Equilibrium geometry of the HCCN triplet ground state: Carbene or allene? An open-shell coupled cluster study including connected triple excitations

Abstract

The molecular structure of the HCCN molecule has been investigated using open-shell coupled cluster methods. The full single and double excitation procedure (CCSD) is used initially. Connected triple excitations were then included perturbatively via the CCSD(T) method. A standard double zeta plus polarization (DZP) basis set, designated H(4s1p/2s1p), C,N(9s5p1d/4s2p1d), was used initially. The predicted carbene H–C–C equilibrium bond angles are 133.8° (CISD), 138.4° (CCSD), and 139.8° with the CCSD(T) method. The corresponding bent-linear energy differences are 5.30, 2.70, and 2.15 kcal/mol, respectively. A much larger basis set was chosen to consider the importance of d functions on hydrogen and f functions on the carbon and nitrogen atoms. Using this H(5s2p1d/3s2p1d), C,N(10s6p2d1f/5s3p2d1f) basis, the H–C–C equilibrium bond angle is 136.3° with the CISD method and the bent-linear energy difference 3.8 kcal/mol. Best estimates of the HCC equilibrium bond angle and singlet–triplet separation are 142° and 0.8 kcal/mol, respectively. The relationship of these theoretical results to several experiments is discussed.