Anion–Cation–Anion\nIon Triplet Characterization\nby Computation and Photoelectron Spectroscopy

Ion triplets of the chloride salts of two commonly used\nweakly\ncoordinating cations are reported (i.e., Cl<sup>–</sup>·NMe<sub>4</sub><sup>+</sup>Cl<sup>–</sup> (<b>1</b><sup><b>–</b></sup>) and Cl<sup>–</sup>·PPh<sub>4</sub><sup>+</sup>Cl<sup>–</sup> (<b>2</b><sup><b>–</b></sup>)). Negative ion photoelectron spectra at 20 K afford vertical\nand adiabatic detachment energies of 5.18 and 5.00 eV (<b>1</b><sup><b>–</b></sup>) and 5.03 and 4.70 eV (<b>2</b><sup><b>–</b></sup>), respectively. These results are\nwell reproduced by coupled cluster calculations with single, double,\nand perturbative triple excitations (CCSD(T)) whereas M06-2X is systematically\ntoo small by ∼0.3 eV (i.e., 7 kcal mol<sup>–1</sup>).\nThe structures of both <b>1</b><sup><b>–</b></sup> and <b>2</b><sup><b>–</b></sup> have five or\nsix C–H···Cl<sup>–</sup> interactions\nthat stabilize these cluster anions by 32 (<b>1</b><sup><b>–</b></sup>) and 27 (<b>2</b><sup><b>–</b></sup>) kcal mol<sup>–1</sup> as given by their chloride dissociation\nenthalpies. These values drop to 7.4 and 3.8 kcal mol<sup>–1</sup> in dichloromethane based up conductor-like polarizable continuum\nmodel calculations and suggest that X<sup>–</sup>·M<sup>+</sup>X<sup>–</sup> ion triplets with a weakly coordinating\ncation maybe the reactive form of salts under some conditions.