Knowledge Bank

There are four different trimeric van der Waals complexes containing carbon dioxide and acetylene: $(CO2)3,(HCCH)3,(CO2)2HCCH$, and $(HCCH)2CO2$. Infrared spectra of $(CO2)3$ and $(HCCH)3$ have been studied and these molecules exhibit planar structures having three fold symmetry axes. The two mixed trimers have not been observed. Potential functions for this series of complexes have been constructed using a model containing repulsion, dispersion, and electrostatic interactions. Each of these terms are distributed among the atoms of the individual molecules. No adjustable parameters are used in the potential functions discussed here. The results for acetylene trimer are in excellent agreement with experiment. The $CO2$ trimer potential contains two stable configurations: one in agreement with the experimental structure, and a second nonplanar configuration. Two different high barrier inversion paths, that permit internal rotation of the monomers within the complex, are found in both $(HCCH)3$ and $(CO3)3$. Numerous local minima are observed in the potential functions for both $(CO2)2HCCH$ and $(HCCH)2CO2$. These configurations represent both planar and nonplanar geometrics. These results suggest that the absence of experimental data for the mixed trimers arises in part from the effects of large amplitude motions sampling multiple configurations. All of the local minimum structures can be understood by considering the trimers as sums of dimeric species.