Comparative Ab initio and Hybrid DFT Studies Relevant to an Experimental Investigation of Neutral and Cationic [Si, P, H2] Isomers

The neutral and the lowest cationic (singlet and triplet) potential-energy surfaces of [Si, P, H2] have been explored by means of ab initio MO calculations at the G2 level of theory as well as the hybrid DFT (B3LYP/6-311G**) method. Contrary to the neutral and triplet surfaces, where the H2SiP+/0 isomers represent the global minima, for the singlet cation a doubly bridged P(H)2Si+ structure has been identified as the most stable isomer, with the non-bridged H2SiP+ species being significantly less stable (ΔE = 31.7 kcal mol−1 at G2). As far as the comparison of the two quantum-chemical methods (i.e. G2 and B3LYP) is concerned, the calculated relative energies ΔE are quite close to each other, with deviations smaller than 4 kcal mol−1. However, while the non-bridged SiPH2+ (1A′) represents a minimum at the G2 level, it could not be located using the hybrid DFT method. The computationally predicted singlet and triplet [Si, P, H2]+ potential-energy surfaces provide insight into the course of the ion/molecule reactions of Si+• with phosphine and P+ with silane, respectively, which were examined experimentally. Thus, the reaction of the doublet species Si+• with phosphine allows access to the non-classical, bridged P(H)2Si+ structure, while in the reaction of the P+ triplet cation with silane, no evidence for Si–P bond formation is obtained.