A first-principles study on the B5O5 (+/0) and B5O5 (-) clusters: The boron oxide analogs of C6H5 (+/0) and CH3Cl.

The concept of boronyl (BO) and the BO/H isolobal analogy build an interesting structural link between boron oxide clusters and hydrocarbons. Based upon global-minimum searches and first-principles electronic structural calculations, we present here the perfectly planar C2v B5O5 (+) (1, (1)A1), C2v B5O5 (2, (2)A1), and tetrahedral Cs B5O5 (-) (3, (1)A') clusters, which are the global minima of the systems. Structural and molecular orbital analyses indicate that C2v B5O5 (+) (1) [B3O3(BO)2 (+)] and C2v B5O5 (2) [B3O3(BO)2] feature an aromatic six-membered boroxol (B3O3) ring as the core with two equivalent boronyl terminals, similar to the recently reported boronyl boroxine D3h B6O6 [B3O3(BO)3]; whereas Cs B5O5 (-) (3) [B(BO)3(OBO)(-)] is characterized with a tetrahedral B(-) center, terminated with three BO groups and one OBO unit, similar to the previously predicted boronyl methane Td B5O4 (-) [B(BO)4 (-)]. Alternatively, the 1-3 clusters can be viewed as the boron oxide analogs of phenyl cation C6H5 (+), phenyl radical C6H5, and chloromethane CH3Cl, respectively. Chemical bonding analyses also reveal a dual three-center four-electron (3c-4e) π hyperbond in Cs B5O5 (-) (3). The infrared absorption spectra of B5O5 (+) (1), B5O5 (2), and B5O5 (-) (3) and anion photoelectron spectrum of B5O5 (-) (3) are predicted to facilitate their forthcoming experimental characterizations. The present work completes the BnOn (+/0/-) series for n = 1-6 and enriches the analogous relationship between boron oxides and hydrocarbons.