Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

The singlet ground (XΣ+1) and excited (Σ−1,Δ1) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum (XΣ+1HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of ω1=3344cm−1, ω2=689cm−1, and ω3=1298cm−1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2kcalmol−1 above the global minimum HCP. The dissociation energy D0[HCP(XΣ+1)→H(S2)+CP(XΣ+2)] of HCP is predicted to be 119.0kcalmol−1, which is very close to the experimental lower limit of 119.1kcalmol−1. Eight singlet excited...