The equilibrium constant of carbon dioxide dimerization and the lifetime of the dimer

Abstract

A new method, similar to previous methods applied to hydrogen bonding in aqueous solutions, is developed to calculate the equilibrium constant of weak dimer complexes and the lifetime of the dimer in the gas phase. In contrast to reactions involving covalent bonds, where, for example, statistical mechanics in the rigid-rotator and harmonic-oscillator approximation yields good results, no such methods are established for weak complexes. There are even problems in defining when approaching monomers form a dimer. In the new method a time correlation function is defined and sampled in molecular dynamics simulations, that shows a slow decay corresponding to real dimers and a fast decay corresponding to unstable collisions or nearby passes, yielding a distinct definition for a dimer. The results obtained for carbon dioxide dimerization are compared to results obtained by two other methods using different approximations. A possible application is to predict the dimer concentration in the atmospheres of Mars and Venus, which mainly consist of carbon dioxide.

Acknowledgement

This work is part of the project 200020-103322 of the Schweizerischer Nationalfonds zur Förderung der Wissenschaften.