Kinetics of radiative/termolecular associations in the low pressure regime: reactions of bare au+ with benzene

Consecutive ion/molecule reactions of gold cation Au+ with (deuterated) benzene are examined by Fourier-transform mass spectrometry. Primary products upon reacting Au+ with C6H6 are due to charge transfer to afford C6H6+· concomitant with neutral gold as well as ligand association to yield the Au(C6H6)+ complex. Subsequent reactions of so-formed Au(C6H6)+ with benzene result in exclusive formation of the corresponding bisligated complex Au(C6H6)2+. In contrast to a previous report by Ho and Dunbar [Int. J. Mass Spectrom. 182/183 (1999) 175], occurrence of charge transfer from benzene to Au(C6H6)+ affording C6H6+· concomitant with neutral Au(C6H6) is rigorously excluded. Our results are supported by ab initio calculations which predict a strong interaction in the Au(C6H6)+ cation, whereas neutral Au(C6H6) is best described as a loose van der Waals complex; i.e. D0(Au+–C6H6) = 61.1 kcal/mol versus D0(Au–C6H6) = 2.0 kcal/mol. Interestingly, the reaction kinetics for the sequential additions of benzene to cationic gold show irregularities which point to a non-negligible role of excess energy in consecutive bimolecular associations at low pressures.