Ultraviolet photodissociation dynamics of the benzyl radical

Abstract

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, <f(T)>, is ∼0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

Benzyl Radical Photodissociation Dynamics at 248 nm

“…The existence of an H loss channel from benzyl has been ascertained by shock tube studies, photodissociation studies, and thermolysis studies. 35,36,39,40,62,63 Although the other fragment measured in this experiment at m/z = 90 has not been unambiguously identified, theoretical work has suggested that it is most likely the fulvenallene molecule. 7,8,41,42 The identification of an H loss channel is corroborated here by the agreement between the Newton diagram in Figure 7 with the angular distribution of signal and by the excellent agreement between the P(E T ) distribution from Song et al, based on measuring the H atom fragment, with our P(E T ) in Figure 8, determined from TOF spectra for the C 7 H 6 counterfragment.…”

Ultraviolet photodissociation dynamics of the n-propyl and i-propyl radicals

“…33,[38][39][40] The pulsed n-propyl and ipropyl radical beams were generated by photolyzing suitable precursors seeded in He or Ar (at ∼5% concentration and a total pressure of 120 kPa) with the 193-nm radiation of an ArF excimer laser focused in front of the pulse nozzle. 1-nitropropane (Aldrich, purity ≥98.5%) and 4-heptanone (Aldrich, purity 98%) were used as precursors for the n-propyl radical, and 2-nitropropane (Aldrich, purity ≥97%) and 2,4dimethyl-3-pentanone (Aldrich, purity 98%) for the i-propyl radical.…”

Isomer‐Selective Generation and Spectroscopic Characterization of Picolyl Radicals

“…[4] Rate constants are required to predict, for example,t otal ignition delays in combustion engines [27] and to understand the formation of PA NHs.I ti s expected that the loss of ahydrogen atom, acetylene,orHCN will compete in the decomposition of picolyl radicals.F or benzyl, Hatom loss leading to fulvenallene is the primary dissociation pathway. [28] Interestingly,t he formation of nitrogen-substituted fulvenallenes from picolyl has been detected at higher pyrolysis temperatures and will be the subject of future work.…”