Fragment mode analysis and its application to the vibrational normal modesof boron trichloride-ammonia and boron trichloride-pyridine complexes

A method for expressing quantitatively the vibrational normal modes of a mo lecule in a basis set consisting of the normal vibrations (plus translation s and rotations) of its constituent fragments is presented. The method is i llustrated by describing the vibrational modes of BCl3-NH3 and BCl3-pyridin e electron donor-acceptor complexes in terms of motions of BCl3 and either NH3 or pyridine. These complexes show examples of mixing between modes loca ted on different fragments, mixing between modes of one fragment due to sym metry lowering, and the transformation of six fragment translations/rotatio ns into vibrations of the complex. Although perturbation theory has been pr oposed to explain such examples of mode mixing, calculations imply that int eractions between fragments of both complexes are too strong for perturbati on theory to be generally applicable. In addition, the transformation of fr agment rotations and/or translations into vibrations of the composite molec ule will always occur and cannot be understood in detail by using perturbat ion theory. For the BCl3-pyridine complex, a band observed at 1107 cm(-1) i s re-assigned as a combination of C-H in-plane bending and a ring-breathing mode of the pyridine fragment. (C) 2001 Elsevier Science B.V. All rights r eserved.