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 th eory (BLYP and B3LYP), complete active space SCF (CASSCF), and the coupled- cluster method with single, double (CCSD), and perturbative triple excitati ons [CCSD(T)] in conjunction with basis sets of up to triple-zeta plus doub le polarization quality. The meta isomer, 1,2,3,5-tetradehydrobenzene, is t he most favorable cyclic isomer, but is 38 kcal mol(-1) less stable than th e hexatriyne C6H2 global minimum. All of the cyclic isomers have singlet gr ound states, but these are less stable for the ortho and para isomers; henc e, the singlet-triplet energy splittings for 1,2,4,5- and 1,2,3,4-tetradehy drobenzene are smaller than for ortho-benzyne. Harmonic vibrational frequen cy 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-tetradehydroben zene at ca. 1675 cm(-1) but that this vibration might be too weak to be see n for 1,4-bis(trifluoromethyl)-2,3,5,6-tetradehydrobenzene.