Normal Coordinate Analysis of Trigonal Bipyramidal XY5 Type Molecules

Normal coordinate analysis has been carried out for trigonal bipyramidal XY5 molecules such as PF5, SbF5, PCl5 and SbCl5. It has been found that the simple Urey-Bradley force field does not provide satisfactory results for the bending modes of these molecules. The results have been improved considerably by introducing an interaction constant between equatorial and axial angle coordinates. In carrying out normal coordinate analysis, the repulsive force constants were first estimated from a Lennard-Jones type potential. Then the stretching and bending force constants were adjusted to fit the observed frequencies. Finally, the new interaction constant was introduced to minimize the errors in the bending frequencies. The force constants thus obtained are consistent through the four molecules studied. Some conflicting band assignments made by previous investigators have been discussed on the basis of the present calculations.