Abstract:
Non-covalent dimer formation and intermolecular bonding features of planar monocyclic carbon rings
showing C4N+2 and C4N configurations have been studied using the meta-GGA DFT method, M06L/6-
311+G(d) for N ¼ 1–8. The C4N+2 show cumulenic structures with equal bond lengths and C4N form
structures with clear bond length alternation. The doubly Huckel aromatic nature of C ¨ 4N+2 is revealed
through two cyclic delocalized p-molecular orbitals and highly negative nucleus independent chemical
shift (NICS) parameters while the doubly Huckel antiaromatic nature of C ¨ 4N is brought out through two
localized p-molecular orbitals and highly positive NICS parameters. Further, the uniform electron
distribution over the delocalized CC bonds in C4N+2 and the alternate electron rich and electron
deficient regions in C4N are assessed on the basis of the critical features of the molecular electrostatic
potential (MESP). The contrasting geometric, electronic and magnetic features of C4N+2 compared to
C4N result in a drastic difference in their intermolecular bonding behaviour. The C4N showed a much
higher tendency than C4N+2 for dimer formation as the former, in general show a 4N number of
intermolecular C/C interactions due to complimentary electrostatic interactions between electron rich
shorter CC bonds and electron deficient longer CC bonds. In C4N dimers, a perfect sandwich
configuration is preferred to maximize the attractive complementary electrostatic interactions while in
C4N+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
(Ehdr) with acetylene. The Ehdr indicated significant stabilization of C4N+2 compared to C4N. The energy
required to open up a carbon ring to the linear form is computed as Eopening and this quantity is used to
estimate the aromatic stabilization of C4N+2 as well as the antiaromatic destabilization of C4N systems.
