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

The singlet ground $(X̃Σ+1)$ 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 $(X̃Σ+1HCP)$ 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 $D0[HCP(X̃Σ+1)→H(S2)+CP(XΣ+2)]$ 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 states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, $Σ−1→A″1$⁠, with excitation energies $Te$ of $101.4kcalmol−1(A″1HCP)$⁠, $104.6kcalmol−1(Σ−1HCP)$⁠, $122.3kcalmol−1(A″1HPC)$⁠, and $171.6kcalmol−1(Σ−1HPC)$ at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the $A″1$ state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state $(ÃA″1)$⁠. The excitation energy predicted for this excitation is $T0=99.4kcalmol−1(34800cm−1,4.31eV)$⁠, in essentially perfect agreement with the experimental value of $T0=99.3kcalmol−1(34746cm−1,4.308eV)$⁠. For the second lowest-lying excited singlet surface, $Δ1→A′1$⁠, four stationary points were found with $Te$ values of $111.2kcalmol−1$ (⁠$2A′1$ HCP), $112.4kcalmol−1$ $(Δ1HPC)$⁠, $125.6kcalmol−1(2A′1HCP)$⁠, and $177.8kcalmol−1(Δ1HPC)$⁠. The predicted CP bond length and frequencies of the $2A′1$ state with a bond angle of 89.8° (⁠$1.707Å$⁠, 666 and $979cm−1$⁠) compare reasonably well with those for the experimentally reported $C̃A′1$ state (⁠$1.69Å$⁠, 615 and $969cm−1$⁠). However, the excitation energy and bond angle do not agree well: theoretical values of $108.7kcalmol−1$ and 89.8° versus experimental values of $115.1kcalmol−1$ and 113°.