Can a single water molecule catalyze the OH+CH2CH2 and OH+CH2O reactions?

Abstract The reaction of OH radicals with ethylene (CH 2 CH 2 ) and formaldehyde (CH 2 O) molecules with and without water have been investigated using ab initio /DFT potential energy surfaces (PESs) at CCSD(T)/aug-cc-pVTZ//BHandHLYP//aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways OH + CH 2 X···H 2 O (X = CH 2 , O) and CH 2 X + H 2 O⋯HO were calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction. The kinetic results show that OH radical adds to C=C bond in CH 2 CH 2 , and abstract the hydrogen atoms from CH 2 O, similar to its isoelectronic analogous OH + CH 2 NH (+H 2 O) reaction. The catalytic effect of a single water molecule on OH + CH 2 X (X = CH 2 , O) reaction system shows that the initial water complexation step is essential in the rate coefficients calculation. The calculated rate coefficient for the OH + CH 2 CH 2 (+H 2 O) reaction at 300K is 6 × 10 −16  cm 3 molecule −1 s −1 and for OH + CH 2 O(+H 2 O) reaction at 300K is 8.1 × 10 −14  cm 3 molecule −1 s −1 . The rate coefficient for OH + CH 2 CH 2 (+H 2 O) reaction is at least two orders of magnitude smaller than OH + CH 2 NH(+H 2 O) reaction (at 300K is 5.1 × 10 −14  cm 3 molecule −1 s −1 ) and rate coefficient for OH + CH 2 O reaction is in good agreement with OH + CH 2 NH reaction. In general, the rate coefficients for OH + CH 2 X (X = CH 2 , O)⋯H 2 O and for CH 2 X + H 2 O⋯HO reactions are ∼3–4 orders of magnitude smaller than reaction without water molecule. Our results predict that catalytic effect of single water molecule on OH + CH 2 CH 2 and OH + CH 2 O reactions can make a negligible contribution to the gas phase removal of CH 2 CH 2 and CH 2 O by OH radicals because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller. The present results provide a better understanding of gas phase catalytic effect of a water molecule on the most important atmospheric and combustion reaction prototypes.