THE 1ST 28 GAS-PHASE PROTON HYDRATION ENERGIES

The successive binding energies DELTA-E(n-1,n) (n = 2-28) of protonated water clusters H+(H2O)n, produced by fast-atom bombardment of ice, have been determined by collision-induced dissociation in a triple-quadrupole mass spectrometer. They decrease dramatically initially, and after n = 9 increase slowly up to n = 28. However, the twenty-first water molecule stands out as bound about 2 kcal/mol more strongly than expected from a simple interpolation, accounting for the well-known "magic" number, n = 21. The trend in DELTA-E(n-1,n) shows that hydrogen bonding quickly becomes a dominant force after the first nine water molecules surround a proton. The DELTA-E(n-1,n) values were used in thermodynamic cycles to show that the "magic" cluster, n = 21, already contains 78% of the total aqueous proton hydration energy.