Azaboracyclooctatetraenes Reveal that the Different Aspects of Triplet State Baird-Aromaticity are Nothing but Different

The Baird-aromaticity of BN/CC cyclooctatetraene isosteres (azaboracyclooctatetraenes) in their lowest triplet states (T1) has been explored through computations of various aromaticity indices describing the different aspects of aromaticity (magnetic, electronic, energetic and geometric). While cyclooctatetraene (COT) is aromatic in its T1 state following Baird’s 4n rule, we now reveal that the degree of Baird-aromaticity of the BN isosteres varies extensively with aromaticity aspect considered. According to energetic and electronic indices, the thermodynamically most stable octagonal B4N4H8 isomer, having alternating B and N pattern (borazocine, B4N4COT-A), is only weakly aromatic or nonaromatic in T1, while the magnetic descriptors suggest it to have about two thirds the Baird-aromaticity of T1 state COT (3COT). The extent of Baird-aromaticity of intermediate BN/CC isosteres also varies markedly with aspect investigated. On the other hand, magnetic indices indicate that borazine (B3N3H6) in its singlet ground state (S0) is weakly aromatic. This opposite features in the magnetic aromaticity descriptors of, respectively, S0 state borazine (1borazine) and 3B4N4COT-A can be linked to differences in the symmetries of the orbitals involved in the virtual transitions from occupied to unoccupied orbitals which describe the response of a molecule in an external magnetic field. For azaboraCOTs, the magnetic aspect of T1 state Baird-aromaticity (response aromaticity) is not related to the electronic and energetic aspects (intrinsic aromaticity), findings that underline earlier observations on differences between the various aspects of the aromaticity phenomenon (or phenomena).