INDEX dipolar interactions

The interactions between the molecule and the environment can lead to distortions in the electrical properties due to the susceptibility of the molecules and the properties of the host matrix. The refractive index of the matrix acts as a screening factor, modifying the optical spectra and interaction between charges or dipoles embedded within it. Local field effects change the interaction with an electromagnetic field and should be considered along with orientation factors in the dipolar interaction. 

In addition to the repulsive electrostatic interactions, two isolated identical particles immersed in a solvent of different index of refraction, experience an attractive interaction, namely, the van der Walls or dispersion forces, which arise from the induced dipolar interactions between the molecules constituting the two particles. This interaction depends on the geometry (the shape of the particles) and on the material of which the particles are made of. For two spherical particles, the van der Waals interparticle potential uyj(r) is given by... 

THz-TDS was also used to study the benchmark of molecular nanomagnetism, Mni2Ac [102-104], but has not been employed to study MNM since. These early studies demonstrated that the ZES of MNMs can be obtained by THz-TDS. Parks et al. then investigated the linewidth in some detail, where they considered contributions from hyperfine interactions and intermolecular magneto-dipolar interactions on the linewidth and concluded that, in addition to these, there must also be a distribution in the Z)-parameter. These investigations also made use of the fact that both amplitude and phase of the THz electric field are obtained, which can be converted to the real and imaginary parts of the index of refraction. 

Dipolar and induced dipolar interactions cause this general and fundamental attractive interaction. The character of this interaction is comparatively weak and comparatively long-range in order. The interaction is, in principal, proportional to the refractive index difference between the phases. The difference between oil and water is large and only fairly high concentrations of dissolved material reduce the van der Waals interactions significantly. 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

The universal interaction is due to the collective influence of the solvent a dielectric medium and depends on the dielectric constant D and the ctive index n of the solvent. Reasonably large environmental pertur-fions may be caused by van der Waals dipolar or ionic fields in solution,... 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

In the work of Famini and Wilson,a molecular volume, Vmc, (units of 100 A ) is used to model the cavity term that measures the energy required to create a solute-molecule sized cavity in the solvent. The dipolarity/polarizability term, which attempts to account for dispersion-type interactions, is modeled by the polarizability index, tij, (unitless). This index is defined as the average molecular polarizability divided by the molecular volume, a/Vmc, and helps account for the correlation between polarizability and molecular volume. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

The overall tendency of compounds to interact through dispersion forces is related to the refractive index values of the compounds (see Karger et al., 1973) the greater the refractive index the stronger the dispersion interactions. Thus, the dipolar aprotic solvents and pyridine have the strongest influences in dispersion interactions of the compounds listed in Table 1. Where refractive index is used to measure the concentration of solute it is, of course, important to maximize the differences in these values between solute and solvent. 

Mancini PME, del Pdrez AC, Vottero LR (2001) Nonspecific solute-solvent interactions in binary solvent mixture containing an aprotic hydrogen-bond acceptor rmd a hydrogen-bond donor dipolarity/polarizabihty rmd refractive index. J Sol Chem 30 695-707... 

The solvatochromic parameters are derived from spectroscopic and other measurements specifically designed to measure only a single interaction. In addition, the values are averages of the results from several solutes for each parameter and somewhat independent of solute identity. The most comprehensive solvatochromic treatment of solvent selectivity are the Tti, ai and Pi parameters of Kamlet and Taft, Table 4.15 [568-570, 578]. The rti value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide = 1. The i scale of hydrogen-bond acidity measures the... 

Many attempts have been made to develop empirical measures of solvent polarity that reflect the interaction of polar molecules with solutes and that correlate well with chemical reactivity, and Katritzky and co-workers discussed 184 such parameters. For example, Kamlet, Taft, and co-workers proposed a general dipolarity/polarizability index, n, to measure the ability of a solvent to stabilize an ionic or polar species by means of its dielectric... 

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