Is the bonding cumulenic or polyacetylenic in nonatetraynylidene?

Abstract

To answer the entitled question, the equilibrium geometry of the linear triplet ground electronic state X∼3Σg- of nonatetraynylidene (1), HC9H, has been theoretically investigated at various levels of theory. The relative energies, optimal geometry parameters, expectation values of S2, rotational constant, spin densities, and harmonic vibrational frequencies with respect to both restricted open-shell Hartree-Fock (ROHF) and unrestricted HF (UHF) at different levels are examined. The methods employed are HF Self-Consistent Field (SCF), Møller-Plesset perturbation level of theory until second-order (MP2), coupled-cluster singles and doubles (CCSD), and CCSD with perturbative triple excitations (CCSD(T)). Correlation consistent polarized valence double and triple zeta (cc-pVXZ; X=D and T) basis sets are used in all calculations and frozen-core approximation is utilized in all post-SCF calculations. Population analysis of the spin densities at CC levels suggest that several valence structures are possible for 1 implying more polyacetylenic character rather than the cumulenic biradical.