Weak Temperature Dependence of the Relative Rates of Chlorination and Hydrolysis of N 2 O 5 in NaCl-Water Solutions.

We have measured the temperature dependence of the ClNO 2 product yield in competition with hydrolysis following N 2 O 5 uptake to aqueous NaCl solutions. For NaCl-D 2 O solutions spanning 0.0054-0.21 M, the ClNO 2 product yield decreases on average by only 4 ± 3% from 5 to 25 °C. Less reproducible measurements at 0.54-2.4 M NaCl also fall within this range. The ratio of the rate constants for chlorination and hydrolysis of N 2 O 5 in D 2 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 - and D 2 O attack on N 2 O 5 has similar energetic barriers despite the differences in charge and complexity of these reactants. In combination with the measured preexponential ratio favoring chlorination of 300 -200 +400 , we conclude that the strong preference of N 2 O 5 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 - and the hydrophobically solvated N 2 O 5 . 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.