structure, as controled by the angular position of neighbouring molecules with respect to each other. 

For more detailed discussion of dipole moments, see L. E. Sutton, in Determination of Organic Structures by Physical Methods, Vbl. 1, E. A. Braude and F. C. Nachod, eds., Academic Press, New York, 1955, Chapter 9 V I. Minkin, O. A. Osipov, and Y. A. Zhdanov, Dipole Moments in Organic Chemistry, Plenum Press, New York, 1970. 

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

In 1882 Baeyer and Oekonomides advanced formula 72 (R = H) for isatin on chemical grounds, but shortly thereafter the dioxo structure 73 (R H) was proposed since the ultraviolet spectrum of isatin resembled that of the N—Me derivative (73, R Me) and not that of the O—Me derivative (72, R = Me). " It was later shown, despite a conflicting report, that the ultraviolet spectrum of isatin is very similar to the spectra of both the O— and N—Me deriva-tives - the early investigators had failed to take into consideration the facile decomposition of the O—Me derivative. Although isolation of the separate tautomers of isatin has been reported, - these claims were disproved. A first attempt to determine the position of the mobile hydrogen atom using X-ray crystallographic techniques was inconclusive, but later X-ray work," dipole moment data, and especially the infrared spectrum demonstrated the correctness of the... 

We conclude that the dipolar models may exhibit many interesting features, such as the influence of the strength of the dipole moment on the nematic-isotropic and smectic-nematic transitions. Determining the structure of the various smectic phases is clearly an area which needs more investigation. However, an appropriate simulation scheme must be used to avoid misunderstanding the behaviour caused by the method rather than the model. This may be as simple as checking for system size dependence, or running... 

The crystal structure of 4-butylphenyl-4 -butylbenzoyloxybenzoate was determined by Haase et al. [101]. The compound forms a nematic phase. The neighbouring phenyl rings of the molecule are twisted by 49 and 62°. The dipole moments of the carbonyloxy groups perpendicular to the long molecular axis are compensated to each other as much as possible. 

Kurogoshi and Hori [ 104] determined the crystal structures of the mesogenic ethyl and butyl 4-[4-(4-n-octyloxybenzoyloxy)benzylidene]aminobenzoates. The compounds have different phase sequences crystal-smectic A-nematic-isotropic and crystal-smectic C-smectic A-nematic-isotropic for the ethyl and butyl compounds, respectively. Both compounds have layer structures in the solid phase. The butyl compound contains two crystallographically independent molecules. Within the layers, adjacent molecules are arranged alternately so as to cancel their longitudinal dipole moments with each other. In the ethyl compound the core moieties are almost perpendicular to the layer plane, while in the butyl compound these moieties are tilted in the layer. 

C09-0087. Determine the Lewis structures of the following compounds, and determine which ones have dipole moments. For each molecule that has a dipole moment, draw a ball-and-stick model and include an arrow to indicate the direction of the dipole moment (a) Sip4 (b) H2 S (c) Xep2 (d) GaCl3 and (e) NP3. 

C09-0123. Determine the Lewis structures for the two possible arrangements of the N2 O molecule, N—N—O and N—O—N. Experiments show that the molecule is linear and has a dipole moment. What is the arrangement of atoms Justify your choice. 

A"-Ray structure determinations (see Chapter 11 for details) have been reported for triphenylphosphine oxide, tri-o-tolylphosphine oxide, sulphide, and selenide, and for cw-2,2,3,4,4-pentamethyl-l-phenylphos-phetan-1-oxide (5). Electron spectroscopic studies of phosphorus oxychloride and thiophosphoryl chloride in the gaseous state, and n.m.r., i.r., and u.v. spectra of phosphine sulphides have appeared. Dipole moments have been used to define the stereochemistry of 2-cyanoethylphosphine oxides, such as (6), which is shown in its preferred conformation. 

Dipole Homents and Kerr Effect. - The structures of two conformers of ethyldifluorophosphane have been determined using a combination of dipole moments and microwave spectroscopy.227 Dipole moments have been used to study the different tuist angles of allenic phosphonates (81),228 and also to deduce the stereochemistry of 1,3,2,5-dioxaboraphosphorinane.22 ... 

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 ... 

In order to determine the structural factors maximizing 2PA cross section values, we analyze (8) from Sect. 1.2.1. For all cyanine-like molecules, symmetrical and asymmetrical, several distinct 2PA bands can be measured. First, the less intensive 2PA band is always connected with two-photon excitation into the main absorption band. The character of this 2PA band involves at least two dipole moments, /

symmetry forbidden for centro-symmetrical molecules, such as squaraines with C, symmetry due to A/t = 0, and only slightly allowed for polymethine dyes with C2V symmetry (A/t is small and oriented nearly perpendicular to /t01). It is important to note that a change in the permanent dipole moment under two-photon excitation into the linear absorption peak, even for asymmetrical D-a-A molecules, typically does not lead to the appearance of a 2PA band. 2PA bands under the main absorption peak are typically observed only for strongly asymmetrical molecules, for example, Styryl 1 [83], whose S0 —> Si transitions are considerably different from the corresponding transitions in symmetrical dyes and represent much broader, less intense, and blue-shifted bands. Thus, for typical cyanine-like molecules, both symmetrical and asymmetrical, with strong and relatively narrow, S (I > S) transitions, we observe... 



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