AB-INITIO STUDIES OF PROTON SPONGES .2. 1,6-DIAZABICYCLO[4.4.4]TETRADECANE

The structures of 1,6-diazabicyclo[4,4.4]tetradecane and its inside-protonated cation have been determined by ab initio geometry optimization. The title compound possesses D-3 symmetry in the gas phase; an N ... N distance of 2.87 Angstrom is obtained at the Hartree-Fock/6-31G** level of theory. Asymmetric protonation at one nitrogen (lowering the symmetry to C-3) is energetically preferred to the symmetric D-3 [N ... H ... N](+) bridged form by some 11 kJ mol(-1), in contrast to the situation in the solid state and in solution. However, the presence of a barrier to proton transfer between nitrogens is in agreement with numerous theoretical studies on the [NH4 ... NH3](+) cation and on 1,8-bis(dimethylamino)naphthalene. The proton affinity, which has so far proved impossible to measure experimentally, is predicted to be 1088 kJ mol(-1), the highest known value for a neutral organic base. A topological analysis of the electron distribution reveals that the two nitrogens are linked by a (3,-1) critical point with a small positive value of del(rho)(2), characteristic of a closed-shell interaction. Bond path analysis indicates that there is relief of strain in the C-C bonds on protonation.