Enhancement of a lewis acid-base interaction via solvation: Ammonia molecules and the benzene radical cation

The interaction between ammonia and the benzene radical cation has been investigated by gas-phase studies of mass selected ion clusters {C ₆H ₆-(NH ₃) _n=0-8} ⁺ via tandem quadrupole mass spectrometry and through calculations. Experiments show a special stability for the cluster ion that contains four ammonias: {C ₆H ₆-(NH ₃) ₄} ⁺. Calculations provide evidence that the first ammonia forms a weak dative bond to the cyclohexadienyl radical cation, (C ₆H ₆-NH ₃) ⁺, where there is a transfer of electrons from ammonia to benzene. Additional solvating ammonia molecules form stabilizing hydrogen bonds to the ring-bound ammonia {C ₆H ₆-NH ₃} ⁺, (NH ₃) _n, which cause cooperative changes in the structure of the cluster complex. Free ammonia is a weak hydrogen bond donor, but electron transfer from NH ₃ to the benzene ring that strengthens the dative bond will increase the hydrogen acidity and the strength of the cluster hydrogen bonds to the added ammonia. A progressive "tightening" of this dative bond is observed upon addition of the first, second, and third ammonia to give a cluster stabilized by three N- ⁺H·N hydrogen bonds. This shows that the energetic cost of tightening the dative bond is recovered with dividends in the formation of stable cluster hydrogen bonds.