Lindemann-Christiansen theory

Note The Lindemann mechanism was also suggested independently by Christiansen. Hence, it is also sometimes referred to as the Lindemann-Christiansen mechanism. The theory of unimolecular reactions was further developed by Hinshelwood and refined by Rice, Rampsberger, Kassel and Marcus. 

Although the theory does need to be improved in a number of details before it can provide a quantitative description of experiment, the observation of fall-off from first order at high pressures to second order at low pressures is correctly explained by the Lindemann-Christiansen mechanism, and modem theories of unimolecular reactions are based on this mechanism. 

Perrin s argument that the very nature of a unimolecular reaction demands independence of collisions, and therefore dependence on radiation, is adequately met both by the theory of Lindemann and by that of Christiansen and Kramers. Both these theories have the essential element in common that the distribution of energy among the molecules is not appreciably disturbed by the chemical transformation of the activated molecules thus the rate of reaction is proportional simply to the number of activated molecules and therefore to the total number of molecules, sinc in statistical equilibrium the activated molecules are a constant fraction of the whole. Thus the radiation theory is not necessary to explain the existence of reactions which are unimolecular over a wide range of pressures. 

This problem was resolved in 1922 when Lindemann and Christiansen proposed their hypothesis of time lags, and this mechanistic framework has been used in all the more sophisticated unimolecular theories. It is also common to the theoretical framework of bimolecular and termolecular reactions. The crucial argument is that molecules which are activated and have acquired the necessary critical minimum energy do not have to react immediately they receive this energy by collision. There is sufficient time after the final activating collision for the molecule to lose its critical energy by being deactivated in another collision, or to react in a unimolecular step. 

C6H5CH21+ and Cl. In fact this general idea of other collisions is part of a much more detailed theory first put forward in the doctoral thesis of J. A, Christiansen in 1921, but later attributed to a more senior worker, F. A. Lindemann, in 1922. 

In his seminal work (as it is frequently called), Kramers treated the escape over a potential barrier by a particle undergoing Brownian motion, i.e. thermal noise-assisted escape [1]. Hence, his focus was on the effect of the medium - solvent or bath gas - on the solute reaction rate. While much of the physical chemistry community was at the time focused on the rate of reaction of an isolated molecule -- and would remain so occupied for many years to come, Kramers work was not completed in a vacuum. Indeed, Lindemann, Rice and Ramsperger, Kassel, Slater, Christiansen, and others had already published their collision-rate-based theories of the role of the bath gas in promoting chemical reactions in low-density gases [3, 4]. Thus, one must ask,... 

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