The opening of carbon nanotubes by the addition of oxygen to form CnO6 has been modelled using the AM I Hamiltonian and the program MOPAC 6.0. Three series of nanotubes based on C-60 were considered. The first series was based on pulling two hemispherical halves Of C60 apart along a fivefold axis and inserting additional carbon atoms between the hemispheres to form C-70, C-80, C-90 and C-100. The second series was obtained similarly by pulling C60 apart along a threefold axis to form the D-3h and D-3 isomers Of C-78 and the D-3d and D-3 isomers Of C-96. The third series was based on the extension Of C60 along a twofold axis to form C-76 and C-92. There is a wide diversity in the bonding patterns in these nanotubes. In general, the hemispherical ends resemble C60 with single bonds on pent-hex edges and double bonds on hex-hex edges, with the exception of C80 in which there is significant double bond character on the pent-hex edges. The six-membered rings on the hemispherical ends have the normal Kekule pattern of single and double bonds. In general, the distinction between single and double bonds is continued into the tube part of the structure but again there are some exceptions. For example, in C-70, D-3-C-78 and C-76 there are some six-membered rings with delocalised bonding. In C-90, D-3h-C-78 and D-3d-C-96 there are six-membered rings composed of six single bonds with six double bonds radiating out from the ring and these rings show significantly different chemical properties. Addition of oxygen occurs as near as possible to the end of the nanotube with the opening of a large hole leading into the interior of the structure. In the most favourable structures, a pair of oxygen atoms adds onto a hex-hex edge replacing the C=C double bond by two C=O ketone groups and the coalescence of two six-membered rings. Multiple additions may be of the same type leading to tris(diketone) structures with the coalescence of four six-membered rings into two different types of 18-membered rings. Alternatively, additional oxygen atoms may add onto pent-hex edges replacing the C-C single bond with a C-O-C ether linkage forming the mixed ether/ketones or the mixed ether/lactones which are generally the most stable types of structure. (C) 2002 Elsevier Science B.V. All rights reserved.