The CH(X2Π) + H2O reaction: two transition state kinetics

Abstract

The reaction of ground state methylidyne (CH) with water vapor (H₂O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H–O bond of H₂O over a submerged barrier via a well-skipping mechanism yielding solely H and CH₂O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC—OH₂) and its subsequent isomerization to activated CH₂OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH₂OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10⁻¹⁰ cm³ s⁻¹ at 50 K, followed by a fairly steep decrease down to 8 × 10⁻¹² cm³ s⁻¹ at 900 K, but increasing again to 5 × 10⁻¹¹ cm³ s⁻¹ at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10⁻¹¹ (T/300 K)^−1.615 exp(−38.45/T) cm³ s⁻¹ for T = 50–400 K k(T, P = 0) = 1.15 × 10⁻¹² (T/300 K)^0.8637 exp(892.6/T) cm³ s⁻¹ for T = 400–1000 K k(T, P = 0) = 4.57 × 10⁻¹⁵ (T/300 K)^3.375 exp(3477.4/T) cm³ s⁻¹ for T = 1000–3500 K.