Thermoneutral isotope exchange reactions in proton-bound complexes of water with organic molecules: correlations with energetics of formation of the corresponding association ions

The efficiencies of the reaction MH+ + D2O → MD+ + HDO or the analogous reaction in which deuteration is reversed have been measured for the cases, M. = formaldehyde, acetaldehyde, methanol, methyl formate, propionaldehyde, dimethyl ether, 1,4-dioxane, acetone, diethyl ether, di-n-propyl ether, and pyridine. A quantitative evaluation of the model used in estimating well depths leads to the conclusion that the energies of association in such complexes are primarily electrostatic in nature, since the model, which considers only electrostatic interactions, predicts well depths in close agreement with those obtained by experiment or ab initio calculations. Use of this approximation also correctly predicts the experimentally determined well depths for the systems studied here and permits one to assign the structure [MH+ H2O] to the most stable configurations of the proton-bound complexes in all cases except the formaldehyde-water system, where the complexes have the structure [H2CO H3O+]; this is verified by the observation that the statistically corrected probability for isotope exchange in this system is unity. In the acetaldehyde-, methanol-, methyl formate-, propionaldehyde-, and dimethyl ether-water systems, the [M H3O+] complexes are prediced to be energetically accessible to separated reactants and, in all these cases, isotope exchange is seen to occur, with a statistically corrected probability of 0.85 to 0.42, depending on the energetics of the proton transfer in the complex. For the 1,4-dioxane-, diethyl ether-, di-n-propyl ether-, and pyridine-water systems, where the model predicts that the [M H3O+] complex is energetically inaccessible, isotope exchange is not observed. The implications of these results to the observed correlation between the heats of formation of association ions and the differences in proton affinities of the complexed molecules are discussed. 29 references