Carbon rings: a DFT study on geometry, aromaticity, intermolecular carbon–carbon interactions and stability. Issue 50 (4th May 2016)

• Record Type: Journal Article
• Authors: Remya, Karunakaran; Suresh, Cherumuttathu H.
• Is Part Of: RSC advances. Volume 6:Issue 50(2016)
• Page Start: 44261
• Page End: 44271
• Publication Date: 2016-05-04
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c6ra06833b
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : Strong complimentary electrostatic interactions lead to large number of C⋯C interactions in doubly antiaromatic C4 N dimers while the delocalized system of π electrons in doubly aromatic C4 N +2 dimers suggest very few intermolecular C⋯C interactions. Abstract : Non-covalent dimer formation and intermolecular bonding features of planar monocyclic carbon rings showing C4 N +2 and C4 N configurations have been studied using the meta -GGA DFT method, M06L/6-311+G(d) for N = 1–8. The C4 N +2 show cumulenic structures with equal bond lengths and C4 N form structures with clear bond length alternation. The doubly Hückel aromatic nature of C4 N +2 is revealed through two cyclic delocalized π-molecular orbitals and highly negative nucleus independent chemical shift (NICS) parameters while the doubly Hückel antiaromatic nature of C4 N is brought out through two localized π-molecular orbitals and highly positive NICS parameters. Further, the uniform electron distribution over the delocalized CC bonds in C4 N +2 and the alternate electron rich and electron deficient regions in C4 N are assessed on the basis of the critical features of the molecular electrostatic potential (MESP). The contrasting geometric, electronic and magnetic features of C4 N +2 compared to C4 N result in a drastic difference in their intermolecular bonding behaviour. The C4 N showed a much higher tendency than C4 N +2 for dimer formation as the former, in general show a 4 N number of intermolecular C⋯C interactions due to complimentary electrostatic interactions between electron rich shorter CC bonds and electron deficient longer CC bonds. In C4 N dimers, a perfect sandwich configuration is preferred to maximize the attractive complementary electrostatic interactions while in C4 N +2 dimers a shifted-parallel stacked arrangement indicated the non-complementary character of interactions arising from smooth aromatic distribution of electrons. The comparative stability of the carbon rings and unsubstituted polyynes is quantified by measuring the homodesmotic reaction energy ( E hdr ) with acetylene. The E hdr indicated significant stabilization of C4 N +2 compared to C4 N . The energy required to open up a carbon ring to the linear form is computed as E opening and this quantity is used to estimate the aromatic stabilization of C4 N +2 as well as the antiaromatic destabilization of C4 N systems.