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 CC 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 (C[triple bond, length as m-dash]C)(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 (Eint); -1.07 kcal mol(-1) for the acetylene dimer to -45.83 kcal mol(-1) for the 50yne dimer. The magnitude of Eint 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 CC 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 CC interactions and lower values of Eint. Furthermore, strong cooperativity in the CC bond strength in tetramers is observed as the interaction energy per monomer (Em) 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.

A theoretical investigation of water–solute interactions: from facial parallel to guest–host structures

“…The (H 2 O) 20 cuboidal structure was confirmed at the MP2 perturbative level of theory [49], while the (H 2 O) 24 24 endohedral complex.…”

“…In summary, the dispersion corrected range-separated-hybrid functional LC-wPBE-GD3BJ combined with a moderate size basis set (6-311++G(d,p)) was found to suitably describe the geometric, energetic and spectroscopic (dipole moments and rotational constants 20,24 complexes are stable with respect to the lowest lying D-and T-cage added to free BPL molecule. Nevertheless, it is interesting to note that these endohedral compounds lie around ten kJmol -1 above the sum of free partners when we take into account the global minimum of water cluster of cuboid structure.…”

“…The interaction of the solute with two larger water clusters is further investigated. The exohedral and endohedral BPL:(H 2 O) 20 isomers are close in energy with each other, whereas the formation of an inclusion complex is energetically more favored than the facial interaction in the case of the BPL:(H 2 O) 24 cluster. The topological analysis suggests that the substantial energetic stability is due to interactions between the solute and almost all oxygen atoms of the water cage.…”

“…First of all, we remind the reader that the global minimum reported in the literature for both (H 2 O) 20 and (H 2 O) 24 cluster are cuboidal [19,[48][49][50][51][52][53] in which the insertion sites are too small to accept a guest polyatomic molecule without opening the water network. The (H 2 O) 20 cuboidal structure was confirmed at the MP2 perturbative level of theory [49], while the (H 2 O) 24 24 endohedral complex.…”

“…For instance it was shown at a molecular level that methane is efficiently encapsulated in a (H 2 O) 20 cage, whereas larger (H 2 O) 24,28 cages lead to more stable complexes when SF 6 is the guest molecule [15]. Smaller guest species such as rare gases, diatomic or triatomic molecules may be efficiently trapped in the (H 2 O) 20 cage with the concomitant formation of multiple water-solute interactions [16], characterized by bond critical points (BCPs) in the Quantum Theory of Atoms in Molecules (QTAIM) approach [17].…”