Anharmonic Effects in Unimolecular Rate Theory. Vibrations and Collisions of Simple Polyatomic Systems

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Abstract
The equations of motion for the following systems were integrated numerically: (1) a four-atom linear molecule with Morse function bond stretching potentials; (2) a pair of Morse function diatomic molecules undergoing collisional interaction via a Lennard-Jones 6–12 potential; (3) a Morse function linear triatomic molecule interacting with an atom via a 6–12 potential; and (4) a Morse function diatomic molecule colliding with an atom via a 6–12 potential. A commonly used approximate method of solution is also used to analyze Case 4. In all cases the systems are constrained to colinear motion. Results are presented graphically. Our conclusions are as follows: (1) The behavior of the four-atom anharmonic molecule shows features quite different from those shown by its harmonic counterpart. (2) Collisional interaction via 6–12 potentials does not lead to efficient transfer of vibrational energy from one molecule to another or to efficient interconversion of translational and vibrational energies. (3) A commonly used assumption in the calculation of the efficiency of collisional vibrational activation of diatomic molecules breaks down badly at high energies.