Dissociative recombination of protonated methanol

Abstract

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH₃OH₂⁺ and CD₃OD₂⁺ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH₃OH₂⁺ and CD₃OD₂⁺, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C–O bond, the major process being the three-body break-up forming CH₃, OH and H (CD₃, OD and D). The overall cross sections are best fitted by σ = 1.2 ± 0.1 × 10⁻¹⁵E^{−1.15±0.02} cm² and σ = 9.6 ± 0.9 × 10⁻¹⁶E^{−1.20±0.02} cm² for CH₃OH₂⁺ and CD₃OD₂⁺, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 ± 0.9 × 10⁻⁷ (T/300)^{−0.59±0.02} cm³ s⁻¹ (CH₃OH₂⁺) and k(T) = 9.1 ± 0.9 × 10⁻⁷ (T/300)^{−0.63±0.02} cm³ s⁻¹(CD₃OD₂⁺) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH₃⁺ and H₂O and subsequent dissociative recombination of the resulting CH₃OH₂⁺ ion to yield methanol and hydrogen atoms is therefore very unlikely.