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The Bergman cyclizations of the enediyne and its four N-substituted analogs [(Z)-pent-2-en-4-ynenitrile, 3-azahex-3-en-1,5-diyne, malenotrile, and 3,4-azahex-3-en-1,5-diyne] have been studied using the complete active space self-consistent field and multiconfigurational second-order perturbation theory methods in conjunction with the atomic natural orbital basis sets. The geometries and energies of the reactants, transition states, and products along both the S(0) (the ground state) and T(1) (the lowest-lying triplet state) potential energy surfaces (PESs) were calculated. The calculated geometries are in good agreement with the available experimental data. The distance between two terminal carbons in enediyne, which was considered as an important parameter governing the Bergman cyclization, was predicted to be 4.319 Å, in agreement with the experimental value of 4.321 Å. Our calculations indicate that the replacements of the terminal C atom(s) or the middle C atom(s) in the C=C bond by the N atom(s) increase or decrease the energy barrier values, respectively. There exist stable ring biradical products on the T(1) PESs for the five reactions. However, on the S(0) PESs the ring biradical products exist only for the reactions of enediyne, (Z)-pent-2-en-4-ynenitrile, and 3-azahex-3-en-1,5-diyne.

We have systematically studied Coulomb explosion of nitrogen and oxygen molecules in intense 8 and 24 fs laser pulses. In the experiment, we explicitly separated all explosion pathways through coincident measurements. The high resolution kinetic energy releases (KERs) and the exotic angular distributions of atomic ions provide direct evidence that Coulomb explosion occurs through non-Coulombic states. In the theory, we calculated dissociation potential energy curves (PECs) of nitrogen and oxygen molecules and their multicharged molecular ions using multiconfiguration second-order perturbation theory. The results indicate that Coulomb potentials are close to the accurate PECs of multicharged molecular ions only when the internuclear distance is larger than 3 Å. In comparison with the experimental observations and the theoretical calculations, we determined the internuclear distance when Coulombic explosion occurs. It is near the equilibrium distance of the neutral molecules in the case of 8 fs laser pulses and expands gradually with the increase of the charge state of the molecular ions in the case of 24 fs laser pulses.

Highest occupied molecular orbital–lowest unoccupied molecular orbital (H-L) energy gaps govern the chemical stabilities of molecules. Molecules with larger H-L gaps are more inert to chemical reactions and thus more experimentally separable. In this sense, H-L gaps play a more important role than energies of formation in determining the structures of the fullerenols that can be experimentally isolated. However, previous structural predictions for fullerenols are mainly based on energies. Recently, we proposed a rule of chemical stability for multiple addition products of fullerenes and endohedral metallofullerenes. In this study, we implement this rule into a computer program and use it to automatically determine large-energy-gap structures for C60 and C82 fullerenols. C60(OH) n (n = 12–48) and C 2v (9)-C82(OH) x (x = 14–58) fullerenols have been studied using this program. All the structures determined have H-L gaps larger than those predicted previously. Some of the structures simultaneously have even more favorable thermodynamic stabilities than the previously predicted structures. Therefore, the results we present here provide an improved description for the structures of fullerenols.