18 and 12 – Member carbon rings (cyclo[n]carbons) – A density functional study

[Display omitted] •A new basic and intriguing theoretical study based on experimental stabilization of 18 atom carbon ring.•18 and 12-member carbon rings are computationally stabilized on graphene sheet.•Bond lengths are equal in both carbon rings and bonds are made of double bonds.•Bonds are covalent and there is no sp-hybridization.•Carbon acquires an electronic configuration 1 s2 2 s4, resonating with 1 s2 2 s2 2p2. Multi-member rings composed of 18 and 12 carbon atoms are designed and stabilised on graphene, the one atom thick carbon substrate, computationally using density functional theory. The resultant structures are intriguing. Bond lengths are found to be equal in both the carbon rings and the atoms are connected through double bonds. The 18-member carbon ring is not circular in shape, whereas the 12-member ring has the circular configuration. The system of 18-member carbon ring on graphene is metallic. Our studies have shown that a weak but definite bonding between the ring structures and the graphene substrate decides the shape of the new carbon allotrope.