Six Low-Lying Isomers of C11H8 Are Unidentified in the Laboratory - a Theoretical Study

The potential energy surface of C₁₁H₈ has been theoretically examined using density functional theory and coupled-cluster methods. The current investigation reveals that 2aH-cyclopenta[cd]indene (2), 7-ethynyl-1H-indene (6), 4-ethynyl-1H-indene (7), 6-ethynyl-1H-indene(8), 5-ethynyl-1H-indene (9), and 7bH-cyclopenta[cd]indene (10) remain elusive to date in the laboratory. The puckered low-lying isomer 2 lies at 11 kJ mol⁻¹ below the experimentally known molecule, cyclobuta[de]naphthalene (3), at the fc-CCSD(T)/cc-pVTZ//fc-CCSD(T)/cc-pVDZ level of theory. 2 lies at 35 kJ mol⁻¹ above the thermodynamically most stable and experimentally known isomer, 1H-cyclopenta[cd]indene (1), at the same level. It is identified that 1,2-H transfer from 1 yields 2H-cyclopenta[cd]indene (14) and subsequent 1,2-H shift from 14 yields 2. Appropriate transition states have been identified and intrinsic reaction coordinate calculations have been done at the B3LYP/6-311+G(d,p) level of theory. Recently, 1-ethynyl-1H-indene (11) has been detected using synchrotron based vacuum ultraviolet ionization mass spectrometry. 2-ethynyl-1H-indene (4) and 3-ethynyl-1H-indene (5) have been synthetically characterized in the past. While the derivatives of 7bH-cyclopenta[cd]indene (10) have been isolated elsewhere, the
parent compound remains unidentified to date in the laboratory. Although C₁₁H₈ is a key elemental composition in reactive intermediates chemistry and most of its isomers are having a non-zero dipole moment, to the best of our knowledge, none of them have been characterized by rotational spectroscopy. Therefore, energetic and spectroscopic properties have been computed and the present investigation necessitates new synthetic studies on C₁₁H₈, in particular 2, 6-10, and also rotational spectroscopic studies on all low-lying isomers.