Dipole moment, determination

It has been shown that the polarizability of a substance containing no dipoles will indicate the strength o/any dispersive interactions that might take place with another molecule. In comparison, due to self-association or internal compensation that can take place with polar materials, the dipole moment determined from bulk dielectric constant measurements will often not give a true indication of the strength of any polar interaction that might take place with another molecule. An impression of a dipole-dipole interaction is depicted in Figure 11. 

FL, and the difference in dipole moments determined from the plot is 2.36 D if the Onsager radius is 0.33 nm [53]. The Onsager cavity radius was obtained from molecular models where the molar volumes were calculated by CAChe WS 5.0 computer program. The simplest method to estimate the cavity radius is to assume a = (3y/47r) 3, where V is the volume of the solute. 

In the gas phase the dipole moment determined through Eq. (4.10) refers to an individual adsorbed particle. This is not so in the electrochemical situation. The dipole moment of an adsorbed species will tend to align neighboring solvent molecules in the opposite direction, thereby reducing the total dipole potential drop (see Fig. 4.3). Only the total change in dipole potential can be measured, and there is no way of dividing this into separate contributions from the adsorbate bond and the reorientation of the solvent. The apparent dipole potential of an ion adsorbed from a solution on a particular metal is often substantially smaller than that of the same ion adsorbed in the vacuum (see Table 4.1), since it contains a contribution from the solvent. For comparison we note that the dipole moments of alkali ions adsorbed from the vacuum are usually of the order of the order of 10 29 C m. 

The electron cloud of an ion subjected to an electric field undergoes deformations that may be translated into displacement of the baricenters of negative charges from the positions held in the absence of external perturbation, which are normally coincident with the centers of nuclear charges (positive). The noncoincidence of the two centers causes a dipole moment, determined by the product of the displaced charge (Z ) and the displacement d. The displacement is also proportional to the intensity of the electrical field (F). The proportionality factor (a) is known as ionic polarizability ... 

Just as two polar molecules, like opposite ends of a magnet, are attracted to each other, a polar molecule may be attracted to an ion. This gives rise to an ion-dipole force. The negative ends of polar molecules are attracted to cations and the positive end to anions. The charge on the ion and the strength of the dipole moment determine the... 

A second use of microwave spectroscopy is the measurement of dipole moments. These are obtained by measuring the frequency shifts of lines in the applied electric field of a Stark-modulated spectrometer. This method of dipole-moment determination is superior to the older method of measuring dielectric constants. For example, impurities in the sample will not affect the dipole moment as measured by microwave spectroscopy. The dipole moment of a substance present to the extent of a few percent can be accurately measured if its microwave spectral lines can be assigned. The components of d can be determined, thus giving its orientation in the molecule, in addition to its magnitude. 

We may conclude that the dipole moments determined 20-30 years ago by standard methods either in solution or in the gas phase are directly comparable to those measured... 

The gaucheltrans conformational equilibria of 2,2 -bi-(l,3-dioxepanyl) 44 and 2,2 -bi-(l,3-dithiepanyl) 45 in CCI4 and benzene have been studied by a dipole moment determination. The data revealed that both 44 and 45 favored the /razw-conformation however, the X-ray crystallography showed that both compounds prefer the tram-conformation in the solid state <2004PCA6874>. 

The measured dipole moments for X and XI in different solvents are summarized in Table III. First, the experimental values of p vary from solvent-to-solvent with a trend to higher values for more polar solvents. This may be partly due to the approximations mentioned above. It is also important to note that no attempt was made to account for the nonspherical shape of the dye molecule. We believe that this approximation is justified, since the local field factor used to calculate the hyperpolarizabilities in the EFISH experiment for the product p/J involves similar approximations. Thus, the effective dipole moment determined in these experiments,... 

Reports on the determination of dipole moments of pyrans and derivatives have fallen off dramatically over the past two decades. This is probably a reflection of the facility with which this property can be calculated by high-level computational studies. Nonetheless, several papers describe important dipole moment determinations. 

Under these circumstances we are very much tempted to conclude that the structural formula suggested by Townsend and Robins for 3-methylguanine is erroneous and that probably this molecule corresponds to a more classical type of tautomer. This could be, for example, the N(3)H-N(7)H one (63). As can be seen in Table XVI the properties of such a tautomer would be more reasonable and from the spectral point of view more in agreement with the observed properties of the substance. The distinction between 63 or the N(3)H-N(9)H tautomer could easily be made on the basis of dipole moment determination. The N(3)H-N(7)H seems to be the more probable of the two. 

The expression contains the van der Waals MM/MM energy given by E J/MM and the polarization term is the second term in Eq. (13-28) which depends explicitly on the induced dipole moments determined as... 

Accordingly, the modifications to the KS operator are twofold (i) a static contribution through the static multipole moments (here charges) of the solvent molecules and (ii) a dynamical contribution which depends linearly on the electronic polarizability of the environment and also depends on the electronic density of the QM region. Due to the latter fact we need within each SCF iteration to update the DFT/MM part of the KS operator with the set of induced dipole moments determined from Eq. (13-29). We emphasize that it is the dynamical contribution that gives rise to polarization of the MM subsystem by the QM subsystem. 

An important and precise traditional method of measuring capacitance for dipole moment determinations is the heterodyne beat method, a particular form of the null method mentioned above. The output of an LC oscillator incorporating the capacitance... 

These results demonstrate two facts. The aromatic hydrocarbons have a strong Interaction with the polar groups of surfactants as evidenced by NMR and calorimetry Investigations. In addition, the light scattering and dipole moment determination show that this Interaction Influences the transition from monomeric aggregates to Inverse micelles. 

The electronic transition dipole moment determines the overall intensity of the transition. The overlap integrals of the vibrational wave... 

Dipole moments determined for simple methylated 1,3-dioxolanes in benzene solution include 2-methyl-, 1.21 D 2,2-dimethyl-, 1.12 D 2,4-dimethyl-, 1.32 D 2,4,4,5,5-pentamethyl-, 1.29 D (37JA1590) and 4,4-dimethyl-, 1.56 D (71JCS(B)I302>. The dipole moment of 1,3-dioxolan-2-one... 

Change in the dipole moment and polarizability upon electronic excitation was determined for various linear polyenes by adopting electric field-induced changes in the optical absorption spectrum [70]. Polyenes studied (Exhibit 2) were diphenylbutadiene (DPB 7), diphenylhexatriene (DPH 8), diphenyloctate-traene (DPO 9), diphenyldecapentaene (DPD 10), and all-tranj-retinal (3). Exhibit 2 describes the experimental values determined. It is proposed that (based on the results obtained) the excited state dipole moments determined on these polyene systems have a role to play in the mechanism of trans-cis photoisomerization. 

The arrow intentionally goes in only one direction, because the left-hand side is a well-defined quantum mechanical entity whereas the right-hand side is not. Thus, the gradient of the dipole moment determines the charge, but the latter cannot be derived independently to predict the computationally observed" dipole gradient. 

The presence on the furan ring of several substituents, each of which can give rise to rotational isomerism, increases the number of possible conformations. Furan-2,5-dicarbaldehyde represents a classic example. In principle four conformations are possible for this compound (59). Analysis of the electric dipole moments of furan-2,5-dicarbaldehyde in solvents of low dielectric permittivity enabled the relative populations of the conformers to be determined <89JST(196)227>. Experimental data suggest that the compound exists as a mixture of mixed ( ,Z)/(Z, )- and ( , )-conformers with ( ,Z) + (Z, ) = 0.68 and 0.96, ( , ) = 0.32 and 0.04, according to the solvent. Molecular orbital AMI-calculated enthalpies of all the conformers confirm the presence of the mixtures with populations of ( ,Z) -H (Z,E) = 0.38 and ( , ) = 0.60. Another conformational analysis of the same derivative, obtained by an experimental dipole moment determination combined with theoretical MNDO calculations <89H(29)657>, indicates the presence of a conformational equilibrium in which the ( ,Z)-conformation has the biggest contribution, and that the two aldehyde groups are not equivalent. Experimental confirmation has been performed for two model reactions monoprotection with MeOH/TsOH and aldolic condensation. 

In 2,5-disubstituted derivatives (60) and (61), the number of possible conformations amount to 8 for (60) and 10 for (61). Conformational analysis of these derivatives has been carried out employing experimental dipole moment determinations together with semiempirical calculations (MNDO and AMI) <92JST(253)179>. This indicates that compounds (60 R = Me, OMe) and (61 R = Me) exist as an equilibrium mixture of the possible conformers with different populations, while (61 R = OMe) exists in only one conformation (62). 

Trifluoroacetonitrile has received attention at the hands of spectroscopists [matrix-diagonalization treatment of /-type resonance in the rotational spectrum, in the states vs = 1 and vg = 2, dipole-moment determination by microwave... 

Plot of Ajj VS. dipole moment. Determination of order parameters in decanol-... 

Signs of electric dipole moments, which must be obtained by techniques other than Stark effect measurements, are listed in the column Varia, remarks . There exist some molecular beam deflection data of electric dipole moments which have reached the accuracy of dipole moments determined by microwave spectroscopy or may even be better. These values have been included in columns 3-"6. If there are experimental dipole moments from other spectroscopic regions they are listed too. The methods are given with abbreviations as explained in part 1. 

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