Knowledge Bank

Reliable descriptions of the vibrational potential energy distributions of simple chiral molecules are useful in the application of empirical and theoretical models for vibrational optical activity (Raman optical activity and infrared circular dichroism). Normal coordinate analyses of one of the simplest chiral molecules, HCBrClF, using general valence and Urey-Bradley type force fields, have produced conflicting descriptions of some normal modes. These conflicts which are most apparent in modes involving significant contributions from several internal coordinates, could not be resolved directly since limited frequency data results in a GVFF that is indeterminant in a large number of off-diagonal force constants. Vapor phase infrared band contours for HCBrClF have been used to establish the components of the dipole moment derivatives. This information has been used to re-evaluate the descriptions of several vibrational modes where the composition in terms of the internal coordinates was previously ambiguous.