Ab initio studies of acetylene tetramer and pentamer

Abstract

Ab initio studies of acetylene tetramer, (C₂H₂)₄, show that the lowest-energy conformation is a cyclic structure, of C_4h symmetry. The potential-energy surface is very flat in the direction of out-of-plane displacements, indicating that it would undergo large-amplitude flexes to the experimentally observed S₄ symmetry ring-puckered structure. Interconversion tunnelling between permutationally distinct configurations is shown to be of no significance in the full description of this cluster. The electrostatic model predicts two structures as possible equilibrium conformations of acetylene pentamer. Ab initio studies also show that out-of-plane ring-puckerings are important in both of these. With no further computation, likely favourable structures of higher-order planar acetylenic clusters are predicted. We assert that single-ring cyclic structures become progressively less favourable for clusters larger than acetylene tetramer.