Polarizability group contributions

Atom dynamics Group contribution and rigid bonds/angels Specific adsorption Dipolar hard sphere SPC, ST2, TIPS Polarizable H Bonds... 

The atomic properties satisfy the necessary physical requirement of paralleling the transferability of their charge distributions - atoms that look the same in two molecules contribute identical amounts to all properties in both molecules, including field-induced properties. Thus the atoms of theory recover the experimentally measurable contributions to the volume, heats of formation, electric polarizability, and magnetic susceptibility in those cases where the group contributions are found to be transferable, as well as additive additive [4], The additivity of the atomic properties coupled with the observation that their transferability parallels the transferability of the atom s physical form are unique to QTAIM and are essential for a theory of atoms in molecules that purports to explain the observations of experimental chemistry. 

The magnitude of the induced dipole moment depends on the electric field strength in accord with the relationship fa = aF, where Jl is the induced dipole moment, F is the electric field strength, and the constant a is called the polarizability of the molecule. The polarizability is related to the dielectric constant of the substance. Group-contribution methods (2) can be used to estimate the polarizability from knowledge of the number of each type of bond within the molecule, eg, the polarizability of an unsaturated bond is greater than that of a saturated bond. 

Influence of U.V. Radiation. Above 180 C the values of s and tan 6 rise significantly more rapidly for the u.v.-resistant LEXAN 143 than for either 141 or 145 and in a manner which seems to be independent of frequency [1]. It is postulated that u.v.-protection in the 143 material is conferred by intimate association of the benzotriazole additive with the polymer molecules, and that above 180 C this additive is liberated from the polymer molecules, freeing additional polarizable groups which contribute to the observed increase in 8, 8 and tan 8. 

Atomic and group properties in the alkanes TABLE 8. Atomic and group contributions to molecular polarizability"... 

Using the experimental results one may determine group contribution in an empirical manner, by subtracting the contribution of ethane (which is itself assumed to equal twice the mean polarizability of a methyl group) from that of propane to obtain the contribution of the methylene group. Following this procedure one obtains a(CHH3) = 15.26 au and... 

The use of several expansion centers surely improves the situation. The averaged value of dipole first polarizability a can be satisfactorily expressed as a sum of transferable group contributions, but the calculations of IND with distributed polarizabilities are rather unstable, probably divergent, and it is convenient to limit the calculation to the first term alone, as is actually done in almost all the practical implementations. Instability and lack of convergence are factors suggesting an accurate examination of the function to fit the opportune values for these distributed a polarizabilities. 

QSAR is an important technique in ligand-stmeture-based dmg design [19], Potency or toxicity of a set of similar dmgs is correlated with a variety of molecular descriptors with the help of QSAR. Empirical formula is used to rapidly calculate multiple descriptors based on the structure and the connectivity of atoms in the molecule. For example, descriptors such as the molecular weight and the number of H-bond acceptors are easily concluded. Some descriptors, such as logP and molecular polarizability, can be approximated from atomic or group contributions. 

Historically, multiple theoretical descriptor-based approaches to H-bond strength ranking were proposed. That includes approaches based on group contribution method [46], electrostatic potentials [47], electrophilic superdelocalizability and self-atom polarizability [48], Quantum Theory of Atoms In Molecules (QTAIM) descriptors [49-51], the two-center shared electron number a and the product of ionization potential [45, 52], and local quantum mechanical molecular parameters, which quantify electrostatic, polarizability, and charge transfer contributions to H-bonding [53, 54],... 

Bishop and Bouferguene have also attempted to partition molecular polarizabilities and hyperpolarizabilities into localized group contributions over the molecular frame. The theory has been applied to a series of push-pull polyenes. An interesting conclusion is that there is little similarity between the distributions for the vibrational and electronic contributions. 

The low polarizability provided by the aUcychc Epiclon moieties leads to Pis with low refractive index. The ellipsometricaly measured values listed in Table 2 show that refractive indices of this type of Pis measured at 632.8 nm correspond to transparent materials, i.e. 1.6-1.7, and are in good agreement with theoretical estimations based on molar refraction and molar volume derived from group contributions theory [29,36]. This approach is based on the assumption that the molar volume, Vu, and the molar refraction, Ru, of the chain repeating unit are additive functions of composition ... 

However, the quantities already include the interactions of the solutes with water, but properties foreign to such interactions are preferable for predictive purposes. Two such properties have been proposed and tested by Marcus [65] the Kamlet-Taft polarity/polarizability index a (Section 3.3.2.1) and the Hildebrand solubility parameter (Section 3.2.2). These as well as the intrinsic volumes of the solutes, expressed as their McGowan volumes are independent of interactions with water, hence true predictive parameters that are known or calculable from group contributions for a large number of solutes. The predictive expression (for ambient temperatures) is ... 

Molecular polarizability and molar refractivity are closely related properties that provide a measure of a molecule s susceptibility to becoming polarized. These descriptors are often useful in situations where dipole-induced dipole and dispersion interactions play an important role. They are readily calculated from refractive index and molar volume however, applications in QSAR and QSPR usually employ empirical estimates based on atomic, bond, or group contributions. A paper by Miller includes a review of techniques that have been used to estimate molecular polarizabilities. Methods for estimating molar refractivity may be found in the literature. ... 

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