Temperature Dependence of the Relative Rates of Chlorination and Hydrolysis of N2O5 in NaCl-Water Solutions.

Abstract

We have measured the temperature dependence of the ClNO<sub>2</sub> product yield in competition with hydrolysis following N<sub>2</sub>O<sub>5</sub> uptake to aqueous NaCl solutions. For NaCl-D<sub>2</sub>O solutions spanning 0.0054 M to 0.21 M, the ClNO<sub>2</sub> product yield decreases on average by only 43 % from 5 to 25 C. Less reproducible measurements at 0.54 M and 2.4 M NaCl also fall within this range. The ratio of the rate constants for chlorination and hydrolysis of N<sub>2</sub>O<sub>5</sub> in D<sub>2</sub>O is determined on average to be 1150±90 at 25 C up to 0.21 M NaCl, favoring chlorination. This ratio is observed to decrease significantly at the two highest concentrations. An Arrhenius analysis reveals that the activation energy for hydrolysis is just 3.0±1.5 kJ/mol larger than for chlorination up to 0.21 M, indicating that Cl<sup>-</sup> and D<sub>2</sub>O attack on N<sub>2</sub>O<sub>5</sub> have similar energetic barriers despite the differences in charge and complexity of these reactants. In combination with the measured pre-exponential ratio favoring chlorination of {300\ }_{-200}^{+400}, we conclude that the strong preference of N<sub>2</sub>O<sub>5</sub> to undergo chlorination over hydrolysis is driven by dynamic and entropic, rather than enthalpic, factors. Molecular dynamics simulations elucidate the distinct solvation between strongly hydrated Cl<sup>-</sup> and the hydrophobically solvated N<sub>2</sub>O<sub>5</sub>. Combining this molecular picture with the Arrhenius analysis implicates the role of water in mediating interactions between such distinctly solvated species and suggests a role for diffusion limitations on the chlorination reaction.