The role of heteroatom substitution in the rigidity and curvature of buckybowls. A theoretical study

Ab initio (Hartree Fock), hybrid density functional (B3LYP), and semiempiri cal SCF (MNDO and AM1) calculations on sumanene (2), trioxa-sumanene (3) an d trithia-sumanene (4) show that the C-3v-bowl structure is a minimum in al l cases, but show dramatic variations in bowl depths and inversion barriers . Calculations on monosubstituted corannulenes C19XH10 (X = N+, B-, P+ and Si) at various levels predict that isoelectronic substituents possessing la rge atomic size increase the bowl-to-bowl inversion barrier at the hub posi tion and decrease it at the rim position. Strain is a guiding factor, which accounts for the relative stability of positional isomers, curvature and b owl rigidity. The most stable positional isomer for a given substituent sho ws the minimum bowl-to-bowl inversion barrier in all cases. Calculations ar e performed on monosubstituted sumanenes derived by replacing skeletal C by isoelectronic atoms on sumanene (2), C20XH12 for X = N+ and Si. The genera l strategy of substituting larger atoms at rim positions flattens the bowl, and at the hub position it makes the bowl deeper. The strategy seems to wo rk well. HF/3-21G and B3LYP/6-31G* computations are in very good agreement with each other, both qualitatively and quantitatively, and the central res ults are reproducible even at semiempirical levels. The performance of MNDO is consistently better than AM1 and becomes the method of choice when ab i nitio and DFT methods are not practical. Homodesmic equations, used to asce rtain the thermodynamic stabilities of the monosubstitutions on corannulene s and sumanenes, show that substitution at appropriate sites imparts stabil ity to the buckybowl framework. Linear correlation is obtained between the curvature, as estimated by the pyramidalization angle (Phi), and the invers ion barrier. It is shown that bowl rigidity, curvature and the relative sta bilities of positional isomers are controlled by the strain energy build up , which depends on the size of the substituent and the site of substitution .