Mean dipole moment derivatives

R. L. A. Haiduke, A. E. de Oliveira, R. E. Bruns. Atomic mean dipole moment derivative and anisotropic contributions to molecular infrared intensity sums. J. Phys. Chem. A-, 108, 6788-6796 (2004). 

Atomic mean dipole moment derivatives and GAPT charges ... 

The function/( C) may have a very simple form, as is the case for the calculation of the molecular weight from the relative atomic masses. In most cases, however,/( Cj will be very complicated when it comes to describe the structure by quantum mechanical means and the property may be derived directly from the wavefunction for example, the dipole moment may be obtained by applying the dipole operator. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

Several nickel derivatives of the type [ANiXfi (A = allyl, X = halogen) are known, but they are very much less stable to air than the palladium compounds (76, 77). The small dipole-moment of bis (ally lnickel bromide) may mean that the nickel-bromine bridges are not coplanar (76). 

For the description of a solution of alanine in water two models were compared and combined with one another (79), namely the continuum model approach and the cluster ansatz approach (148,149). In the cluster approach snapshots along a trajectory are harvested and subsequent quantum chemical analysis is carried out. In order to learn more about the structure and the effects of the solvent shell, the molecular dipole moments were computed. To harvest a trajectory and for comparison AIMD (here CPMD) simulations were carried out (79). The calculations contained one alanine molecule dissolved in 60 water molecules. The average dipole moments for alanine and water were derived by means of maximally localized Wannier functions (MLWF) (67-72). For the water molecules different solvent shells were selected according to the three radial pair distributions between water and the functional groups. An overview about the findings is given in Tables II and III. 

Chapter E is devoted to the mean-square dipole moment and mean rotational relaxation time derived from dielectric dispersion measurements. Typical data, both in helieogenic solvents and in the helix-coil transition region, are presented and interpreted in terms of existing theories. At thermodynamic equilibrium, helical and randomly coiled sequences in a polypeptide chain are fluctuating from moment to moment about certain averages. These fluctuations involve local interconversions of helix and random-coil residues. Recently, it has been shown that certain mean relaxation times of such local processes can be estimated by dielectric dispersion experiment. Chapter E also discusses the underlying theory of this possibility. 

Nitrone derivatives of a unique structure were prepared recently130. The two stereoisomers 43a and 43b are in equilibrium in solution configuration was determined by means of NMR spectroscopy. Dipole moments were calculated by MNDO for both stereoisomers, but were not measured. In our opinion one suitable approach has thus been omitted. 

The IETS intensities for the methyl group vibrations of this species are shown in Fig. 9. The theoretical predictions of Kirtley and Hall (34) using KSH, and taking methyl group dipole derivatives from infrared measurements of ethane, assuming the C-S bond normal vs parallel to the interface, are also shown in Fig. 9. Note that for an orientation with the C-S bond normal, the symmetric C-H modes ( 2 and 9 ), which have net dipoles parallel to the C-S bond, are favored over the anti-symmetric modes ( 4,7, and 11), which have net dipole moments perpendicular to the C-S bond, but that for the C-S bond parallel to the surface the situation is reversed. The better, although by no means perfect, agreement between theory and experiment for the C-S bond normal tends to support the proposed orientation of Hall and Hansma. 

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