C21HZ9 (Z = -3 to +3):: a theoretical study on the redox behaviour of C3 symmetric fragment of C60

B3LYP/6-31G* calculations were done on seven C21H9Z fragments, where the charge, Z, is varied from -3 to +3; doublet states of the open shell systems and both singlet and triplet states of closed shell species are considered. Among them, only the D-3h, and C-3v forms of the singlet states of C21H93- and C21H91+, and triplet-C21H(9)(1-) correspond to the bowl-to-bowl inversion transition states and minimum energy structures, respectively. 6-31 + G* basis set, which employs a set of diffuse functions on the non.-hydrogen atoms, was employed on anionic systems. The rest encountered symmetry breaking problem, which necessitated us to go for lower symmetry structures to obtain the local minima and the bowl-to-bowl inversion transition states. The curvature, ascertained by pi-orbital axis vector angles (POAV), is lowest for the trianion and highest for the cations. The triplet states of the closed shell species are found to be competitive in energy with the singlet states when Z = -1 and + 3, whereas for Z = -3 and + 1, the triplet states lie above the corresponding singlet states over 30 kcal/mol. The inversion barriers vary from 12.2 to 40.2 kcal/mol with C21H93- and C21H92+ having the lowest and highest inversion barriers, respectively. The electron affinities and ionization energies were calculated to assess the electron receiving and donating ability of the C-3-fragment in comparison to corannulene. The curvature and redox behavior of the C21Hg9 fragment are compared to those of C-60. (C) 2004 Elsevier B.V. All rights reserved.