Evidence for concerted ring opening and C–Br bond breaking in UV-excited bromocyclopropane

Abstract

Photodissociation of gaseous bromocyclopropane via its A-band continuum has been studied at excitation wavelengths ranging from 230 nm to 267 nm. Velocity-map images of ground-state bromine atoms (Br), spin-orbit excited bromine atoms (Br(∗)), and C3H5 hydrocarbon radicals reveal the kinetic energies of these various photofragments. Both Br and Br(∗) atoms are predominantly generated via repulsive excited electronic states in a prompt photodissociation process in which the hydrocarbon co-fragment is a cyclopropyl radical. However, the images obtained at the mass of the hydrocarbon radical fragment identify a channel with total kinetic energy greater than that deduced from the Br and Br(∗) images, and with a kinetic energy distribution that exceeds the energetic limit for Br + cyclopropyl radical products. The velocity-map images of these C3H5 fragments have lower angular anisotropies than measured for Br and Br(∗), indicating molecular restructuring during dissociation. The high kinetic energy C3H5 signals are assigned to allyl radicals generated by a minor photochemical pathway which involves concerted C-Br bond dissociation and cyclopropyl ring-opening following single ultraviolet (UV)-photon absorption. Slow photofragments also contribute to the velocity map images obtained at the C3H5 radical mass, but the corresponding slow Br atoms are not observed. These features in the images are attributed to C3H5 (+) from the photodissociation of the C3H5Br(+) molecular cation following two-photon ionization of the parent compound. This assignment is confirmed by 118-nm vacuum ultraviolet ionization studies that prepare the molecular cation in its ground electronic state prior to UV photodissociation.

Multi-mass velocity-map imaging studies of photoinduced and electron-induced chemistry

“…One of the most important outcomes of this study is that it demonstrates the ability of VMI to provide insight into 'real' chemical reactions of interest to a wide audience of chemists. Multimass VMI has since been used to investigate ring-opening dynamics in bromocyclopropane 43 and 2-bromothiophene 44 and the UV photodissociation dynamics of dimethylsulphide 45 , and will no doubt be used increasingly to perform detailed mechanistic studies on reactions of interest in synthetic organic photochemistry.…”

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

“…The probe laser setup has been discussed in detail previously. 40 A time delay of 10-20 ns was set between pump and probe pulses to minimize unwanted background signals. The typical probe laser energy was o1 mJ per pulse during Br/Br* EBB-44 c a The 5-C channel is ignored because of a higher activation energy (see Section 4.1).…”

Dissociative Ionization and Coulomb Explosion of Molecular Bromocyclopropane in an Intense Femtosecond Laser Field

“…S. Pandit et.al. studied the photodissociation of bromocyclopropane in its A-band by the violate excitation (230–267 nm) and proposed three dissociative pathways, in which the C–Br bond fission and the cyclopropyl ring opening are involved [13]. However, the ionization and dissociation of molecular BCP is rarely investigated under the intense femtosecond laser field.…”