At low temperatures, the reaction of OH + ethene proceeds through an addition mechanism of the hydroxyl radical to the double bond of the ethene, forming the  CH2CH2OH adduct. Adducts with sufficient energy can undergo subsequent isomerization or dissociation reactions. In an effort to study the barrier heights and branching ratios for these reaction pathways, we generate the CH2CH2OH radical from the photodissociation of 2-bromoethanol and use a velocity map imaging apparatus to measure the velocity distributions of the radical and its momentum-matched bromine co-fragment. The bromine atoms are state-selectively photoionized and detected with resonance enhanced multiphoton ionization. Using the measured bromine atom velocity distribution and conservation of momentum, we calculate the velocity energy distribution of all nascent CH2CH2OH radicals. Stable radicals are detected using single photon photoionization at 118 nm. Comparison of the total recoil translational energy distribution for all nascent CH2CH2OH + Br to that for stable CH2CH2OH + Br shows that the two distributions diverge at 42 kcal/mol.  Conservation of energy allows the calculation of an internal energy distribution of the radicals, but a significant portion of the internal energy is partitioned into rotational energy and thus ineffective in surmounting  energetic barriers. We have developed an impulsive model to predict the distribution of rotational energy imparted to the  CH2CH2OH radicals upon C-Br bond fission. The model considers the range of impact parameters accessed by the photolytic precursor as it vibrates along a certain normal mode of vibration. We test our model by  predicting the fraction of the C-Br bond fission events across the entire translational energy distribution that produce radicals with vibrational energy below the theoretically predicted barrier calculated by Senosiain et al (J. Phys. Chem. A 2006, 110, 6960),  then comparing it to the experimentally determined distribution. The excellent  agreement suggests that our model is an accurate method for representing the rotational energy partitioned to the radical upon dissociation. We derive a vibrational energy distribution of nascent CH2CH2OH radicals that spans the barriers to  subsequent reactions.

Modeling the rovibrationally excited CH<sub>2</sub>CH<sub>2</sub>OH radicals from the dissociation of 2-bromoethanol at 193nm

240th National Meeting (2010)

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Modeling the rovibrationally excited CH2CH2OH radicals from the dissociation of 2-bromoethanol at 193nm

Modeling the rovibrationally excited CH₂CH₂OH radicals from the dissociation of 2-bromoethanol at 193nm