Effect of ammonia and water molecule on OH + CH3OH reaction under tropospheric condition

Abstract

The rate coefficients for OH + CH₃OH and OH + CH₃OH (+ X) (X = NH₃, H₂O) reactions were calculated using microcanonical, and canonical variational transition state theory (CVT) between 200 and 400 K based on potential energy surface constructed using CCSD(T)//M06-2X/6-311++G(3df,3pd). The results show that OH + CH₃OH is dominated by the hydrogen atoms abstraction from CH₃ position in both free and ammonia/water catalyzed ones. This result is in consistent with previous experimental and theoretical studies. The calculated rate coefficient for the OH + CH₃OH (8.8 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹), for OH + CH₃OH (+ NH₃) [1.9 × 10⁻²¹ cm³ molecule⁻¹ s⁻¹] and for OH + CH₃OH (+ H₂O) [8.1 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹] at 300 K. The rate coefficient is at least 8 order magnitude [for OH + CH₃OH(+ NH₃) reaction] and 3 orders magnitude [OH + CH₃OH (+ H₂O)] are smaller than free OH + CH₃OH reaction. Our calculations predict that the catalytic effect of single ammonia and water molecule on OH + CH₃OH reaction has no effect under tropospheric conditions because the dominated ammonia and water-assisted reaction depends on ammonia and water concentration, respectively. As a result, the total effective reaction rate coefficients are smaller. The current study provides a comprehensive example of how basic and neutral catalysts effect the most important atmospheric prototype alcohol reactions.