Molecular Vibrations of the Trigonal Bipyramidal Model MXY2Z2: Pentacoordinated Molecules. XI

A vibrational analysis of the trigonal bipyramidal model of C2v symmetry neglecting anharmonicity is given in terms of the FG matrix system. The angular dependence of the secular equations is derived in expanded form. A normal-coordinate analysis of CH3PF4, PClF4, and PCl2F3 based on recent vibrational data provides a partial description of the potential function for this symmetry. The resulting force constants show that equatorial P–F bonds are considerably ``stronger'' than axial P–F bonds in these molecules.