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 1(mu) in terms of internal vibrational modes v(n) typical of molecular fragments or s tructural subunits is developed. Essential for this concept is the amp litude An(mu) that provides the basis for a quantitative comparison of modes 1(mu) and v(n) by this, facilitates the extraction of chemical information out of vibrational spectra. Twelve possibilities of defini ng 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 parame ters chosen to describe the molecule, and (c) the amount of chemical i nformation transferred by A. The two most promising candidates for a g enerally applicable amplitude A are based on adiabatic internal modes and a comparison of 1(mu) and v(n) with the help of mass or force cons tant matrix. For the practical testing of amplitude A three different criteria are developed. (C) 1998 John Wiley & Sons, Inc. Int J.