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Charge Variations in Substituted Alkanes:  Evidence for a Through-Space Effect

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journal contribution
posted on 05.11.2000, 00:00 by Elizabeth M. Nolan, R. G. Linck
Ab initio quantum mechanical calculations at the 6-31+G* basis set are reported on over 75 substituted ethanes and higher alkanes. The charges on the carbon and hydrogen atoms of these species are determined by topological- and orbital-based methods. The relative change in charge is used to determine how a substituent group affects atoms remote from it. Arguments are presented to account for the charge on the hydrogen atoms in substituted ethanes and higher alkanes. Fluorine atoms within about 2.7 Å of hydrogen atoms cause an increase in positive charge on those hydrogen atoms. Charge is moved to the periphery of the molecule when a charged substituent is present. A model that involves through-space interactions between the charged group and the alkane framework accounts for these data. The charge calculations indicate a substituted carbon atom has a charge that is more dependent on the nature of the atom to which it is attached than on that atom's charge or on other atoms in the substituent. No evidence of systematic charge variation on nonsubstituted carbon atoms was found. Energy changes for some isodesmic reactions are explained on the basis of the model of charge distribution. The results are applicable to studies of the Hammett equation.