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The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CH3CH2) [D. V. Chicharro et al., Chem. Sci., 2019, 10, 6494] is interrogated in the photodissociation of the ethyl isotopologues CD3CD2, CH3CD2 and CD3CH2 through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated in situ from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the Cα-H/D and Cβ-H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/D-atoms become accessible. High-level ab initio calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes – the so-called vibrational promoting modes – is discussed. [ABSTRACT FROM AUTHOR]

The C–C bond dissociation channel of the ethyl (CH 3 –CH 2 ) radical is investigated at 197.4, 199.98 and 201 nm by velocity map imaging and resonance enhanced multiphoton ionisation (REMPI) detection of the produced CH 3 fragments. On the light of high-level abinitio calculations performed in this work, up to four dissociation pathways are identified leading, respectively, to ground state CH 3 radicals in correlation with CH 2 fragments in the first four electronic states. The major pathway is associated with threshold dissociation over an exit barrier located at the point of a conical intersection between the initially populated 3 p A ′ state and a dissociative valence state. As discussed in the text, the concomitance of both features opens the narrow window in which C–C bond dissociation occurs for ethyl and, presumably, for other similar alkyl radicals. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics and photofragment alignment of bromoiodomethane (CH 2 BrI) have been studied at 193 nm using a double experimental and theoretical approach. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectrum of gas phase CH 2 BrI has been measured in the photon energy range of 5-11 eV using the VUV Fourier transform spectrometer (FTS) at the VUV beamline DESIRS of the synchrotron SOLEIL facility. The slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of the Br( 2 P J ) and I( 2 P J ) (with J = 3/2 and 1/2 for Br/I and Br*/I*, respectively) atomic photofragments have been used to produce experimental translational energy and angular distributions, which were analyzed to deliver, on one hand, the partitioning of the available energy among the different degrees-of-freedom of the photofragments and, on the other, the photofragment polarization in terms of a q k ( p ) alignment parameters. The experimental measurements were rationalized in terms of high-level ab initio calculations of vertical excitation energies, transition dipole moments and potential energy curves (PECs) along different reaction coordinates to provide a complete picture of the photodissociation dynamics. The results indicate that for excitation at 193 nm, prompt C-X cleavage (with X being either halogen atom, Br or I) competes with fast internal conversion and consequent stochastic dissociation in lower electronic states. In the case of the CH 2 Br + I( 2 P 3/2 )/I*( 2 P 1/2 ) channels, the dynamics are greatly biased toward the stochastic dissociation process due to both the particular PECs landscape and the unfavored excitation of the CH 2 BrI ensemble with respect to the C-I molecular axis at this excitation energy. The ab initio PECs provide a tentative path for the fast dissociation process in either case. For the C-Br bond breakage, excitation to the 13 A ' electronic state and predissociation through the 11 A '/11 A ″ or 12 A '/12 A ″ states, leading to direct dissociation through the 10 A '/9 A ″ states, appear as the most consistent dynamics. For the C-I channel, predissociation does not become a reliable possibility and a fast internal conversion may precede dissociation through the repulsive 6 A '/6 A ″ and 4 A '/4 A ″ states. The large content of rotational and vibrational excitation of the polyatomic cofragments is justified through the soft impulsive model and the geometrical changes produced along the dissociation pathway. Strikingly, the a q k ( p ) alignment parameters obtained for the Br( 2 P 3/2 ) and I( 2 P 3/2 ) photoproducts indicate that the rotational angular momentum of the CH 2 X (X = I or Br) cofragment appears highly constrained along the recoil direction. Finally, this work presents a highly plausible explanation for the branching ratio of secondary dissociation processes in the photodynamics of CH 2 BrI at 193 nm.