216 research outputs found
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(<sup>2</sup>P<sub>3/2</sub>) and CD<sub>3</sub> + HCl → CD<sub>3</sub>H + Cl(<sup>2</sup>P<sub>3/2</sub>) have been studied by photo-initiation (by CH<sub>3</sub>I or CD<sub>3</sub>I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(<sup>2</sup>P</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 CH<sub>3</sub> (and CD<sub>3</sub>) radicals formed in coincidence with I(<sup>2</sup>P<sub>3/2</sub>) and I(<sup>2</sup>P<sub>1/2</sub>) atoms from CH<sub>3</sub>I (CD<sub>3</sub>I) photodissociation, giving overlapping Newton diagrams with displaced centre of mass velocities. The relative reactivities to form Cl atoms are greater for the slower CH<sub>3</sub> speed group than the faster group by factors of ~1.5 for the reaction of CH<sub>3</sub> and ~2.5 for the reaction of CD<sub>3</sub>, consistent with the greater propensity of the faster methyl radicals to undergo electronically adiabatic reactions to form Cl(<sup>2</sup>P<sub>1/2</sub>). 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 CH<sub>3</sub> 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
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
Velocity map imaging of the dynamics of reactions of Cl atoms with neopentane and tetramethyl silane
Distinguishing Population and Coherence Transfer Pathways in a Metal Dicarbonyl Complex Using Pulse-Shaped 2DIR Spectroscopy
Solvent response to fluorine-atom reaction dynamics in liquid acetonitrile
Solvent restructuring and vibrational cooling follow exothermic fluorine-atom reactions in acetonitrile.</p
Differential and integral cross sections for the rotationally inelastic scattering of methyl radicals with H-2 and D-2
Ultrafast Observation of a Photoredox Reaction Mechanism:Photoinitiation in Organocatalyzed Atom-Transfer Radical Polymerization
Photoredox
catalysis has driven a revolution in the field of organic
chemistry, but direct mechanistic insights into reactions of genuine
synthetic utility remain relatively scarce. Herein we report ultrafast
time-resolved spectroscopic observation of a bimolecular organocatalyzed
photoredox reaction, from catalyst photoexcitation through to photoinduced
electron transfer (PET) and intermediate formation, using transient
vibrational and electronic absorption spectroscopy with sub-picosecond
time resolution. Specifically, the photochemical dynamics of initiation
in organocatalyzed atom-transfer radical polymerization (O-ATRP) are
elucidated for two complementary photoredox organocatalysts (<i>N</i>,<i>N</i>-diaryl-5,10-dihydrophenazines). Following
photoexcitation, a dissociative bimolecular electron transfer is observed
from the first excited singlet state of both photocatalysts to methyl
2-bromopropionate in dichloromethane, toluene, and dimethylformamide.
The photocatalyst excited donor state, ground state, and radical cation
are tracked in real time alongside the debrominated radical fragment.
Our work challenges previously proposed mechanisms of initiation in
O-ATRP and indicates that PET from short-lived excited singlet states
can exert control of polymer molecular weight and dispersity by suppressing
the steady-state concentration of the reactive debrominated radical.
More broadly, we aim to demonstrate the potential of ultrafast absorption
spectroscopy to observe directly transient, open-shell intermediates
in mechanistic studies of photoredox catalysis
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