Activation energy, apparent collision theory

But a new difficulty arose from the apparent insufficiency of the collisions to provide energy at the required absolute rate. The way out was provided by the now very natural idea that multiple internal degrees of freedom can be drawn upon to contribute to the activation process. The theory of reaction rates now becomes correlated with the study of normal modes of vibration of complex molecules. Fresh questions about the dependence of transformation probability on energy excess or energy distribution arise and the subject enters its specialized phase— where there are still some unsolved problems. 

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