| Abstract
| - The interaction between ammonia and the benzene radical cation has been investigated by gas-phase studiesof mass selected ion clusters {C6H6-(NH3)n=0-8}+ via tandem quadrupole mass spectrometry and throughcalculations. Experiments show a special stability for the cluster ion that contains four ammonias: {C6H6(NH3)4}+. Calculations provide evidence that the first ammonia forms a weak dative bond to the cyclohexadienylradical cation, {C6H6−NH3}+, where there is a transfer of electrons from ammonia to benzene. Additionalsolvating ammonia molecules form stabilizing hydrogen bonds to the ring-bound ammonia {C6H6−NH3}+·(NH3)n, which cause cooperative changes in the structure of the cluster complex. Free ammonia is a weakhydrogen bond donor, but electron transfer from NH3 to the benzene ring that strengthens the dative bondwill increase the hydrogen acidity and the strength of the cluster hydrogen bonds to the added ammonia. Aprogressive “tightening” of this dative bond is observed upon addition of the first, second, and third ammoniato give a cluster stabilized by three N−+H·N hydrogen bonds. This shows that the energetic cost of tighteningthe dative bond is recovered with dividends in the formation of stable cluster hydrogen bonds.
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