Ligation kinetics as a probe for relativistic effects: Ligation of atomic coinage metal cations with ammonia

Abstract

The kinetics for ammonia ligation of the three (d10) transition metal coinage cations Cu+, Ag+ and Au+ were measured in an attempt to assess the role of relativistic effects in reaction kinetics. Measurements of several main-group cations were included for comparison: the alkali (s0) cations K+, Rb+ and Cs+, the alkaline-earth (s1) cations Ca+, Sr+ and Ba+ and the p0 atomic cations Ga+, In+ and Tl+. Measurements were performed at room temperature in helium bath gas at 0.35Torr using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. The atomic cations are produced at ca. 5500K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A strong enhancement in the rate of addition of NH3 to Au+ was observed for the coinage metal cation period in contrast to the continuous decline in rate down the periodic table that was seen for the main group s0, s1 and p0 cations. We attribute this rate enhancement to the enhancement in the Au+-NH3 binding energy expected from relativistic effects. Comparisons are made with the periodic trends that we have reported previously for the rates of ligation of the coinage metal cations with O2, D2O, N2O, CO2, CS2, CH3F and SF6 and that we measured with pyridine. For seven of the nine ligands that were investigated, rate enhancement with Au+ provided an indirect experimental measure of relativistic effects.