Boronyls as key structural units in boron oxide clusters: B(BO)2- and B(BO)3-.

BOis isoelectronic with CN-.1 However, in comparison to CN-, which is an important ligand in inorganic and biomolecules, the chemistry of BOis relatively unknown. The electron affinity (EA) of BO (2.51 eV)2,3 is much smaller than that of CN (3.86 eV),4 which may explain the fact that CNis a stable anion in solution, but BOis not. However, the electronic structure and bond strength of BOare similar to those of CN-, suggesting that it may be a robust chemical unit and can retain its structural integrity in chemical compounds. In a recent study, we indeed found that BO behaves like a monovalent structural unit in its bonding to Au in AunBO (n ) 1-3) clusters.5 Theoretical calculations also suggested that carbon boronyls (CBO)n (n ) 3-7) are stable species on the potential-energy surfaces.6 Here we report a photoelectron spectroscopy (PES) and theoretical study on two boron oxide clusters, B3O2 and B4O3, which are shown to possess a D∞h (Σg) linear and D3h (A2′′) triangular structure, respectively, and can be viewed as two and three boronyl groups bonded to a single B atom. Bulk boron oxide (B2O3) is a highly stable glassy material, and the combustion of boron and boranes has received persistent interest over the past 50 years, primarily aimed at the development of energetic boron-based propellants.7 However, the electronic and structural properties of boron oxide clusters remain poorly understood.8 One of our research goals is to remedy this deficiency. Following our previous work on BOand BO2, in this Communication we focus on B3O2 and B4O3. The experiment was done using a magnetic-bottle PES apparatus equipped with a laser vaporization cluster source (see Supporting Information). A 10B-enriched target was used with a He carrier gas seeded with 0.01% O2, producing BxOy clusters with a variety of compositions. B3O2 and B4O3 were mass-selected for photodetachment. Figure 1 shows the PES spectra at several photon energies. The spectra for both species are similar and appear unusually simple, each showing only one vibrationally resolved band even at the highest detachment photon energy (6.424 eV). The 0-0 transition in each species defines an accurate adiabatic detachment energy (ADE): 2.94 eV for B3O2 and 3.64 eV for B4O3 (Table 1), which also represent the EA’s of the neutral clusters. The resolved vibrational frequencies are also similar: 1950 cm-1 for B3O2 and 1980 cm-1 for B4O3. The 355 nm spectrum of B3O2 (insert of Figure 1a) reveals a partially resolved lowfrequency vibration. We did calculations at the B3LYP level with the augmented Dunning’s all-electron basis set (aug-cc-pVTZ) (see Supporting Information). Our structural searches for B3O2 and B4O3 started from the well-characterized bare D3h B3 and D2h B4. O atoms were attached terminally to the bare clusters, but optimization led to a linear B(BO)2 (D∞h,Σg) and a triangular B(BO)3 (D3h, A2′′) (Figure 2). Similar structures were obtained for the corresponding neutrals with very little structural change. The calculated total energies and vibrational frequencies for the two B(BO)n species and their neutrals are given in Table S1. A variety of alternative structures were also optimized and summarized in Figures S1S4. However, all the other structures are substantially higher in energy, suggesting that the boronyl-containing B(BO)2 (D∞h) and B(BO)3 (D3h) are extremely stable clusters. To compare with the PES results, we further calculated the ground and excited-state electron detachment energies. The calculated ADE and VDE values from the D∞h B(BO)2 and D3h B(BO)3 are in excellent agreement with the experimental data (Table 1). The first excited-state is predicted to be beyond 6.4 eV for both species (Table S2), consistent with the simple PES pattern (Figure 1). The calculated BO stretching frequencies for the neutral B(BO)2 and B(BO)3 are also in excellent agreement with the experimental † Washington State University. ‡ Pacific Northwest National Laboratory. § Xinzhou Teachers’ University. Figure 1. Photoelectron spectra of B3O2 at (a) 266 nm (4.661 eV) and (b) 193 nm (6.424 eV) and B4O3 at (c) 266 and (d) 193 nm. Inset in panel a shows the 355 nm (3.496 eV) spectrum of B3O2. Vertical bars represent the resolved vibrational structures.