ABINITIO STUDY OF LOW-LYING ELECTRONIC STATES OF XP (X=LI-B,NA-SI)

The equilibrium geometries and harmonic vibrational frequencies of the low-lying electronic states of several as yet unobserved species (LiP, BeP, BP, NaP, MgP, AlP, and SiP) were calculated at the SCF/6-31G*, MP2(full)/6-31G*, and MP2(full)/6-311+G* levels of theory. For each of these species, equilibrium structures of the ground state and lowest excited states were also optimized at the QCISD(T)/6-311+G(2df)level. For all but SiP, the bonding is found to involve high-spin ground electronic states (3SIGMA- for Lip, 4SIGMA- for BeP, 3PI for BP, 3SIGMA- for NaP, 4SIGMA- for MgP, 3SIGMA- for AlP) in which two electrons occupy pi orbitals lying primarily on the P atom (expect for BP, where three electrons are in pi orbitals), and the number of valence sigma electrons ranges from four to six. In each case, the number of unpaired electrons is larger than that anticipated based on covalent P-X sigma-bond formation. These unusual state orderings are consistent with our earlier findings and with the bonding picture we used to explain them. SiP has two lowest electronic states 2SIGMA+ and 2PI with close energies. The bond dissociation energies were also computed and found to be 37.7 (LiP), 23.6 (BeP), 70.5 (BP), 27.4 (NaP), 12.2 (MgP), 47.2 (AlP), and 76.3 kcal/mol (SiP), with all data referring to the QCISD(T)/6-311+G(2df)+ZPE level. The corresponding calculated bond lengths are 2.342 (LiP), 2.081 (BeP), 1.758 (BP), 2.690 (NaP), 2.546 (MgP), 2.426 (AlP), and 2.007 angstrom (SiP).