Molar polarizability Molecular weight

A property for which generally good experimental data are available is molar refraction (MR), a composite of refractive index, density, and molecular weight. Because of its relation to polarizability, it has been used in many QSAR relations as a regression variable. It is observed that the relation between MR and skeletal variation is very different from the relation between heat of atomization and structure. The heat of atomization for a set of alkane isomers is generally ranked rather well by the x index the most stable isomer in a set generally has the smallest value of the chi-one index. Such is not the case with MR the MR values in an isomer set follow a much different pattern with x- We expect, then, a rather different QSAR for MR than for AH.,. For alkanes the following QSAR is obtained ... 

Comparison of Parameters Used in Structure-Activity Studies -Leo et al. reported a comparison of parameters currently used in studies of structure-activity relationships. The activities of a variety of molecular types, as measured in four different biological systems, were correlated with octanol/ water partition coefficients, polarizabilities, molar attraction constants, parachors, adjusted parachors, and molecular weights. In the systems investigated the octanol/water partition coefficient was found to correlate best with biological activity while adjusted parachor and molar attraction constant gave the next best correlations. ... 

The simplest structural descriptors are just the number of carbon atoms, or molecular weight, that is linearly related to the increase in gas chromatographic retention among a homologous series of compounds. This simple relationship led to the development of the Kovits retention index scale. A linear relationship between Kovits retention index and carbon number for homo-logues has been shown to hold for a number of chemical groups. Molar volume, molar refractivity, and molecular polarizability are other simple descriptors of molecular bulk that have been used in QSRR studies. 

The alternatives to mathematical descriptors derived from molecular graphs or molecular geometry are the traditional QSAR (quantitative structure-activity relationship) descriptors and quantum chemically computed parameters. The former include the partition coefficient for oil/water (often octanol/water) (log P), the Hammet sigma value (electronic parameter that measures the electron withdrawal from and the electron release to the aromatic ring by a substituent, the Taft s parameters for the electronic effects of substituents in aliphatic compounds (a ), and a steric parameter for the proximity of substituents on reaction sites (Es)- Also selected molecular properties, such as molar refractivity (MR), polarizability (a), molecular weight (MW), and density (d), have been used. 

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