Non covalent bonds in some bromo-substituted aromatic anions

Four bromo-substituted derivatives of naphthalene and anthracene were studied by means of Dissociative Electron Attachment Spectroscopy. Long-lived molecular anions with lifetimes ranging from 25 to 144 μs were observed in all molecules under investigation. In all cases molecular anion fragmentation is poor: only the Brˉ, [M-Br]ˉ and (with very small intensity) [M-H]ˉ species were observed, except for the case of 9,10-Br 2 -anthracene. It was shown that the presence of long-lived molecular anions in 1-Br- and 2-Br-naphthalene (τ a =26 μs and τ a =25 μs, respectively) does not contradict the fact that their dissociation rates measured by the pulse radiolysis method are fairly large (1.0 ×10 10 s −1 and 1.8 ×10 10 s −1 , respectively). Scanning the potential energy surface of anions in the process of positioning a bromine anion around a polarized aromatic radical revealed the presence of a series of local minima separated by potential barriers. It is inferred that the most energetically favorable structures of the 1-Br- and 2-Br-naphthalene anions should be interpreted as complexes of the bromide anion bound to the polarized aromatic radical by non-covalent Br--H bonds. Similar local minima were found in the 9-Br- and 9,10-Br 2 -anthracene anions, but the energies of these structures are significantly higher than those of the “standard” anionic structures with a C-Br bond length of ~1.93 Å. The EA a s obtained with DFT CAM-B3LYP/6–311 +G(d,p) calculations are in acceptable agreement with the estimates made within the framework of the simple Arrhenius model from the lifetimes of molecular anions.