The carbon dioxide dimer

Ab initio calculations with a large basis set (5s 4p 2d), including correlation by second-order Møller-Plesset perburbation theory (MP2), are performed for the dimer of carbon dioxide. The energies, structures and IR frequencies are all calculated at this level. It is found that the T-shaped conformation is a transition state between two slipped parallel dimers of such low energy that even at relatively low temperatures a planar geared motion is possible. (Similar behaviour has been found previously for the acetylene dimer.) The experimental data available are in excellent agreement with our results.