THE INFRARED INTENSITY SUM RULE AND EFFECTIVE CHARGES
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Publisher:Ohio State University
It is shown that, within the linear dipole-harmonic oscillator approximation, the sum over all active modes of the integrated infrared intensities of a molecule does not depend explicitly upon molecular geometry. This sum is given by a simple expression depending upon the number and kind of atoms in the molecule, a combination of dipole derivatives, called effective charges, associated with each atom, and the permanent molecular dipole moment. Because the intensity sum is independent of geometry, it can be used directly to compare the effective charges for the same kind of atom in different molecules, without introducing models or additional assumptions. An analysis of intensities reported for several hydrocarbons reveals that the effective charges for the hydrogen or the carbon atoms are essentially equal in methane, ethane, ethylene and benzene. Acetylene seems to form an exceptional system and possible reasons for this will be mentioned.
Author Institution: Metcalf Research Laboratory, Brown University