Characterization of the X̃2Π and Ã2Σ+ Electronic States of the Phospaethyne cation (HCP+)
Temelso, B.; Richardson, N. A.; Sari, L.; Yamaguchi, Y.; Schaefer, H. F. Characterization of the X̃2Π and Ã2Σ+ Electronic States of the Phospaethyne cation (HCP+). J. Theor. Comput. Chem. 2005, 4 Special Issue, 707-724.
The electronic ground state (X̃2Π) and first excited state (Ã2Σ+) of phosphaethyne cation (HCP+) have been systematically investigated using ab initio electronic structure theory. The total energies, geometries, rotational constants, dipole moments, harmonic vibrational frequencies, and parameters for Renner-Teller splittings were determined using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster (CC) with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], CC with single, double, and iterative partial triple excitations (CCSDT-3), and CC with single, double, and full triple excitations (CCSDT) methods and eight different basis sets. Some of the largest full triples coupled cluster computations to date are reported. Degenerate bending frequencies for the Ã2Σ+ state were determined using the equation-of-motion (EOM)-CCSD technique. The two states have been confirmed to have linear equilibrium structures. At the full CCSDT level of theory with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set, the classical X̃ — Ã splitting (Te value) is predicted to be 47.7 kcal/mol (2.07 eV, 16,700 cm-1) and the quantum mechanical splitting (T0 value) to be 48.1 kcal/mol (2.08 eV, 16,800 cm-1), which are in excellent agreement with the experimental values of Te = 47.77 kcal/mol (2.072 eV, 16,708 cm-1) and T0 = 47.94 kcal/mol (2.079 eV, 16,766 cm-1). The excitation energies predicted by the CCSDT-3 and CCSD(T) methods differ from the full triples CCSDT result by 0.38 and 0.45 kcal/mol, respectively. With the aug-cc-pVQZ CCSDT-3 method the Renner parameter and the averaged harmonic bending vibrational frequency are determined to be ɛ= -0.0390 and ω̃2=656 cm-1 for the ground state of HCP+, which are reasonably consistent with the experimental values of ɛ=-0.0415 and ω̃2≃636 cm-1. The predicted dipole moments are 1.30 Debye (X̃2Π state, polarity-hydrogen atom positive) and 0.06 Debye (Ã2Σ+ state, polarity-phosphorus atom positive).
Journal of Theoretical and Computational Chemistry