Theoretical study of the molecular properties and the formation kinetics of the FS(O₂)OCO radical

Abstract

Molecular and kinetics properties of the FS(O₂)OCO radical have been studied. Equilibrium structure, harmonic vibrational frequencies, conformational mobilities, enthalpy of formation of this radical and the energetic of the reaction FSO₃ + CO → FS(O₂)OCO have been investigated at different levels of the density functional theory and of the Gaussian composite models. The standard enthalpy of formation for FS(O₂)OCO is predicted to be ΔH_f,298 = −155.0 kcal mol⁻¹. In very good agreement with reported experimental values, a rate coefficient at 296 K of 3.6 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ and an activation energy of 7.6 kcal mol⁻¹ have been calculated.

Introduction

Kinetic studies of fluorosulfate radical FSO₃ reactions have received a considerable deal of attention. Early steady-state mechanistic studies [1], [2], [3] were more recently followed by theoretical and real-time resolved investigations [4], [5], [6], [7], [8], [9], [10]. From another point of view, the interest for the spectroscopy of this radical has been recently renewed. In fact, the first systematic spectroscopic studies of the vibrational, rotational and electronic spectra of FSO₃[11] have been very recently improved by more detailed and sophisticated studies [12], [13]. In addition, ozone depleting catalytic cycles with the participation of FSO, FSO₂ and FSO₃ radicals might be operating in rich SO₂ industrialized regions, active volcanic areas and in the marine biosphere [14].

A number of recombination reactions of the FSO₃ radical with F [4], FSO₃[5], [7], Cl [6], FC(O)O [8], [9], CO [8], [9] and FSO₂[10] have been experimentally and theoretically investigated. In addition, indirect experimental data for the reaction (1) have been reported in Refs. [2], [3].

$${\text{FSO}}_3+\text{CO}\to \text{FS}({\text{O}}_2)\text{OCO}$$

Continuing with these studies, a theoretical analysis of both, the FS(O₂)OCO radical properties and its reaction kinetics is presented here.

The activated process (1) has been postulated to explain the mechanism of the thermal reaction between the peroxide FS(O₂)OO(O₂)SF and CO over the 263–298 K temperature range [2]. Under these conditions, the established FS(O₂)OO(O₂)SF ⇔ 2FSO₃ equilibrium [1], [5] provides FSO₃ radicals, being the reaction products exclusively FS(O₂)O(O₂)SF and CO₂. The analysis of the reaction mechanism leads to an activation energy for reaction (1) of (7 ± 1) kcal mol⁻¹. In the presence of O₂, the catalytic oxidation of CO is operative and the only reaction product is CO₂[3]. More recently, reaction (1) has been directly observed in laser flash photolysis studies of FS(O₂)OF in the presence of CO and O₂ at 296 K [8], [9]. Besides of the determination of the rate coefficient for the formation of the novel peroxide FC(O)OO(O₂)SF from FSO₃ and FC(O)O radicals, a limiting high pressure rate coefficient for reaction (1) of k_∞,1 = (4.3 ± 0.9) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ was measured. In qualitative agreement with the experiments of Ref. [2], this small value suggests the presence of an activated process. The potential importance of FS(O₂)OCO radical in atmospheric chemistry is mainly related to the relevance of the afore-mentioned ozone destruction cycle induced by FSO_x (x = 1–3) radicals [14].

This Letter is concerned with a quantum-chemical and kinetic study of FS(O₂)OCO radical. In particular, reliable molecular structures, conformational mobilities, harmonic vibrational frequencies, the enthalpy of formation and the rate coefficient for reaction (1) have been calculated by using recent formulations of the density functional theory (DFT), high-level composite model chemistries and transition state theory.