Ito Chao

Fullerene isomerism : Isolation of C2v-C78 and D3-C78

Early reports on the formation of the higher fullerenes C76, C78, C84, C90, and C94 by resistive heating of graphite stimulated theoretical calculations of possible cage structures for these all-carbon molecules. Among the five fullerene structures with isolated pentagons found for C78, a closed-shell D3h-isomer was predicted to form preferentially. Two distinct C78-isomers were formed in a ratio of ∼5:1 and could be separated by high-performance liquid chromatography. The carbon-13 nuclear magnetic resonance (NMR) spectrum of the major isomer is uniquely consistent with a C2v-structure. The NMR data also support a chiral D3-structure for the minor isomer. The isolation of specifically these two isomers of C78 provides insight into the stability of higher fullerene structures and into the mechanism for fullerene formation in general.

Solvent effects in molecular recognition

Synthetic cyclophane hosts form stable and highly structured inclusion complexes with organic molecules in aqueous solutions. The solution geometries of these complexes are determined in a conformational analysis using Monte Car10 methods. Solvation-desolvation processes are a central factor in determining the stability of apolar inclusion complexes. The tight binding of small aromatic solutes in water is entropically unfavorable and is predominantly enthalpy-driven. A large part of the favorable enthalpy term for strong complexation in water results from its specific contributions. Electron donor-acceptor interactions stabilize complexes between electron-rich cyclophane hosts and electron-deficient aromatic substrates; however, they may be masked by specific solvation effects. Computer liquid phase simulations are undertaken to evaluate at a microscopic level the origin of such solvation effects. The progress in the modeling studies is described. Apolar complexation also occurs in organic solvents. Solvents like 2,2,2-trifluoroethanol and ethylene glycol come close to water in their ability to promote apolar complexation. Binding strength decreases from water to polar protic to dipolar aprotic and to apolar solvents. Complexation strength in solvents of all polarity including water and in binary aqueous solvent mixtures is predictable according to a linear free energy relationship between the complexation free energy and the empirical solvent polarity parameter ET(30).