On the electronic structure of the cations, radicals and anions of H 2SiP, H 2PSi and related compounds

Abstract

The structure of H 2SiP + and of H 2PSi + are investigated with ab initio calculations at a double-Λ level. The former structure adopts preferentially a triplet ground state while the latter prefers a single state with C s symmetry. A bridged structure, as is known for H 2Si 2 is only observed at MP2 electron correlation level as an energy minimum. At QCISD level of optimization a classical singlet with a slightly pyramidalized phosphorus atom is more stable than the asymmetrically bridged structure. The electron affinities of NCH 2, PCH 2, NSiH 2 and PSiH 2 are calculated for the cations and doublets. Anion and cation stabilities are also estimated by group transfer reactions. In reference to the cations, for NSiH 2 and PSiH 2 the anions are stabilized via hyperconjugation with concomitant shortening of the NSi and PSi bonds. The formation of lithium structures to rings and cubes is studied for the anions under consideration. The essential differences for coordination of lithium to nitrogen or phosphorus are outlined. The energies for ring (cube) formation do not correlate with the anion stabilities in the gas phase. The relative stabilities are determined rather by coordination of lithium to (the more electronegative) nitrogen versus coordination to (the less electronegative) phosphorus.