Stanford Synchrotron Radiation Lightsource

Speaker:  Basile F.E. Curchod and Andrew J. Orr-Ewing, University of Bristol

Program Description:

Pyruvic acid (CH3C(O)CO2H) is the simplest example of an a-keto acid, and is formed in the Earth’s atmosphere by oxidation of isoprene emitted in high quantities from trees and plants.  It is present in the troposphere both in the gas phase and dissolved in aqueous cloud and aerosol droplets.   Because of its weak p* ¬ n absorption bands at wavelengths longer than 290 nm, pyruvic acid is photochemically active at wavelengths of solar radiation that penetrate the stratospheric ozone layer and reach the troposphere. The known photochemical pathways of pyruvic acid differ in the gas and aqueous phases, and for the protonated and deprotonated forms of the carboxylic acid group. For example, in the gas phase, pyruvic acid photodissociates to CO2 and hydroxymethylcarbene via electron-coupled proton transfer and C-C bond cleavage on singlet potential energy surfaces [1-4].  In contrast, in aqueous solution, oligomerization pathways have been identified following intersystem crossing to the manifold of excited triplet states and intermolecular hydrogen-atom transfer reactions [5].  Here, we will present computational and experimental studies of the photochemical dynamics of pyruvic acid initiated by absorption of wavelengths longer than 300 nm to contrast the non-adiabatic excited state dynamics in the gas phase and in solution.  The computational studies use electronic structure methods to characterize the ground and excited state potential energy surfaces and their crossings, combined with trajectory surface hopping simulations of the nuclear dynamics.  The experimental measurements use transient absorption spectroscopy on timescales from 100 fs – 1 ms to observe the pH-dependent relaxation mechanisms of photoexcited pyruvic acid in water.   

[1] A.E. Reed-Harris et al., J. Phys. Chem. A 2016, 120, 10123

[2] X.P. Chang et al., J. Chem. Phys. 2014, 141, 154311

[3] B.R. Samanta et al., Phys. Chem. Chem. Phys. 2021, 23, 4107

[4] L. Hutton and B.F.E. Curchod, ChemPhoto-Chem 2022, 6, e202200151

[5] R.J. Rapf et al. J. Phys. Chem. A 2017, 121, 4272