A new way of analyzing vibrational spectra. III. Characterization of normal vibrational modes in terms of internal vibrational modes

The concept of characterizing normal vibrational modes lμ in terms of internal vibrational modes vn typical of molecular fragments or structural subunits is developed. Essential for this concept is the amplitude 𝒜nμ that provides the basis for a quantitative comparison of modes lμ and vn and, by this, facilitates the extraction of chemical information out of vibrational spectra. Twelve possibilities of defining amplitude 𝒜 are tested with regard to (a) the physical basis of the definition of 𝒜, (b) the dependence of 𝒜 on the set of internal parameters chosen to describe the molecule, and (c) the amount of chemical information transferred by 𝒜. The two most promising candidates for a generally applicable amplitude 𝒜 are based on adiabatic internal modes and a comparison of lμ and vn with the help of mass or force constant matrix. For the practical testing of amplitude 𝒜, three different criteria are developed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 29–40, 1998