The singlet ground (X̃Σ+1) and excited (Σ1,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=3344cm1, ω2=689cm1, and ω3=1298cm1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2kcalmol1 above the global minimum HCP. The dissociation energy D0[HCP(X̃Σ+1)H(S2)+CP(XΣ+2)] of HCP is predicted to be 119.0kcalmol1, which is very close to the experimental lower limit of 119.1kcalmol1. 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, Σ1A1, with excitation energies Te of 101.4kcalmol1(A1HCP), 104.6kcalmol1(Σ1HCP), 122.3kcalmol1(A1HPC), and 171.6kcalmol1(Σ1HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the A1 state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state (ÃA1). The excitation energy predicted for this excitation is T0=99.4kcalmol1(34800cm1,4.31eV), in essentially perfect agreement with the experimental value of T0=99.3kcalmol1(34746cm1,4.308eV). For the second lowest-lying excited singlet surface, Δ1A1, four stationary points were found with Te values of 111.2kcalmol1 (2A1 HCP), 112.4kcalmol1(Δ1HPC), 125.6kcalmol1(2A1HCP), and 177.8kcalmol1(Δ1HPC). The predicted CP bond length and frequencies of the 2A1 state with a bond angle of 89.8° (1.707Å, 666 and 979cm1) compare reasonably well with those for the experimentally reported C̃A1 state (1.69Å, 615 and 969cm1). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7kcalmol1 and 89.8° versus experimental values of 115.1kcalmol1 and 113°.

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