KINETIC-ENERGY DISTRIBUTION OF 2,2'-BIPYRIMIDINE

A method is proposed for the vibrational assignment of large molecules , based on the kinetic energy distribution among different groups of i nternal coordinates. It is related to the treatment of constrained mot ion and requires a coordinate change for a transformation of the whole vibrational space into two orthogonal subspaces. The normal mode desc ription obtained by this procedure is compared with that suggested by the familiar potential energy distribution, using 2,2'-bipyrimidine as a test molecule. It is found that a criterion based on kinetic energy distribution gives a correct representation of the low frequency mode s, associated with translation or rotation of the pyrimidine rings. It also attributes a proper weight to internal displacements associated with large diagonal force constants and is not influenced by the value of the vibrational frequencies.