EQUILIBRIUM GEOMETRY OF THE HCCN TRIPLET GROUND-STATE - CARBENE OR ALLENE - AN OPEN-SHELL COUPLED CLUSTER STUDY INCLUDING CONNECTED TRIPLE EXCITATIONS

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 doubel zeta plus polarization (DZP) basis set, designated H(41p/2s1p), C,N(9s5p1d/4s2p1d), was used initially. The predicted carbene H-C-C equilibrium bond angles are 133.8-degrees (CISD), 138.4-degrees (CCSD), and 139.8-degrees 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-degrees 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-degrees and 0.8 kcal/mol, respectively. The relationship of these theoretical results to several experiments is discussed.