A theoretical study of low-lying electronic states of aminonitrene, phosphinonitrene, and phosphinocarbene

• Published in: The Journal of chemical physics 1991-06, Vol.94 (12), p.8029-8039
• Main Authors: HOFFMANN, M. R, KUHLER, K
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Electron correlation calculations for atoms and molecules; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Physics
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.460137

The recently formulated multiconfiguration-based unitary coupled electron pair approximation (UCEPA) is compared with multireference configuration interaction (MR-CISD) calculations, including all single and double excitations, for the molecules in this study. The electronic states of the molecules in this study are not only of experimental interest, but represent a challenge to any formalism to accurately predict the energy separations of the low-lying electronic states. The equilibrium geometries and fundamental vibrational frequencies of the three lowest electronic states (i.e., 1A1, 3A″, and 1A″) of aminonitrene H2N2, and phosphinonitrene, H2PN, have been determined using a split-valence basis with polarization functions on the heavy atoms and a small complete active space self-consistent-field (CASSCF) description of the active space. Both MR-CISD and UCEPA calculations have been performed at the equilibrium structures using larger basis sets to accurately determine the relative energetics of the electronic states. The equilibrium geometries and vibrational frequencies of the two lowest electronic states (i.e., 1A′ and 3A″) of phosphinocarbene, H2PCH, have been determined using a larger than double zeta basis set, augmented with polarization and diffuse functions, and a CASSCF description of the active space. Both MR-CISD and UCEPA calculations were performed on the equilibrium structures and predict that the singlet lies between 10.4 and 11.8 kcal/mol lower in energy than the triplet. The use of a generalized valence bond (GVB) reference function within UCEPA is introduced and is shown to be a useful approximation.