EFFECTS OF ELECTRON CORRELATION ON THE STRUCTURES OF 1-H-1-BH2-PHOSPHIRENE

In search of novel geometrical and electronic structures for phosphorus-containing heterocyclic compounds, ah initio geometry optimization and frequency calculations at the HF/3-21G(d on P) level were carried out on some substituted 1-H-phosphirenes. A number of different substituent groups at the C- and/or P-sites have been considered, and it was found that with a BH2 group at the P-site, the barrier to planarity was lowered considerably. Further HF and correlated calculations were carried out with the standard 6-31G* basis set on 1-BH2-1-H-phosphirene. The different correlation methods employed include the CASSCF, CISD, and MP2 methods. Two different conformers of C-s symmetry, with the BH2 group in, and perpendicular to, the symmetry planes have been obtained, and they were both confirmed to be true minima on the MP2/6-31G* hypersurface by analytical second-derivative calculations. The lower energy conformer is the one with the BH2 group perpendicular to the mirror plane of the molecule and has a pyramidal PCCB skeletal structure (the angle between the BP bond and the three-membered ring is ca. 60 degrees) at the MP2 level. CASSCF(8,8) geometry optimization for this conformer however, gave a structure similar to that obtained at the HF level, with the angle between the BP bond and the ring being ca. 1000. These observations suggest that, apart from the B-P (C=C) bonding HOMO being an obvious stabilizing factor of this uniquely bonded structure, dynamic electron correlation is the major stabilization for the pyramidal 1-BH2-1-H-phosphirene. The energy difference between the two structures is rather small. The use of a larger basis set stabilizes the pyramidal structure (relative to the opened form), and this structure remains the most stable 1-BH2-1H-phosphirene at the highest level attempted here (CCSD(T)/6-311G**//MP2/6-311G**). If the BH2 group is placed in the mirror plane and on top of the PCC ring, having an initial pyramidal structure, geometry optimization at both the HF and MP2 levels leads to a heterobicyclobutene structure, in which the PCC and BCC rings share a CC single bond. In order to describe this system adequately, a multiconfigurational wave function was needed, as shown by the large changes in both geometry and planarization barrier when the CASSCF method is used. Two isomers, with the PH and BH in cis and trans positions, of this double three-membered-ring system, have been studied, and each of them is a true minimum at both the HF and MP2 levels, with the cis conformer having the lowest computed energies among all the 1-BH2-1-H-phosphirene isomers considered in this study at the HF, MP2, and CASSCF levels.