High quality structure–property regressions. Boiling points of smaller alkanes

Abstract

For the correlation of the boiling points of smaller alkanes we outline the use of the variable connectivity index ¹χ^f involving a parameter x, to be determined during the regression fitting. The optimal values of the variable x is determined by minimizing the standard error of the regression. With the value x=0.65 we obtained a quadratic regression characterized by a standard error of 2°C, when out of 21 structures one structure is eliminated as an outlier (having a standard error over 6°C). This result, based on a single descriptor, is better than hitherto reported regressions on the same set of compounds using other descriptors. Using as an additional descriptor the product P₃W₃, where P₃ are the paths of length three and W₃ are the walks of length three, the standard error was further reduced to 1.56°C. We include also the regression results derived for orthogonalized descriptors and clarify the alternative approaches to orthogonalization of descriptors.