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 ClNO2 product yield in competition with hydrolysis following N2O5 uptake to aqueous NaCl solutions. For NaCl-D2O solutions spanning 0.0054 to 0.21 M, the ClNO2 product yield decreases on average by only 5% from 5 to 25 ˚C. Less reproducible measurements at 0.54 and 2.4 M NaCl also fall within this range. The ratio of the rate constants for chlorination and hydrolysis of N2O5 in D2O is determined to be 1147 ± 65 at 25 °C, favoring chlorination. An Arrhenius analysis reveals that the activation energy for hydrolysis is just 3.0 ± 1.8 kJ/mol larger than for chlorination. In combination with the measured pre-exponential ratio favoring chlorination of 419 (-215) (+542), we conclude that the strong preference of N2O5 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 N2O5. 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.