Formation of the oxonium phenol ion in the stepwise hydration of the phenyl cation in the gas phase

Abstract

This study provides the first evidence for the formation of the oxonium phenol ion and its hydrated clusters by the sequential addition of water molecules onto the phenylium ion in the gas phase. The oxonium phenol ion exhibits thermal stability at higher temperatures up to nearly 575 K while the hydrated phenyl cation dissociates by the loss of water at temperatures below 400 K. A second water molecule is attached reversibly to each of the oxonium phenol and the hydrated phenyl ions and equilibrium thermochemical measurements in the low temperature range yield an average −ΔH° of 15.5 kcal/mol reflecting 35% contribution from the hydrated oxonium phenol C6H5OH2+(H2O) and 65% contribution from the second hydration step of the phenyl cation C6H5+(H2O)2. In the high temperature range (373–423 K), the measured average −ΔH° of 18.7 kcal/mol reflects 45% and 55% contributions from the hydrated oxonium phenol and the second hydration step of the phenyl cation, respectively. DFT calculations at the B3LYP/6-311++G** level show that the sequential hydration of the oxonium phenol ion results in the formation of externally hydrated clusters C6H5OH2+(H2O)n where the incoming water molecules form a hydrogen bonding network attached to the oxonium site, and the sequential hydration energy decreases from 25.9 kcal/mol for n = 1 to 11.5 kcal/mol for n = 5. For the hydrated phenyl cation clusters C6H5+(H2O)n, the water molecules are attached at two CHδ+ sites of the phenyl cation, and the sequential hydration energy decreases slowly from 11.3 kcal/mol for n = 1 to 8.1 kcal/mol for n = 5.