Molecular connectivity index third-order

The main characteristic of cluster-type indices is that all bonds are connected to the common, central atom (star-type structure). The third-order cluster molecular connectivity index (3yc) is the first, simplest member of the cluster-type indices where three bonds are joined to the common central atom [102-104, 111-113,152-154,166,167,269]. The simplest chemical structure it refers to is the non-hydrogen part of ferf-butane. This index is then calculated using Eq. (43) ... 

Each of nine phenols was combined individually with three concentrations of lead and these combinations were tested in Photobacterium phosphoreum. The result indicated that the combined toxicity of lead and phenol is not only dependent on the Pb concentrations but also on the positions of substituted groups of phenols. QSARs were built from the combined toxicity and physicochemical descriptors of phenols in the different Pb concentrations. The combined toxicity was related to water solubility and the third order molecular connectivity index QX) in low Pb concentration, to solute excess molar refractivity (E) and ionization constant (pA a) in medium Pb concentration, and to dipolarity/polarizability (S) in high Pb concentration. 

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