Tetradehydrobenzenes: Singlet-triplet energy separations and vibrational frequencies

The energies and structures of the three tetradehydrobenzene (C6H2) 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-zeta 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 C6H2 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 v(as)(C equivalent to 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.