Ab initio calculations on the diacetylene dimer: HCCCC(H)C(H)CCCH

Abstract

Geometry optimizations were performed for singlet, triplet, and quintet states on the planar structures (in C2h and C2v symmetries) of the diacetylene dimer, using restricted open-shell Hartree–Fock (ROHF), unrestricted Hartree–Fock (UHF), and unrestricted hybrid density functional theory (UB3LYP) methods, with 6-31G(d) and 6-311G(d, p) basis sets. The 1Ag state of the planar van der Waals dimer is lower in energy than are any covalently bonded dimers. At our best B3LYP/6-311G(d, p) level, the most stable covalently bonded diacetylene dimer is the 3Bu state in C2h symmetry, 11 kcal mol−1 above the van der Waals dimer, followed by the 3B2 state in C2v symmetry with 13 kcal mol−1 above the van der Waals dimer. Both structures were confirmed to be local minima. The two diacetylene monomers of these structures are bridged through a single bond and they exhibit a small bend at the neighboring carbons to the bridge, trans to the hydrogens. The 1Bu and 5Ag states in C2h and the 1B2 and 5A1 states in C2v are between 39 and 43 kcal mol−1 above the van der Waals dimer. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 189–202, 1998