COMPARATIVE AB-INITIO AND HYBRID DFT STUDIES RELEVANT TO AN EXPERIMENTAL INVESTIGATION OF NEUTRAL AND CATIONIC [SI, P, H-2] ISOMERS

The neutral and the lowest cationic (singlet and triplet) potential-en ergy surfaces of [Si, P, H-2] 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 sing let cation a doubly bridged P(PI)(2)Si+ structure has been identified as the most stable isomer, with the non-bridged H2SiP+ species being s ignificantly less stable (Delta E = 31.7 kcal mol(-1) at G2). AS far a s the comparison of the two quantum-chemical methods (i.e. G2 and B3LY P) is concerned, the calculated relative energies Delta E are quite cl ose to each other, with deviations smaller than 4 kcal mol(-1). Howeve r, while the non-bridged SiPH2+ ((1)A') represents a minimum at the G2 level, it could not be located using the hybrid DFT method. The compu tationally predicted singlet and triplet [Si, P, H-2](+) potential-ene rgy surfaces provide insight into the course of the ion/molecule react ions of Si+. with phosphine and P+ with silane, respectively, which we re examined experimentally. Thus, the reaction of the doublet species Si+. with phosphine allows access to the nonclassical, bridged P(H)(2) Si+ structure, while in the reaction of the P+ triplet cation with sil ane, no evidence for Si-P bond formation is obtained.