Hydrated copper ion chemistry: guided ion beam and computational investigation of Cu2+(H2O)n (n=7-10) complexes

Cross sections for the threshold collision-induced dissociation of Cu2+(H2O)(n), where n = 8-10, are measured using a guided ion beam tandem mass spectrometer. The primary dissociation pathway is found to be loss of a single water molecule followed by the sequential loss of additional water molecules until n = 8, at which point charge separation to form CuOH+(H2O)(4) + H+(H2O)(3) is observed to occur at a slightly lower energy than loss of a water molecule. Competition from charge separation prohibits the formation of appreciable amounts of the n = 7 or smaller complexes as reactants in the source. These findings indicate that Cu2+ has a critical size of 8. Analysis of the data using statistical modeling techniques that account for energy distributions and lifetime effects yields primary and sequential bond dissociation-energies (BDEs) for loss of one and two water molecules from n = 8-10 complexes as well as the barrier for charge separation from n = 8. More speculative analysis extends the thermochemistry obtained down to n = 5 and 6. Theoretical BDEs are determined from quantum chemical calculations using structures optimized at the B3LYP/6-311+ G(d, p) level along with the lowest-energy isomers suggested by single point energies at the MP2(full), M06, B3LYP and B3P86 levels of theory using a 6-311+ G(2d, 2p) basis set. BDEs at 0 K are converted to 298 K thermodynamic values using a rigid rotor/harmonic oscillator approximation. Experimental and theoretical entropies of activation suggest that a third solvent shell forms at n = 9, in accord with previous findings. The present work represents the first experimentally determined hydration enthalpies for the Cu2+(H2O) n system.