Theoretical verification and extension of the McKean relationship between bond lengths and stretching frequencies

Vibrational spectra contain explicit information on the electronic structur e and the bonding situation of a molecule, which can be obtained by transfo rming the vibrational normal modes of a molecule into appropriate internal coordinate modes, which are localized in a fragment of the molecule and whi ch are associated to that internal coordinate that describes the molecular fragment in question. It is shown that the adiabatic internal modes derived recently (Int. J. Quant. Chem., 67 (1998) 1) are the theoretical counterpa rts of McKean's isolated CH stretching modes (Chem. Sec. Rev., 7 (1978) 399 ). Adiabatic CH stretching frequencies obtained from experimental vibration al spectra can be used to determine CH bond lengths with high accuracy. Con trary to the concept of isolated stretching frequencies a generalization to any bond of a molecule is possible as is demonstrated for the CC stretchin g frequencies. While normal mode frequencies do not provide a basis to dete rmine CC bond lengths and CC bond strengths, this is possible with the help of the adiabatic CC stretching frequencies; Measured vibrational spectra a re used to describe different types of CC bonds in a quantitative way. For CH bonds, it is also shown that adiabatic stretching frequency leads to the definition of an ideal dissociation energy, which contrary to the experime ntally determined dissociation energy is a direct measure of the bond stren gth. The difference between measured and ideal dissociation energies gives information on stabilization or destabilization of the radicals formed in a dissociation process. (C) 1999 Elsevier Science B.V. All rights reserved.