Electronic Compressibility and Polarizability:  Origins of a Correlation

Recently, two new chemical concepts have been introduced, namely, the atomic compressibility β‘ = r 4/Z eff and the corresponding group (molecular) compressibility Gβ‘ = . Here, r is the radius of the atom, Z eff is the effective nuclear charge, n is the number of atoms of a given element i, and N is the total number of different elements in the molecule. The physical meaning of these two compressibility parameters is examined briefly in this work. A fundamental difficulty with the derivation of the primary relationship β‘ = r 4/Z eff is pointed out. We have investigated the origins of an observed linear variation of Gβ‘ with molecular polarizability for a series of predominantly organic molecules. We show that a linear correlation is to be expected for a plot of Gβ‘ vs if certain conditions are satisfied for all members of a given set of molecules. In particular, if the average value of r/Z eff for the constituent atoms is similar for each molecule in the series, then a linear variation is to be expected for Gβ‘ vs .