Vibrational spectra and scaled quantum-mechanical molecular force fields

A number of questions connected with the use of scaled quantum-mechanical force fields in interpreting molecular vibrational spectra are considered briefly. The Pulay method of scaling (congruent transformation of the force constant matrix) is noted to be applicable in the case where the relative accuracies of the determination of diagonal and off-diagonal quantum-mechanical force constants are approximately equal. This requirement is satisfied for a quantum-mechanical force field determined close to the Hartree–Fock limit. Then it is possible to carry out its correction, preserving to a maximum the features inherent to the molecule under investigation. Several examples of predicting the vibrational spectra of isotopomers, rotational isomers, and related compounds are given. Unlike using force fields obtained by the traditional method of solving the inverse vibrational problem and the additive scheme for transferring the force constants, scaled quantum-mechanical force fields make it possible to perform well-grounded theoretical vibrational analyses of all the related compounds of a molecule.