Knowledge Bank

The usual FG matrix method for the calculation of fundamental frequencies and normal co-ordinates must be modified before application to a molecule with free internal rotation. The need for a modification arises because in general for such a molecule the elements of the F and G matrices, and therefore the fundamental frequencies and normal co-ordinates, depend on the torsional angle $\gamma$. Thus an FG calculation must be performed for several values of $\gamma$ in order to determine how the fundamental frequencies and normal co-ordinates vary with $\gamma$. A numerical normal co-ordinate calculation has been performed on the dimethylacetylene and methylsilylacetylene molecules using an IBM 360 computer. The results of these calculations show how the normal co-ordinates and fundamental frequencies vary with $\gamma$ on introducing a slight $\gamma$-dependence into some of the elements of the F-matrix, symmetry co-ordinates having been chosen so that the G-matrix is independent of $\gamma$. The results also show how the values of the fundamental frequencies and species of the normal co-ordinates change with the force field. It is also hoped to present the results of calculations of the Coriolis coupling constants and centrifugal distortion constants.