Velocity map imaging of the dynamics of the CH3 + HCl -> CH4 + Cl reaction using a dual molecular beam method

International audienceThe reactions CH3 + HCl → CH4 + Cl(²P_3/2) and CD₃ + HCl → CD₃H + Cl(²P_3/2) have been studied by photo-initiation (by CH₃I or CD₃I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(²P</sub>3/2</sub>) atoms were detected using resonance enhanced multi-photon ionisation and velocity map imaging, revealing product translational energy and angular scattering distributions in the centre-of-mass frame. Image analysis is complicated by the bimodal speed distribution of CH₃ (and CD₃) radicals formed in coincidence with I(²P_3/2) and I(²P_1/2) atoms from CH₃I (CD₃I) photodissociation, giving overlapping Newton diagrams with displaced centre of mass velocities. The relative reactivities to form Cl atoms are greater for the slower CH₃ speed group than the faster group by factors of ~1.5 for the reaction of CH₃ and ~2.5 for the reaction of CD₃, consistent with the greater propensity of the faster methyl radicals to undergo electronically adiabatic reactions to form Cl(²P_1/2). The average fraction of the available energy becoming product translational energy is = 0.48 ± 0.05 and 0.50 ± 0.03 for reaction of the faster and slower sets of CH₃ radicals, respectively. The Cl atoms are deduced to be preferentially forward scattered with respect to the HCl reagents, but the angular distributions from the dual beam imaging experiments require correction for under-detection of forward scattered Cl products

Perspective:How can ultrafast laser spectroscopy inform the design of new organic photoredox catalysts for chemical and materials synthesis?

Photoredox catalysis of chemical reactions, using light-activated molecules which serve as electron donors or acceptors to initiate chemical transformations under mild conditions, is finding widespread use in the synthesis of organic compounds and materials. The transition-metal-centred complexes first developed for these photoredox-catalysed applications are steadily being superseded by more sustainable and lower toxicity organic photocatalysts. While the diversity of possible structures for photoredox-active organic molecules brings benefits of design flexibility, it also presents considerable challenges for optimization of the photocatalyst molecular architecture. Transient absorption spectroscopy over timescales from the femtosecond to microsecond domains can explore the detailed mechanisms of activation and reaction of these organic photocatalysts in solution and, by linking their dynamical properties to their structures, has the potential to establish reliable design principles for future development of improved photocatalysts

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors using Time-Resolved Infrared Spectroscopy

[Image: see text] The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide–solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C–H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm(–1)) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by −19 cm(–1) for interactions with ethanol, −14 cm(–1) for chloroform, −10 cm(–1) for dichloromethane, −9 cm(–1) for acetonitrile or cyclohexane, and −16 cm(–1) for Li(+) coordination, allowing the time evolution of the ylide–solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp(2) character found for lithium-ion coordination