Experimental and theoretical studies of atomic oxygen reactions with terminal alkenes: Relevance of the formaldehyde product channel for atmospheric organic aerosol growth

The primary products and branching ratios of the combustion and atmosphere relevant reactions of O(3P) with alkenes are not easy to guess because intersystem crossing (ISC) from the triplet to the underlying singlet potential energy surface (PES) can occur, opening up other reaction channels not accessible on the triplet PES. For this reason, we have undertaken a systematic experimental investigation of this class of multichannel nonadiabatic reactions by the crossed molecular beam technique with mass spectrometric detection (CMB-MS), exploiting soft electron ionization. The capabilities of this technique have been illustrated in the recent study of the O+C2H4 reaction where ISC was found to be very efficient and to account for about 50% of the reaction products (5 competing channels were characterized). The experimental results were validated by accurate quasiclassical trajectory-surface hopping calculations on reliable, global, coupled ab initio PES. The reactions with higher alkenes (propene and 1-butene) are more complex because of the presence of additional alkyl groups. For instance, in O+C3H6 there are 9 possible reaction channels, while for the reaction O+C3H8 there are 11 possible channels (without counting isomeric products). We have studied the reactions of O(3P) with propene and 1-butene by combining the CMB-MS method with high-level ab initio electronic structure calculations of the underlying triplet/singlet PES and statistical computations of branching ratios, including ISC, as a function of temperature and pressure. We have found that the reactive interaction of O(3P) with these terminal alkenes mainly breaks apart the carbon atom chain, producing a variety of both radical and molecular product (for instance, in O+propene, CH3+CH2CHO (32%), C2H5+HCO (9%), C2H4/3CHCH3+H2CO (44%), with the fraction of formaldehyde channel being very significant also in O+ethene and O+1-butene). The observation that the O(3P) reactions with terminal alkenes lead to significant formation of formaldehyde, an important pollutant, is an unexpected result that can have practical implications in combustion as well as atmosphere chemistry. In particular, this new H2CO formation route can contribute significantly to the amount of small oxidized organics containing carbonyl functional groups that are commonly present in atmospheric secondary organic aerosols.