Non-covalent intermolecular carbon–carbon interactions in polyynes

Abstract

Polyynes, the smaller analogues of one dimensional infinite chain carbon allotrope carbyne, have been studied for the type and strength of the intermolecular interactions in their dimer and tetramer complexes using density functional theory. The nature of end group functionalities and the chain length of the polyynes are varied to assess their role in modulating the non-covalent interaction energy. As seen in molecular electrostatic potential analysis, all the polyyne complexes showed a multitude of non-covalent C⋯C interactions, resulting from complementary electrostatic interactions between relatively electron rich formal triple bond region of one monomer and the electron deficient formal single bond region of the other monomer. This type of paired (CC)⋯(C–C) bonding interaction, also characterized using quantum theory of atoms-in-molecules, increases with increase in the monomer chain length leading to substantial increase in interaction energy (E_int); −1.07 kcal mol⁻¹ for the acetylene dimer to −45.83 kcal mol⁻¹ for the 50yne dimer. The magnitude of E_int increases with substitutions at end positions of the polyyne and this effect persists even up to 50 triple bonds, the largest chain length analyzed in this paper. The role of C⋯C interactions in stabilizing the polyyne dimers is also shown by sliding one monomer in a dimer over the other, which resulted in multiple minima with a reduced number of C⋯C interactions and lower values of E_int. Furthermore, strong cooperativity in the C⋯C bond strength in tetramers is observed as the interaction energy per monomer (E_m) of the polyyne is 2.5–2.8 times higher compared to that of the dimer in a test set of four tetramers. The huge gain in energy observed in large polyyene dimers and tetramers predicts the formation of polyyne bundles which may find use in the design of new functional molecular materials.