Structures and Binding Energies of Proton-Bound Pairs of HCN and CH 3 CN with NH 3 , H 2 O, HF, CH 3 NH 2 , CH 3 OH, and CH 3 F

In this study, 12 nitrile-containing proton-bound dimers were investigated with ab initio molecular orbital calculations: (HCN)(NH 3 )H +, (HCN)(H 2 O)H +, (HCN)(HF)H +, (HCN)(CH 3 NH 2 )H +, (HCN)(CH 3 OH)H +, (HCN)(CH 3 F)H +, (CH 3 CN)(NH 3 )H +, (CH 3 CN)(H 2 O)H +, (CH 3 CN)(HF)H +, (CH 3 CN)(CH 3 NH 2 )H +, (CH 3 CN)(CH 3 OH)H +, and (CH 3 CN)(CH 3 F)H + . The geometries of these dimers were optimized at the MP2/6-31+G(d) level of theory, and their binding energies (relative to the lowest energy dissociation products) were calculated with the G2, G2(ZPE=MP2), G2(MP2), and G2(MP2,SVP) methods. The trends in the binding energies follow the absolute value of the difference in proton affinity of the constituent monomers; the larger the PA difference, the smaller the binding energy. An empirical relationship has been derived that may prove useful in predicting the binding energies of other nitrile-containing cluster ions. The results are compared to experimental values where available.