Molecular Charges and Dipoles

Chemical Concepts Required to Understand the Significance of Lipophilicity 12.1.1.1 Molecular Charges and Dipoles 

In a molecule, the presence of charges is the result of the formation of bonds that cause an electron flow from the original atoms to the new bonded atoms, and thus 

Molecular Drug Properties. Measurement and Prediction. R. Mannhold (Ed.) Copyright 2008 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31755-4 

An example of quantum mechanical schemes is the oldest and most widely used Mulliken population analysis [1], which simply divides the part of the electron density localized between two atoms, the overlap population that identifies a bond, equally between the two atoms of a bond. Alternatively, empirical methods to allocate atomic charges to directly bonded atoms in a reasonable way use appropriate rules which combine the atomic electronegativities with experimental structural information on the bonds linking the atoms of interest. A widely used approach included in many programs is the Gasteiger-Hiickel scheme [1]. 

Two additional concepts of relevance and related to charges are the dipole moment and the induced dipole. 

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The latter expression reduces to a familiar expression for the dipole moment in a neutral molecule consisting of two point charges, = + C nd = -C(F g. 1.1). In this case, we have +Qri+i-Q)r2=Q Ti-T2)=QR, which is the conventional expression for the dipole moment of a pair of opposite charges + Q separated by the vector R. By convention, R points toward the positive charge. For molecules characterized by electric charge distributions p(r) instead of point charges, the expressions for the molecular charge and dipole moment are superseded by... 

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