The reactions of the water solvated ammonia radical cation [NH 3 ⅐ϩ , H 2 O] with a variety of aldehydes and ketones were investigated. The reactions observed differ from those of low energy aldehydes and ketones radical cations, although electron transfer from the keto compound to ionized ammonia is thermodynamically allowed within the terbody complexes initially formed. The main process yields an ammonia solvated enol with loss of water and an alkene. This process corresponds formally to a McLafferty fragmentation within a complex. With aldehydes, another reaction can take place, namely the transfer of the hydrogen from the CHO group to ammonia, leading to the proton bound dimer of ammonia and water, and to the NH 4 ϩ cation. Comparison between the available experimental results leads to the conclusion that the McLafferty fragmentation occurs within the terbody complex initially formed, with no prior ligand exchange, the water molecule acting as a spectator partner. (J Am Soc Mass Spectrom 2004, 15, 966 -971) T he study of the unimolecular and bimolecular reactivity of solvated radical cations requires access to such kind of species. Of course, by direct reactions between an ion and a solvent molecule in the cell of an ion cyclotron resonance mass spectrometer (FT-ICR), an encounter complex is formed in which reactions can take place. However, the solvated radical cations formed in this way possess a high energetic content (at least equal to the solvation energy) and are too short lived to be isolated and studied. Furthermore, since these species are hot ions, their fragmentations correspond to the unimolecular reactions which are entropically favored. Therefore, the reactions observed are not necessarily the same as those of long lived solvated radical cations, as demonstrated in the case of the CH 2 CHOH ⅐ϩ /CH 3 CHO system [1], where the acetaldehyde solvated enol ion is produced by reaction of the enol ion with paraldehyde (eq 1). In this case, the hot intermediate reacts by H ϩ transfer, whereas low energy collision induced dissociation of the solvated complex leads to H ⅐ abstraction [1]. CH 2 CHOH ⅐ϩ ϩ (CH 3 CHO) 3 3 [CH 2 CHOH ⅐ϩ , CH 3 CHO] ϩ (CH 3 CHO) 2 (1)

The thermochemical properties of some small clusters such as the (H2O)2*+ dimer have already been investigated by both experimental and theoretical methods. The recent method to selectively prepare the ammonia-water ionized dimer [NH3, H2O]*+ (and not its proton transfer isomer [NH4+, OH*]) allowed us to study its chemical reactivity. This study focuses on the charge and proton transfer pathways: Ion-molecule reactions in the cell of an FT-ICR mass spectrometer were carried out with a range of organic compounds. Examination of the reactivity of the [NH3, H2O]*+ ionized dimer versus ionization energy and proton affinity of the neutral reagents shows a threshold in the reactivity in both instances. This leads to a bracketing of thermochemical properties related to the dimer. From these experiments and in agreement with ab initio calculations, the adiabatic recombination energy of the [NH3, H2O]*+ dimer was evaluated at -9.38 +/- 0.04 eV. The proton affinity bracketing required the reevaluation of two reference gas-phase basicity values. The results, in good agreement with the calculation, lead to an evaluation of the proton affinity of the [NH2*, H2O] dimer at 204.4 +/- 0.9 kcal mol(-1). These two experimental values are respectively related to the ionization energy of NH3*+ and to the proton affinity of NH2* by the difference in single water molecule solvation energies of ionized ammonia, of neutral ammonia, and of the NH2* radical.