Speaker
Description
Following UV photoexcitation, organic diazo compounds can eliminate N$_{2}$ by C=N bond dissociation to form highly reactive singlet carbene intermediates. In solution, these carbenes rapidly react with nucleophilic solvents such as alcohols or ethers to form zwitterionic ylides. If the starting molecules are $\alpha$-diazocarbonyl compounds (with a carbonyl group adjacent to the diazo group), a second reactive pathway competes in which Wolff rearrangement makes a ketene containing a >C=C=O group. The N$_2$ dissociation and Wolff rearrangement are either concerted, or occur in a stepwise fashion via a carbene intermediate.
Here, outcomes will be presented from ultrafast time-resolved infrared (TRIR) spectroscopy studies of the competing rearrangement and reaction pathways for UV-photoexcited $\alpha$-diazocarbonyl compounds in various organic solvents. The TRIR measurements reveal both ketene production via Wolff rearrangement and carbene reactions with the solvent to form ylides [1]. Subsequent shifts in the ylide TRIR spectral bands show the response of the solvent to the reactions making these zwitterionic solute molecules [2]. These studies of $\alpha$-diazocarbonyl photochemistry in solution will be contrasted with the ultrafast structural dynamics observed for gas-phase molecules using time-resolved X-ray scattering at the LCLS-II X-ray free electron laser, and with computational simulations of the non-adiabatic photochemical dynamics using trajectory surface hopping methods.
Acknowledgements
We are grateful for the award of CXI beamtime L-10227 at LCLS-II in November 2023, and to our many collaborators who contributed to the collection of the ultrafast X-ray scattering data. The Bristol group thanks EPSRC for funding via Programme Grant EP/V026690/1.
References
[1] R. Phelps and A.J Orr-Ewing, J. Am. Chem. Soc. 2020, 142, 7836
[2] R. Phelps and A.J Orr-Ewing, J. Am. Chem. Soc. 2022, 144, 9330
