DEUTERIUM-ISOTOPE FRACTIONATION WITHIN PROTONATED WATER CLUSTERS IN THE GAS-PHASE

Ion product distributions have been analyzed for the collision-activated loss of H2O, HOD, or D2O from the water clusters (L2O)nL+ (L = H, D; = 2-4). The ionic products of collision-induced dissociation (CID) are observed to be depleted in deuterium with respect to the statistical product distributions predicted for complete randomization of H and D. The measured isotope distributions in the CID product ions are independent of collision energy within experimental error, suggesting that the observed depletion of deuterium is not the result of a kinetic effect in the unimolecular decomposition, but rather a reflection of the individual cluster structures. An equilibrium isotope effect model is proposed wherein the deuterium in the cluster preferentially migrates to the peripheral positions and localizes on the neutral water molecules in the solvent shell, rather than occupying sites in the cluster ion core, such as those on the core hydronium (lyonium) ion. Deuterium enrichment in the neutral water component of each cluster ion results in enhanced loss of deuterated neutral water upon collisional activation. The present isotope fractionation results are compared with literature data for bimolecular gas-phase H/D-exchange reactions and with condensed-phase isotope fractionation data. The observation of isotope fractionation as an equilibrium effect in stabilized gas-phase water cluster ions suggests that isotope fractionation in the bimolecular reactions between (H2O)nH+ ions and D2O results primarily from a nonstatistical distribution of hydrogen and deuterium in the transient reaction intermediates. © 1990, American Chemical Society. All rights reserved.