Failure of net atomic charge models to represent the van der Waals envelope electric potential of n‐alkanes

Abstract

AbstractTraditional net atomic charge models were found unsatisfactory for representing the molecular electric potential (MEP) of n‐alkanes ethane through decane in their van der Waals envelopes. The MEP of these molecules was calculated by ab initio methods. Mulliken atomic charges were erratic and gave root mean square (rms) relative errors of fit to the MEP ranging from 152 to 607% with the 6‐31 + + g** basis set. Fitting the MEP with potential‐derived net atomic charges (PD charges) gave errors ranging from 51 to 62% with the same basis set. The use of larger basis sets, inclusion of electron correlation, use of more MEP data points, or relaxation to optimal structural geometry did not improve significantly the representation of the MEP by net atomic charges. In many cases PD charges of hydrogen atoms were negative, and carbon atoms were positive. To improve the representation of the MEP of n‐alkanes, augmentation of the model with nonatomic sites was investigated with the program PDM93. Models with additional charge sites located between hydrogens, on a line bisecting the CH2 group, achieved fits to the MEP with errors reduced to 8% or less, except for n‐butane, where the fitting error was 16%. The PD models augmented with methylene bisector sites generally show positive hydrogen charges. They also show CH2 group charge alternation along the n‐alkane chains. The augmented charge models were consistent with the observed quadrupole moment of ethane and gave satisfactory predictions for the crystal structures of pentane and octane. © 1994 by John Wiley & Sons, Inc.