Investigations of the gas-phase reactivity of Cu+ and Ag+ glycine complexes towards CO, D2O and NH3

The room-temperature reactivities of complexes of Cu+ and Ag+ with glycine have been investigated using an inductively coupled plasma-selected ion flow tube (ICP-SIFT)/multicollision-induced dissociation (CID) tandem mass spectrometer. These complexes were produced in a flow tube, collisionally thermalized and then allowed to react with CO, D2O or NH3. The measured reactivities of the CuGly(+) and AgGly(+) complexes have been compared with those of the bare metal cations towards the same neutral reagents. All observed reactions resulted in adduct formation, with helium buffer gas presumably acting as a stabilizing agent. Reaction rate enhancements of up to three orders of magnitude and lower extents of ligation were the main characteristics of the reactions initiated by the metal cation-glycine adducts as compared with those initiated by the bare metal cations. The kinetic information combined with equilibrium analyses and CID results suggest that, irrespective of the reagent, ligation to CuGly(+) is stronger than to AgGly(+) and, in both instances, ligation of carbon monoxide and water has comparable strengths; while much stronger coordination is achieved with ammonia. The structures of the complexes have been investigated computationally using density functional theory (DFT) at the B3LYP level employing the DZVP basis set. Optimized structures and the free energy changes associated with ligation have been computed. A good correlation was obtained between the experimental reaction efficiencies and the calculated free energies of bonding. Also, results are presented for the potential energy surfaces for the conversion of the "charge solvated" forms into the "metal salt" forms of CuGly(+) and AgGly(+) and of their adducts with CO or NH3. The computations indicate a modest catalytic effect of the CO and NH3 on this conversion. (C) 2003 Elsevier Science B.V. All rights reserved.