Ab initio models for multiple‐hydrogen exchange: Comparison of cyclic four‐ and six‐center systems

High‐level ab initio calculations {QCISD(T)/6‐311 +G**//MP2(fu)/6‐31 +G**, with corrections for higher polarization [evaluated at MP2/6‐311 +G(3 df ,2 p )] and ΔZPE//MP2(fu)/6‐31 +G**, i.e., comparable to Gaussian‐2 theory} indicate concerted mechanisms for double‐ and triple‐hydrogen exchange reactions in HF and HCl dimers and trimers, in mixed dimers and trimers containing one NH 3 , and in mixed dimers of HF, HCl, and NH 3 with formic acid. All these reactions proceed via cyclic four‐ or six‐center transition structures, the latter being generally more favorable. Calculated activation barriers (Δ H d̊ at 0 K, kcal/mol) are 42.3 for (HF) 2 , 20.3 for (HF) 3 , 41.2 for (HCl) 2 , 25.6 for (HCl) 3 , 36.0 for NH 3 ‐HF, 10.6 for NH 3 (HF) 2 , 19.9 for NH 3 ‐HCl, 2.3 for NH 3 (HCl) 2 , 9.7 for HCO 2 H‐HF, 7.0 for HCO 2 H‐HCl, and 11.3, for HCO 2 H‐NH 3 . The barriers are lower for the more ionic systems and when more ion pair character is present. © John Wiley & Sons, Inc.