Anion-Cation-Anion Ion Triplet Characterization by Computation and Photoelectron Spectroscopy.

Ion triplets of the chloride salts of two commonly used weakly coordinating cations are reported (i.e., Cl-·NMe4+Cl- (1-) and Cl-·PPh4+Cl- (2-)). Negative ion photoelectron spectra at 20 K afford vertical and adiabatic detachment energies of 5.18 and 5.00 eV (1-) and 5.03 and 4.70 eV (2-), respectively. These results are well reproduced by coupled cluster calculations with single, double, and perturbative triple excitations (CCSD(T)) whereas M06-2X is systematically too small by ∼0.3 eV (i.e., 7 kcal mol-1). The structures of both 1- and 2- have five or six C-H···Cl- interactions that stabilize these cluster anions by 32 (1-) and 27 (2-) kcal mol-1 as given by their chloride dissociation enthalpies. These values drop to 7.4 and 3.8 kcal mol-1 in dichloromethane based up conductor-like polarizable continuum model calculations and suggest that X-·M+X- ion triplets with a weakly coordinating cation maybe the reactive form of salts under some conditions.