Ab initio and TDDFT study of the structural and spectroscopic properties of buckybowls: Corannulene and two methylcorannulenes

Optimization of the C60 fullerene fragment, corannulene, by HF and DFT methods at 6-31++G∗∗ and 6-311+G(2d,p) levels confirms its curved (bowl shaped) structure. The calculated charge density distribution and bond orders support the assumption [W.E. Barth, R.G. Lawton, J. Am. Chem. Soc. 88 (1966) 380] that the corannulene structure is a polar resonance hybrid involving two concentric rings with (4n + 2)π-electrons. Geometry optimization also reveals that methylation at peri positions causes flattening of the bowl, in conformity with a previous general analysis [Seiders et al., J. Am. Chem. Soc. 123 (2001) 517] and the dipole moment vector along the C5-axis of corannulene is more and more tilted towards the methyl groups with increasing extent of methylation. The UV-vis spectra of corannulene, methyl- and 2,3-dimethyl corannulene in CHCl3 were calculated by time dependent density functional theory under the polarizable continuum model and the results are in agreement with the experimentally observed red shift on progressive methyl substitution. The theoretically calculated IR and NMR spectra of corannulene and its two methyl derivatives agree well with reported experimental data. 13C NMR shift calculation explicitly shows the presence of three sets of C-atoms in corannulene. The CH vibrational modes have been classified group theoretically and their calculated frequencies agree well with experimental data.