Is HCCP linear, bent or cyclic? Structures and energies of its low-lying states

Ab initio and density functional theory (DFT) calculations have been carried out to predict the structures and to estimate the energies of the low-lying states of HCCP. The geometrical parameters, harmonic vibrational frequencies, IR intensities, dipole moments and rotational constants of the energetically favorable isomers have been estimated with the (U)HF, (U)MP2, ROHF, CAS(2,2) and DFT methods using the 6-31G ∗ [(U)HF, ROHF and CAS(2,2)], 6-31G(2df,p) (UHF), cc-pVDZ [(U)MP2] and cc-pVTZ[(U)MP2 and DFT] basis sets. At the B3LYP/cc-pVTZ geometries, single point energy calculations were performed in the same basis set at the CISD (single and double excitation configuration interaction) and the CCSD(T) (coupled-cluster with all single and double substitutions, and a quasi-perturbative estimate for the effect of connected triple excitations) higher levels of theory. Linear HCCP ( 3 Σ − ) is found to be the global minimum at all the theoretical levels used. The second most stable isomer with a 1 A′ electronic state is predicted to have a cyclic CCP structure in which the hydrogen atom is attached to the carbon atom. This state lies 18 and 14 kcal/mol above the triplet linear HCCP at the CISD and CCSD(T) levels, respectively. However, the other isomer of which hydrogen atom is attached to the phosphorous atom (CCPH, 1 A′) is found to be much higher in energy than the ground state by 45 and 39 kcal/mol, respectively.