Structural Increment System for 6-Vertex nido-Phosphacarboranes

Abstract

Boron hydride clusters can incorporate a variety of heteroatoms in their clusters. These compounds are called heteroboranes. Qualitative rules for the placement of heteroatoms in heteroaborane clusters provide a good general picture of heteroatom placement in closo-clusters. However, for the nido-heteroborane clusters, quantitative rules are needed to accurately illustrate the relative energies of heteroborane isomers. These quantitative rules are referred to as structural increment system. In this thesis, a structural increment system was established for a set of sixty three isomers of 6- vertex nido-phosphacarboranes. The 6-vertex nido-phosphacarborane isomers were constructed by substituting the phosphorus and carbon heteroatom with the boron atoms in the 6-vertex nidoborane cluster. All constructed isomers were optimized using density functional theory (RB3LYP/6-31+G*) calculations. For all isomers, the disfavoring structural features were determined that destabilize the 6-vertex nido-phosphacarborane clusters. The amount of destabilization due to each structural feature was quantified. A relative stability order (Einc_ret) for all isomeric 6-vertex nido-phosphacarboranes was determined based on these disfavoring structural features. This relative stability order obtained from the structural increment system quite accurately reproduced the relative stability order obtained from the DFT calculations (ЕвзІYР). А total 17 structural features were identified in optimized isomers of 6-vertex nidophosphaboranes, which can reproduce the DFT-computed relative stability order of 63 isomers. Comparison of Einc_rel and the ERB3LYP of all sixty three structures is well in agreement of ±5.0 kcal mol¹, which is a requirement of the structural increment approach.