Thermochemistry and Electronic Structure of Small Boron Clusters (Bn, n = 5−13) and Their Anions

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Abstract
Thermochemical parameters of a set of small-sized neutral (Bn) and anionic (Bn−) boron clusters, with n = 5−13, were determined using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, and Q) basis sets extrapolated to the complete basis set limit (CBS) plus addition corrections and/or G3B3 calculations. Enthalpies of formation, adiabatic electron affinities (EA), vertical (VDE), and adiabatic (ADE) detachment energies were evaluated. Our calculated EAs are in good agreement with recent experiments (values in eV): B5 (CBS, 2.29; G3B3, 2.48; exptl., 2.33 ± 0.02), B6 (CBS, 2.59; G3B3, 3.23; exptl., 3.01 ± 0.04), B7 (CBS, 2.62; G3B3, 2.67; exptl., 2.55 ± 0.05), B8 (CBS, 3.02; G3B3, 3.11; exptl., 3.02 ± 0.02), B9 (G3B3, 3.03; exptl., 3.39 ± 0.06), B10 (G3B3, 2.85; exptl., 2.88 ± 0.09), B11 (G3B4, 3.48;, exptl., 3.43 ± 0.01), B12 (G3B3, 2.33; exptl., 2.21 ± 0.04), and B13 (G3B3, 3.62; exptl., 3.78 ± 0.02). The difference between the calculated adiabatic electron affinity and the adiabatic detachment energy for B6 is due to the fact that the geometry of the anion is not that of the ground-state neutral. The calculated adiabatic detachment energies to the 3Au, C2h and 1Ag, D2h excited states of B6, which have geometries similar to the 1Ag, D2h state of B6−, are 2.93 and 3.06 eV, in excellent agreement with experiment. The VDEs were also well reproduced by the calculations. Partitioning of the electron localization functions into π and σ components allows probing of the partial and local delocalization in global nonaromatic systems. The larger clusters appear to exhibit multiple aromaticity. The binding energies per atom vary in a parallel manner for both neutral and anionic series and approach the experimental value for the heat of atomization of B. The resonance energies and the normalized resonance energies are convenient indices to quantify the stabilization of a cluster of elements.

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