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

The reaction of OH radicals with ethylene (CH2CH2) and formaldehyde (CH2O) 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 + CH2X center dot center dot center dot H2O (X = CH2, O) and CH2X + H2O center dot center dot center dot 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 CH2CH2, and abstract the hydrogen atoms from CH2O, similar to its isoelectronic analogous OH + CH2NH (+ H2O) reaction. The catalytic effect of a single water molecule on OH + CH2X (X = CH2, 0) reaction system shows that the initial water complexation step is essential in the rate coefficients calculation. The calculated rate coefficient for the OH + CH2CH2(+H2O) reaction at 300K is 6 x 10(-16) ems molecule(-1) s(-1) and for OH + CH2O(+H2O) reaction at 300K is 8.1 x 10(-14) cm(3) molecule(-1) s(-1). The rate coefficient for OH + CH2CH2(+ H2O) reaction is at least two orders of magnitude smaller than OH + CH2NH(+H2O) reaction (at 300K is 5.1 x 10(-14) cm(3) molecule(-1) s(-1)) and rate coefficient for OH + CH2O reaction is in good agreement with OH + CH2NH reaction. In general, the rate coefficients for OH + CH2X (X = CH2, O)center dot center dot center dot H2O and for CH2X + H2O center dot center dot center dot HO reactions are similar to 3-4 orders of magnitude smaller than reaction without water molecule. Our results predict that catalytic effect of single water molecule on OH + CH2CH2 and OH + CH2O reactions can make a negligible contribution to the gas phase removal of CH2CH2 and CH2O 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.