ABSTRACT
We investigate the photolytic production of two radical intermediates in the reaction of OH with propene, one from addition of the hydroxyl radical to the terminal carbon and the other from addition to the center carbon. In a collision-free environment, we photodissociate a mixture of 1-bromo-2-propanol and 2-bromo-1-propanol at 193 nm to produce these radical intermediates. The data show two primary photolytic processes occur: C-Br photofission and HBr photoelimination. Using a velocity map imaging apparatus, we measured the speed distribution of the recoiling bromine atoms, yielding the distribution of kinetic energies of the nascent C3H6OH radicals + Br. Resolving the velocity distributions of Br((2)P(1/2)) and Br((2)P(3/2)) separately with 2 + 1 REMPI allows us to determine the total (vibrational + rotational) internal energy distribution in the nascent radicals. Using an impulsive model to estimate the rotational energy imparted to the nascent C3H6OH radicals, we predict the percentage of radicals having vibrational energy above and below the lowest dissociation barrier, that to OH + propene; it accurately predicts the measured velocity distribution of the stable C3H6OH radicals. In addition, we use photofragment translational spectroscopy to detect several dissociation products of the unstable C3H6OH radicals: OH + propene, methyl + acetaldehyde, and ethyl + formaldehyde. We also use the angular momenta of the unstable radicals and the tensor of inertia of each to predict the recoil kinetic energy and angular distributions when they dissociate to OH + propene; the prediction gives an excellent fit to the data.
ABSTRACT
This study photolytically generates, from 2-bromoethanol photodissociation, the 2-hydroxyethyl radical intermediate of the OH + ethene reaction and measures the velocity distribution of the stable radicals. We introduce an impulsive model to characterize the partitioning of internal energy in the C(2)H(4)OH fragment. It accounts for zero-point and thermal vibrational motion to determine the vibrational energy distribution of the nascent C(2)H(4)OH radicals and the distribution of total angular momentum, J, as a function of the total recoil kinetic energy imparted in the photodissociation. We render this system useful for the study of the subsequent dissociation of the 2-hydroxyethyl radical to the possible asymptotic channels of the OH + ethene reaction. The competition between these channels depends on the internal energy and the J distribution of the radicals. First, we use velocity map imaging to separately resolve the C(2)H(4)OH + Br((2)P(3/2)) and C(2)H(4)OH + Br((2)P(1/2)) photodissociation channels, allowing us to account for the 10.54 kcal/mol partitioned to the Br((2)P(1/2)) cofragment. We determine an improved resonance enhanced multiphoton ionization (REMPI) line strength for the Br transitions at 233.681 nm (5p (4)P(1/2) <-- 4p (2)P(3/2)) and 234.021 nm (5p (2)S(1/2) <-- 4p (2)P(1/2)) and obtain a spin-orbit branching ratio for Br((2)P(1/2)):Br((2)P(3/2)) of 0.26 +/- 0.03:1. Energy and momentum conservation give the distribution of total internal energy, rotational and vibrational, in the C(2)H(4)OH radicals. Then, using 10.5 eV photoionization, we measure the velocity distribution of the radicals that are stable to subsequent dissociation. The onset of dissociation occurs at internal energies much higher than those predicted by theoretical methods and reflects the significant amount of rotational energy imparted to the C(2)H(4)OH photofragment. Instead of estimating the mean rotational energy with an impulsive model from the equilibrium geometry of 2-bromoethanol, our model explicitly includes weighting over geometries across the quantum wave function with zero, one, and two quanta in the harmonic mode that most strongly alters the exit impact parameter. The model gives a nearly perfect prediction of the measured velocity distribution of stable radicals near the dissociation onset using a G4 prediction of the C-Br bond energy and the dissociation barrier for the OH + ethene channel calculated by Senosiain et al. (J. Phys. Chem. A 2006, 110, 6960). The model also indicates that the excited state dissociation proceeds primarily from a conformer of 2-bromoethanol that is trans across the C-C bond. We discuss the possible extensions of our model and the effect of the radical intermediate's J-distribution on the branching between the OH + ethene product channels.
