title: Low-lying triplet states of diphosphene and diphosphinylidene.
abstract: In this research, six low-lying triplet states of diphosphene (HPPH) and disphosphinylidene (PPH(2)) are systematically investigated starting from self-consistent field theory and proceeding to multireference coupled cluster methods using a wide range of basis sets. For each structure, the geometry, energy, dipole moment, harmonic vibrational frequencies, and infrared intensities are predicted. The triplet potential energy surface (PES) of P(2)H(2) is presented, based on systematically extrapolated coupled cluster energies and accounting for core-valence correlation, zero-point vibrational energy, and diagonal Born-Oppenheimer effects. Both (3)A'' pyramidal PPH(2) and (3)B skewed HPPH are minima on the triplet PES and lie 27.4 ± 0.3 and 32.4 ± 0.3 kcal mol(-1) above the global minimum structure closed-shell (1)A(g) trans-HPPH, respectively. The energy barrier for the isomerization reaction [(3)B skewed HPPH → (3)A'' pyramidal PPH(2)] is predicted to be 16.4 ± 0.3 kcal mol(-1). On this triplet PES, two equivalent (3)B skewed HPPH are converted via the (3)B(u) trans-HPPH transition state with a barrier of 9.1 ± 0.3 kcal mol(-1) or via the (3)B(2) cis-HPPH transition state with a barrier of 11.1 ± 0.3 kcal mol(-1). Moreover, the two equivalent (3)A'' pyramidal PPH(2) structures are connected through the (3)A(2) planar PPH(2) transition state with a barrier of 18.6 ± 0.3 kcal mol(-1). The energy crossing of the singlet and triplet adiabatic PES is studied using Mukherjee multireference coupled cluster method with the cc-pVQZ basis set, which predicts that the (3)B skewed HPPH is 1.4 kcal mol(-1) lower in energy than the corresponding (1)A skewed HPPH at the (3)B skewed HPPH optimized geometry.
authors.affiliation: Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, USA.
authors.forename: Tongxiang
authors.initials: T
authors.lastname: Lu
authors.forename: Qiang
authors.initials: Q
authors.lastname: Hao
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Simmonett
authors.forename: Francesco A
authors.initials: FA
authors.lastname: Evangelista
authors.forename: Yukio
authors.initials: Y
authors.lastname: Yamaguchi
authors.forename: De-Cai
authors.initials: DC
authors.lastname: Fang
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer