On use of the variable connectivity index 1chi(f) in QSAR: toxicity of aliphatic ethers

The number of non-hydrogen atoms in a molecule, N, appears as a very good molecular descriptor for the toxicity of aliphatic ethers, despite the fact that it does not differentiate among isomers. The regression based on N as a descriptor for the toxicity in mice of 21 alkyl ethers was reported to yield the regression coefficient r = 0.9751. The simple connectivity index 1chi produced for the same data a less satisfactory regression: r = 0.9548. To see if variable connectivity index 1chi(f) can improve the regression characterized by N we examined the same data using the variable connectivity index 1chi(f) as a molecular descriptor. By varying x, y, the variables that discriminate between carbon and oxygen atoms, we obtained regression which approaches in quality the best reported regression using weighted paths as descriptor and which is marginally better than the regression based on N.