Tetradehydrobenzenes:  Singlet−Triplet Energy Separations and Vibrational Frequencies

The energies and structures of the three tetradehydrobenzene (C 6 H 2 ) isomers (“ortho”, “meta”, and “para”) were studied employing density functional theory (BLYP and B3LYP), complete active space SCF (CASSCF), and the coupled-cluster method with single, double (CCSD), and perturbative triple excitations [CCSD(T)] in conjunction with basis sets of up to triple-ζ plus double polarization quality. The meta isomer, 1,2,3,5-tetradehydrobenzene, is the most favorable cyclic isomer, but is 38 kcal mol - 1 less stable than the hexatriyne C 6 H 2 global minimum. All of the cyclic isomers have singlet ground states, but these are less stable for the ortho and para isomers; hence, the singlet−triplet energy splittings for 1,2,4,5- and 1,2,3,4-tetradehydrobenzene are smaller than for ortho -benzyne. Harmonic vibrational frequency analyses [up to CCSD(T)/TZ2P] suggest that the ν as (C⋮C) vibration should be observable in the IR spectra of 1,2,4,5-tetradehydrobenzene at ca. 1675 cm - 1 but that this vibration might be too weak to be seen for 1,4-bis(trifluoromethyl)-2,3,5,6-tetradehydrobenzene.