Five-coordinate molecules shape

When a polyatomic molecule dissociates, most vibrational modes correlate with modes of vibrational character in the products while other modes disappear, meaning that they correlate with rotation or translation of the separated fragments. As we have seen in Section 6.1.4.1 (and in Problems F and G), the transitory modes are those that determine the shape of the barrier. Taking as an example the dissociation ofC2H6 to two CH3 radicals, there are 18 vibrations ofthe parent. One vibration, the C-C stretch, becomes the reaction coordinate. Five other vibrations correlate to free rotations ofthe CH3 radicals, see Problem F. These are the torsion mode of C2H6 and the CH3 rocking modes. Twelve vibrations are conserved, meaning that they correlate to the vibrations of CH3. 

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