Visualizing Raman and Raman optical activity generation in polyatomic molecules

A simple and effective procedure is proposed to characterize vibrational modes of polyatomic molecules, as calculated by ab initio procedures, via their integral circular difference and circular sum scattering cross sections. Placzek polarizability theory type expressions of these scalar molecular quantities are broken down into invariant mono- and di-nuclear terms. The quasi-atomic and group contribution patterns which we define by suitable combinations of them give a numerical or pictorial representation, for each normal mode, of the regions in a molecule where Raman and Raman optical activity (ROA) intensities originate. The similarly defined nuclear and group coupling matrices, on the other hand, are expected to become a powerful tool for establishing ROA sign rules. It is moreover shown that mono- and di-nuclear terms, for each molecular invariant, can be further decomposed into isotropic, anisotropic, and anti-symmetric local coupling terms which reflect, in a highly characteristic manner, the local symmetry of the various vibrational modes. (C) 2001 Elsevier Science B.V. All rights reserved.