PROTON AFFINITY LADDERS FROM VARIABLE-TEMPERATURE EQUILIBRIUM MEASUREMENTS .1. A REEVALUATION OF THE UPPER PROTON AFFINITY RANGE

An interlocking ladder of relative proton affinities of 47 compounds, over a range of 54 kcal/mol, was obtained from the enthalpies of proton-transfer equilibria, by using variable-temperature pulsed high-pressure mass spectrometry. From C3H6 (PA = 179.5 kcal/mol) to i-C4H8 (PA = 195.9 kcal/mol) the results agree well with tabulated values, but for the upper PA range the present values are increased significantly. For example, the present vs literature PA values are NH3, 208.3 vs 204.0; CH3NH2, 219.6 vs 2l4.1; and t-C4H9NH2, 229.2 vs 220.8 kcal/mol. The new value for t-C4H9NH2 is also confirmed by the association thermochemistry of t-C4H9+ with NH3. The entropies of protonation are in reasonable agreement with expected rotational symmetry changes. Aromatics and olefins show a positive entropy of protonation of 3-6 cal/mol K due to structural effects, but no anomalous effects are observed that would indicate a dynamic proton.