Exploring the features on the OH + SO₂ potential energy surface using theory and testing its accuracy by comparison...

Area of Science:

• Atmospheric chemistry
• Chemical kinetics
• Theoretical chemistry

Background:

• The reaction between hydroxyl radical (OH) and sulfur dioxide (SO2) is crucial for atmospheric chemistry.
• Understanding the reaction mechanism, including pre-reaction complexes and transition states, is essential for accurate atmospheric modeling.

Purpose of the Study:

• To identify and characterize the pre-reaction complex in the OH + SO2 reaction.
• To investigate the influence of this complex on the reaction kinetics using theoretical methods.
• To refine understanding of the reaction mechanism and thermodynamic properties.

Main Methods:

• Ab initio quantum chemical calculations were employed to determine the binding energy of the pre-reaction complex.
• Reaction rate theory, utilizing the MESMER master equation package, was applied to simulate the reaction kinetics.
• Experimental kinetic data were used to fit theoretical models and validate findings.

Main Results:

• The binding energy of the pre-reaction complex was calculated to be 7.2 kJ mol⁻¹.
• Simulations indicated that the pre-reaction complex has a negligible impact on the overall reaction kinetics above 250 K.
• The reaction kinetics are predominantly controlled by the inner transition-state leading to HOSO2 formation.
• Fitting to experimental data revealed a submerged inner transition state with a barrier of -0.25 kJ mol⁻¹.
• The enthalpy of reaction (ΔR1H0K) was determined to be -(109 ± 5.6) kJ mol⁻¹.
• A refined expression for the high-pressure rate constant, k∞1(T) = (5.95 ± 0.83) × 10⁻¹³ × (T/298)⁻⁰.¹¹±⁰.²⁷ cm³ molecule⁻¹ s⁻¹, was derived.

Conclusions:

• The pre-reaction complex in the OH + SO2 reaction does not significantly influence thermal kinetics.
• The inner transition-state is the rate-determining factor for this reaction.
• The pre-reaction complex may play a role in energy transfer from vibrationally excited OH radicals.
• The study provides a more accurate kinetic expression and thermodynamic data for the OH + SO2 reaction.