Structure, Stabilities, Thermodynamic Properties, and IR Spectra of Acetylene Clusters (C2H2)n=2-5

There are no clear conclusions over the structures of the acetylene clusters. In this regard, we have carried out high-level calculations for acetylene clusters (C2H2)(2-5) using dispersion-corrected density functional theory (DFT-D), Moller-Plesset second-order perturbation theory (MP2); and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] at the complete basis set limit. The lowest energy structure of the acetylene dimer has a T-shaped structure of C-2v symmetry, but it is nearly isoenergetic to the displaced stacked structure of C-2h symmetry. We find that the structure shows the quantum statistical distribution for configurations between the T-shaped and displaced stacked structures for which the average angle (vertical bar(0) over tilde vertical bar) between two acetylene molecules would be 53-78 degrees, close to the T-shaped structure. The trimer has a triangular structure of C-3h symmetry. The tetramer has two lowest energy isomers of S-4 and C-2h symmetry in zero-point energy (ZPE) uncorrected energy (Delta E-e) but one lowest energy isomer of C-2v symmetry in ZPE-corrected energy (Delta E-0). For the pentamer, the global minimum structure is C-1 symmetry with eight sets of T-type pi-H interactions and a set of pi-pi interactions. Our high-level ab initio calculations are consistent with available experimental data.