Article pubs.acs.org/JPCA Radical Intermediates in the Addition of OH to Propene: Photolytic Precursors and Angular Momentum Effects M. D. Brynteson,† C. C. Womack,† R. S. Booth,† S. -H. Lee,‡ J. J. Lin,§ and L. J. Butler†,* †Department of Chemistry and the James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States ‡National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China §Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan, Republic of China *S Supporting Information 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 2 2 radicals + Br. Resolving the velocity distributions of Br( P1/2) and Br( P3/2) 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. ■ INTRODUCTION In our study of the reaction between OH and propene, we The oxidation of unsaturated hydrocarbons by the hydroxyl directly generate the radical intermediates formed upon radical is an important reaction in both atmospheric and com- addition of the hydroxyl radical by photodissociating two bustion chemistry. Our experiment focuses on the reaction halogenated precursors. Using 193 nm radiation, photolysis of a mixture of 2-bromo-1-propanol and 1-bromo-2-propanol of OH with propene, a simple unsaturated hydrocarbon with fi two sites at which addition of the OH may occur. To date, results in C−Br bond ssion, yielding the radical intermediates many studies on the reaction rate over a wide range of tem- of interest. These intermediates are formed in a collision-free peratures have been performed on this system, both experi- environment with a range of vibrational and rotational energies. mental1−31 and theoretical.32−34 These studies have shown that This results in a portion of radical intermediates stable to the addition of OH to propene dominates at temperatures further dissociation and a portion having enough energy to below 500 K. Beyond 500 K, hydrogen abstraction becomes an surmount various barriers to dissociation, the lowest of which is important part of the mechanism and begins to dominate at the barrier to OH and propene. much higher temperatures. In addition, ethene and propene We present results of two predictive models, one for treating flame studies have given evidence for the presence of enols, the rotational energy in the nascent radicals and one for which has increased interest in the product branching resulting estimating the energy imparted to relative kinetic energy when 35,36 the radicals dissociate to OH + propene. The first model is an from OH-initiated oxidation. ff 42 43 The addition of the hydroxyl radical to propene offers a extension of one used by Ratli et al. and Womack et al., which is used to predict the rotational energy imparted to the wider variety of products than the addition of OH to ethene as fi the addition to propene may occur via two pathways: addition nascent radicals from the C−Br photo ssion. As in the prior to the terminal carbon and addition to the center carbon. work, the model uses the measured recoil kinetic energy distribu- Experimental results show the branching between the center tion and the geometry of the precursor in the Franck−Condon and terminal carbon addition favors the terminal carbon, with approximately 65−75% of additions leading to terminal carbon Received: November 5, 2013 addition.37−39 Theory has given similar results, yielding Revised: March 6, 2014 predictions of ∼65% of additions to the terminal carbon.40,41 Published: April 23, 2014 © 2014 American Chemical Society 3211 dx.doi.org/10.1021/jp4108987 | J. Phys. Chem. A 2014, 118, 3211−3229 The Journal of Physical Chemistry A Article region of the repulsive excited state to estimate the distribu- fundamental in a β-barium borate crystal to produce vertically tion of angular momenta in the nascent radicals. We use an polarized photons at the REMPI wavelength. After focusing extension44 of this model that uses the full inertia tensor to with a 25.4 cm focusing lens, this light crossed the molecular correctly account for the change in rotational energy of the beam at a right angle in the main chamber of the imaging nascent radicals as they progress along the dissociation reaction apparatus. We determine the spin−orbit branching ratio, 2 2 coordinate. This allows us to predict which radicals have N[Br( P1/2)]/N[Br( P3/2)], by integrating the total ion signal 2 enough internal energy at the centrifugally corrected transition from Br atoms in each spin−orbit state, S[Br( P1/2)] and 2 state to dissociate to OH + propene and which radicals do not. S[Br( P3/2)], in images accumulated while scanning the The second model, developed herein, builds on the previous Doppler profiles ±0.008 nm from line center and weighting model and uses the angular momentum in the radicals to this result by the experimental REMPI line strength, k = 0.32 ± predict the resulting speed and angular distributions of the OH 0.02,42 as shown in eq 1. The experimental conditions for and propene fragments. These models are shown to be in good REMPI detection of the bromine atoms (i.e., laser power, focal agreement with experimental results and yield useful length) were the same as those in ref 42. information about the energy partitioned between rotation 2 2 and vibration in the radicals generated by our photolysis N[Br( P1/2 )] S[Br( P1/2 )] method and the energy partitioned to relative kinetic energy 2 = k 2 N[Br( P )] S[Br( P )] (1) when those nascent radicals dissociate. 3/2 3/2 In addition to our characterization of the above energy To detect propene from the major dissociation channel of partitioning, we present evidence for three of the radicals’ the C3H6OH radicals, we use 10.5 eV ionization generated dissociation channels: OH + propene, methyl + acetaldehyde, by tripling the 355 nm output of a pulsed Nd:YAG laser and ethyl + formaldehyde. The calculated barrier heights to (Continuum Surelite I-20). The 355 nm light passed through a these dissociation channels suggest they are the most likely, and beam expander (focal length = 150 mm and focal length = fl − our time-of- ight spectra and speed distributions of these 300 mm at 588 nm) and then two lenses to focus the light into fragments evidence momentum-matched products for each of a 21 cm low-pressure gas cell filled with 22 Torr of high purity these dissociation channels. Our spectra also evidence close-to- Xe (>99.995%). The gas cell, mounted on the main vacuum symmetric forward−backward scattering which indicates the chamber, ends with a MgF2 lens (focal length = 120.3 mm at rotational period of the radical intermediates is much shorter 193 nm) that served as the barrier between the cell and the than their lifetime. chamber. This lens recollimated the 355 nm light while focusing the 118 nm light. ■ EXPERIMENTAL METHODS Following photodissociation and photoionization, the 1. Velocity Map Imaging. This work uses a 2-D velocity electrostatic lens optics with repeller and extractor voltages in map imaging apparatus, described previously,45−48 to detect the a ratio of 1.404:1 (2000 and 1424 V for Br atoms and 3932 and speed distributions of Br atoms, m/z = 42 fragments, and 2800 V for the detection of m/z = 42 and m/z = 31 fragments) m/z = 31 fragments. A supersonic beam of the 70/30 mixture accelerated the spherically expanding ions down a ∼577 mm of 1-bromo-2-propanol and 2-bromo-1-propanol was created by grounded time-of-flight tube toward the detector. The Burle seeding the vapor pressure of the liquid sample (∼7 Torr) in 3040FM detector is a position sensitive Chevron microchannel He to a concentration of ∼1.4%. (The individual brominated plate (MCP) assembly coupled to a P20 phosphor screen. An isomers are not commercially available in pure form, nor can 80 ns, −750 V pulse on the front plate of the MCP gates the the 70/30 mixture be separated with distillation.) At a total ions based on arrival time. A cooled charge-coupled device stagnation pressure of 500 Torr, the molecular beam was (CCD) camera (La Vision Imager 3) with a standard 35 mm supersonically expanded through a heated General Valve Iota lens recorded the images of the ions appearing on the phosphor One pulsed valve with an orifice diameter of 0.8 mm and a screen, which remained at 3.3 kV above the potential of the rear temperature of 80 °C.