Low-lying isomers of the B9− boron cluster: The planar molecular wheel versus three-dimensional structures

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Abstract
The B9− cluster was found previously to be an unprecedented molecular wheel containing an octacoordinate planar boron with D8h symmetry in a combined photoelectron spectroscopy (PES) and theoretical study [H. J. Zhai et al., Angew. Chem., Int. Ed. 42, 6004 (2003)]. However, the PES spectra of B9− exhibit minor features that cannot be explained by the global minimum D8h structure, suggesting possible contributions from low-lying isomers at finite temperatures. Here we present Car–Parrinello molecular dynamics with simulated annealing simulations to fully explore the potential energy surface of B9− and search for low-lying isomers that may account for the minor PES features. We performed density functional theory (DFT) calculations with different exchange-correlation functionals and ab initio calculations at various levels of theory with different basis sets. Two three-dimensional low-lying isomers were found, both of Cs symmetry, 6.29 (Cs−2) and 10.23 (Cs−1)kcal∕mol higher in energy than the D8h structure at the highest CCSD(T) level of theory. Calculated detachment transitions from the Cs−2 isomer are in excellent agreement with the minor features observed in the PES spectra of B9−. The B9− cluster proves to be a challenge for most DFT methods and the calculated relative energies strongly depend on the exchange-correlation functionals, providing an excellent example for evaluating the accuracies of various DFT methods.

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