The Silicon Hydride Clusters Si₃H<i>_n</i> (<i>n</i> ≤ 8) and Their Anions:  Structures, Thermochemistry, and Electron Affinities

Abstract

The molecular structures, electron affinities, and dissociation energies of the Si 3 H n /Si 3 H n - ( n ≤ 8) species have been examined using five hybrid and pure density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. These methods have been carefully calibrated ( Chem. Rev. 2002, 102, 231). The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA ad ), the vertical electron affinity (EA vert ), and the vertical detachment energy (VDE). The first Si−H dissociation energies D e (Si 3 H n →Si 3 H n - 1 +H) for the neutral Si 3 H n and D e (Si 3 H n - →Si 3 H n - 1 - +H) for the anionic Si 3 H n - species have also been reported. In the prediction of bond lengths, the BHLYP predicts the most reliable Si−Si bond lengths and the B3LYP predicts the most reliable Si−H bond lengths. The most reliable EA ad, obtained at the B3LYP and BPW91 levels of theory, are 2.34 or 2.32 eV (Si 3 ), 2.56 eV (Si 3 H), 1.73 or 1.74 eV (Si 3 H 2 ), 2.46 or 2.45 eV (Si 3 H 3 ), 1.95 or 1.93 eV (Si 3 H 4 ), 2.24 or 2.23 eV (Si 3 H 5 ), 1.41 or 1.30 eV (Si 3 H 6 ), and 2.14 eV (Si 3 H 7 ). For Si 3 H 8, there are no reliable EA ad but there are reliable VDE. The values of VDE for Si 3 H 8 are 1.03 eV (B3LYP) or 1.10 eV (BPW91). The first dissociation energies (Si 3 H n →Si 3 H n - 1 +H) predicted by all of these methods are 2.65∼2.74 eV (Si 3 H), 2.82∼3.12 eV (Si 3 H 2 ), 2.13∼2.23 eV (Si 3 H 3 ), 2.92∼3.08 eV (Si 3 H 4 ), 2.63∼2.95 eV (Si 3 H 5 ), 3.29∼3.53 eV (Si 3 H 6 ), 2.19∼2.54 eV (Si 3 H 7 ), and 3.45∼3.62 eV (Si 3 H 8 ). For anion clusters (Si 3 H n - → Si 3 H n - 1 - +H), the dissociation energies predicted are 2.89∼2.95 eV (Si 3 H - ), 2.01∼2.27 eV (Si 3 H 2 - ), 2.83∼2.96 eV (Si 3 H 3 - ), 2.37∼2.68 eV (Si 3 H 4 - ), 2.96∼3.10 eV (Si 3 H 5 - ), 2.36∼2.74 eV (Si 3 H 6 - ), 3.03∼3.16 eV (Si 3 H 7 - ), and 1.44∼1.54 eV (Si 3 H 8 - ).