Correlation of vibrational frequencies for rotational isomers and isotopomers of molecules

Two approaches which can be used to correctly correlate the vibrational fre quencies of rotational isomers of molecules or the vibrational frequencies of isotopomers with inverse isotope shifts are considered. One of these is based on calculating a matrix analogous to the Dushinskii matrix for the vi bronic spectrum of a molecule. This approach is applicable to direct vibrat ional problem calculations with a scaled quantum-mechanical force field for each rotational isomer or isotopomer but cannot be used to correlate vibra tional frequencies obtained in solving the inverse vibrational problem. The other approach is based on analyzing the isotope shifts for isotopomers wi th fictitious atomic masses.