The Reaction Rate Constant of Chlorine Nitrate Hydrolysis

Abstract

The first-order rate constant for the decomposition of chlorine nitrate (ClONO2) by water in a cyclic 1:3 complex at stratospheric temperatures is shown to be close to the values for the hydrolysis rate coefficient of chlorine nitrate on an ice surface determined in the laboratory. On the other hand the rate constants calculated for the cyclic 1:1 and 1:2 complexes are much lower than the experimental results. From the mechanistic point of view the reaction is found to be similar to a SN2 mechanism and coupled with water-mediated proton transfer in accordance with the intriguing findings of Bianco and Hynes [R. Bianco, J. T. Hynes. J. Phys. Chem. A 1998, 102, 309-314]. The function of additional water molecules is to act as a catalyst, that is, to accelerate the hydrolysis process. Quantum-mechanical tunneling is negligible above 125 K in the 1:3 complex and above 175 K in the 1:2 complex. At temperatures below these limits all involved protons tunnel through the barrier at energies at least 5 kcalmol(-1) below the barrier-top in a concerted, but asynchronous manner.

Toward elimination of discrepancies between theory and experiment: The gas-phase reaction of N2O5 with H2O

“…Thus, information on the potential energy surface beyond the barrier height is required. It turned out that tunneling can increase the reaction rate by many orders of magnitude [60][61][62][63] if there is at least one proton transfer involved. Therefore we determined tunneling contributions on the reaction rates for the described reactions.…”

Modeling the heterogeneous reaction probability for chlorine nitrate hydrolysis on ice

“…Figure 1 shows the chemical reaction mechanism, denoted by n = 4, that was found on minimizing the electronic energy as a function of the positions of the nuclei for a system containing four water molecules and one molecule of chlorine nitrate (as implemented by Frisch et al [1998]). In comparison to the molecular systems studied previously by two of us (see Figure 1, n = 1 to n = 3) [ Loerting and Liedl , 2001], there is one crucial difference: The reaction barrier is considerably lower, and amounts to 5.61 kcal mol −1 (G2MP2 level of theory), which is a reduction of the barrier by 14.34 kcal mol −1 as compared to the system with just one molecule of water less ( n = 3). This reaction barrier agrees well with the best estimate given in the literature of 3–7 kcal mol −1 [ Tabazadeh et al , 1994].…”

“…In particular G2(MP2) is employed to calculate the reaction barrier, and B3LYP/6‐31+G(d) energies along the minimum energy path are interpolated to the G2(MP2) barrier. The details for the calculation on the full‐dimensional PES (45 vibrational degrees of freedom for the n = 4 system) can be found elsewhere [ Loerting and Liedl , 2001]. In brief, k rxn ( T ) (in s −1 ) is calculated from quantum mechanical variational transition state theory [ Chuang et al , 1999; Corchado et al , 1999; Frisch et al , 1998]: where κ( T ) is the enhancement factor of the reaction due to quantum mechanical tunnelling along all possible directions of the PES (of course dominated by contributions corresponding to proton motion) and Δ G rxn ‡ ( T ) is the free energy barrier approximated by the zero‐point corrected energy barrier along the minimum energy path (MEP) on the PES [ Truhlar and Garrett , 1984; Truhlar et al , 1985; Truhlar and Gordon , 1990; Truhlar , 1995].…”

“…In particular G2(MP2) is employed to calculate the reaction barrier, and B3LYP/6-31+G(d) energies along the minimum energy path are interpolated to the G2(MP2) barrier. The details for the calculation on the full-dimensional PES (45 vibrational degrees of freedom for the n = 4 system) can be found elsewhere [Loerting and Liedl, 2001]. In brief, k rxn (T) (in s À1 ) is calculated from quantum mechanical variational transition state theory [Chuang et al, 1999;Corchado et al, 1999;Frisch et al, 1998]:…”

“…In the following years the hydrolysis has been studied extensively using various experimental techniques, e.g., flow tubes coupled to mass spectrometers, and employing different surfaces, e.g., nitric acid hydrates [ Moore et al , 1990; Abbatt and Molina , 1992; Molina et al , 1993; Zondlo et al , 1998], sulfuric acid hydrates [ Tolbert et al , 1988; Molina et al , 1993; Zhang et al , 1994], supercooled ternary solutions of H 2 SO 4 /HNO 3 /H 2 O [ Molina et al , 1993; Zhang et al , 1995; Del Negro et al , 1997], vapor‐deposited ice [ Leu et al , 1991; Berland et al , 1997], smooth ice [ Lee et al , 1999], and sodium chloride [see, e.g., Finlayson‐Pitts et al , 1989] or surfaces obtained from vapor codeposition on substrates [ Koch et al , 1997]. Concomitantly, also a lot of theoretical work has been done to resolve important details about the reaction mechanism [ Ying and Zhao , 1997; Bianco and Hynes , 1998; Bianco et al , 1998; McNamara and Hillier , 1999; McNamara et al , 1999; Xu and Zhao , 1999; McNamara and Hillier , 2000; Bianco et al , 2001; McNamara and Hillier , 2001; Loerting and Liedl , 2001; Bianco and Hynes , 2006]. These studies involved mainly molecular dynamics simulations and gas‐phase cluster calculations by employing density functional theory or ab initio methods such as Møller‐Plesset perturbation theory (MP2).…”