Weak interactions among simple triple bonded hydrocarbons

The primary focus of this work has been the investigation and modeling of the weak interactions occurring between triple bonded hydrocarbons. Ab initio calculations were carried out for critical point regions of the acetylene dimer potential surface as well as for dimers of acetylene and a polyyne chain with 2 to 5 triple bonds. The results for the acetylene dimer were used to guide the development of a model interaction potential using MMC. Using a diffusion quantum Monte Carlo (DQMC) treatment of the ground vibrational state dynamics, the parameter selections were refined such that the calculated rotational constants matched the experimentally determined values. Comparison of features of the resulting surfaces showed that similar surfaces could be obtained from the different ab initio starting points. Next, the acetylene-acetylene interaction potential was applied to small and large clusters of acetylene molecules. Using DQMC analysis of the ground vibrational dynamics, calculated rotational constants for the trimer and tetramer compared well with experimental values which give a very good assessment for the use of the interaction potential with more than two interacting molecules. Calculations performed on clusters of up to 391 molecules reveal trends in the energetics of aggregation and in the preferred arrangement of acetylenes. This arrangement is based upon the ability of the T-shape pairings to offer stability through favorable quadrupole-quadrupole interactions. Further application and refinement of the potential was accomplished for acetylene-polyyne dimers. The slice of the intermolecular potential surface for sliding an acetylene molecule along the polyyne chain in a T-shaped orientation was mapped and showed shallow corrugation over a large region