Imaging of Ultrafast Molecular Elimination Reactions Wim G. Roeterdink, Anouk M. Rijs, and Maurice H. M. Janssen* Contribution from the Laser Centre and Department of Chemistry, Vrije UniVersiteit de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands Received August 18, 2005; E-mail: mhmj@chem.vu.nl Abstract: Ultrafast molecular elimination reactions are studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation. A pump laser is used to initiate the dissociative reaction, and after a predetermined time delay a probe laser “interrogates” the molecular system. Ionic fragments are detected with a two-dimensional velocity map imaging detector providing detailed information about the energetic and vectorial properties of mass selected photofragments. In this paper we discuss the ultrafast elimination of molecular iodine, I 2, from IF2C-CF2I, where the iodine atoms originate from neighboring carbon atoms. By varying the femtosecond delay between pump and probe pulse, it is found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. Energetic considerations suggest that the crucial step in this fragmentation process is an electron transfer between the two iodine atoms in the parent molecule, which leads to Coulombic attraction and the creation of an ion-pair state in the molecular iodine fragment. I. Introduction In an elementary chemical reaction, the molecular system evolves continuously through a sequence of intermediate species that are neither reactants nor products, but the former turning into the latter. 1 Much research is focused on unraveling the transition states, because these intermediate states control the details of the reactant-to-product conversion process. One of the most elementary chemical reactions is the dissociation of a molecule, which can readily be triggered using light excitation. Indeed, photodissociation is particular suitable for experimental study, because the chemical reaction can be triggered at a preselected place and time in a highly controlled manner by a well-defined laser pulse. When a molecule photodissociates in the gas phase, its fragments fly apart in space. The trajectories and energies of these fragments are fully determined by the potential energy surfaces involved in the bond breaking process. Very detailed information on photodissociation dynamics has been obtained by mapping the angular distributions of fragments, after the dissociation is induced by a pulsed laser, by the ion imaging technique. 2,3 Most results of this type have been obtained with nanosecond lasers; however, the critical inter- mediate states evolve on a much shorter time scale, i.e. femtoseconds. In this paper, we study the elimination of molecular iodine from C 2 F 4 I 2 employing the femtosecond pump-probe technique in combination with time-of-flight velocity map imaging. Velocity map imaging measures the kinetic energy and the angular distribution of the photoions providing information about the dissociation pathways. 4 When molecular bromine or chlorine adds to an alkene, the alkene polarizes the halogen bond and the reaction is classified as an electrophilic addition of the halogen cation; see Figure 1. In an electrophilic addition reaction, the π electron attacks the positive end of the polarized halogen molecule after which a bridged cation is formed. 5 A second mechanism involves radical formation, and in general there is a competition between the two; see, e.g., ref 6. Both these mechanisms are nonconcerted, i.e., a sequential addition of two halogen atoms. Using molecular beam techniques, the pathways relating reactants to products can be observed in either direction. Thus the two nonconcerted mechanisms have been studied extensively, in reverse, in the form of photodissociation reactions. Nathanson et al. 7 showed that the two-center elimination reaction C 2 F 4 I 2 f C 2 F 4 + I + I is of nonconcerted character by using picosecond excitation of the dissociative A band, in combination with kinetic energy resolved time-of-flight mass spectrometry. The first iodine atom splits off within 200 fs. It takes 25 ps for the second iodine atom to be eliminated, from which a barrier of 0.6 eV was estimated. Zhong et al. 8 studied the elimination of halogen atoms from C 2 F 4 I 2 using femtosecond pulses centered at 277 nm to dissociate the parent molecule. A Resonance-Enhanced Multi- Photon Ionization (REMPI) scheme centered at 304 nm detect- ing the I atomic fragment was used in combination with kinetic energy resolved time-of-flight (TOF) spectroscopy. The recoil anisotropy was determined using polarized femtosecond pulses. These measurements conclusively excluded the possibility of breaking the two C-I bonds concertedly after excitation to the dissociative A band. This is in contrast to photodissociation at 248 nm of C 2 H 4 Br 2 , where an asynchronous concerted mech- anism was observed. 9 The structural dynamics of transient (1) Zewail, A. H. J. Phys. Chem. A 2000, 104, 5660. (2) Chandler, D. W.; Houston, P. L. J. Chem. Phys. 1987, 87, 1145. (3) Rakitzis, T. P.; van den Brom, A. J.; Janssen, M. H. M. Science 2004, 303, 1852. (4) Eppink, A. T. J. B.; Parker, D. H. ReV. Sci. Instrum. 1997, 68, 3477. (5) McMurry, J. Organic Chemistry; Brooks/Cole Publishing: 1992. (6) Yang, Z. Y. J. Org. Chem. 2003, 68, 5419. (7) Nathanson, G. M.; Minton, T. K.; Shane, S. F.; Lee, Y. T. J. Chem. Phys. 1989, 90, 6157. (8) Zhong, D.; Ahmad, S.; Zewail, A. H. J. Am. Chem. Soc. 1997, 119, 5978. (9) Lee, Y. R.; Chen, C. C.; Lin, S. J. Chem. Phys. 2003, 118, 10494. Published on Web 12/15/2005 576 9 J. AM. CHEM. SOC. 2006, 128, 576-580 10.1021/ja055658x CCC: $33.50 © 2006 American Chemical Society