Conformations of [10]annulene: More bad news for density functional theoryand second-order perturbation theory

The molecular structures and relative energies of several of the lowest-ene rgy conformations of [10]annulene (C10H10) have been investigated using the Hartree-Fock (HF) method, density functional theory (DFT), second-order Mo ller-Plesset perturbation theory (MP2), and (for the first time) coupled cl uster singles and doubles with a perturbative inclusion of connected triple excitations [CCSD(T)]. For some years the HF method has been known to inco rrectly favor bond-length-alternating structures for [10]annulene, and stan dard forms of DFT are now seen to incorrectly favor aromatic structures. Fo r the naphthalene-like conformation, the B3LYP method is shown to require a large basis set before the geometry becomes properly bond-localized, i.e., similar to the predictions of CCSD(T) using even a modest basis set. With a basis set of 170 functions, B3LYP and BP86 predict that the aromatic hear t-shaped conformation is 9.11 and 12.11 kcal mol(-1), respectively, lower t han the bond-alternating twist form, while with the same basis set CCSD(T) places the heart-shaped conformation 6.29 kcal mol(-1) above the twist. Fur ther large-scale CCSD(T) computations involving 340 basis functions predict that the twist conformation is lowest in energy, and the naphthalene-like and heart-shaped conformations lie higher than the twist by 1.40 and 4.24 k cal mol(-1), respectively. Implications of the computed structures and ener getics for the interpretation of previous experiments are discussed.