The Singlet-Triplet Gap of Pyruvic Acid

Abstract

Understanding the gas-phase photochemistry of pyruvic acid is crucial to assessing its role and evolution in the atmosphere and relies on knowledge of the relative energy gaps between the atmospherically relevant excited electronic states of the molecule. However, accurate determination of these gaps, particularly between the lowest excited singlet and triplet states, has remained elusive due to the challenge of directly interrogating triplet states of the isolated molecule. In this work, we combine anion photoelectron spectroscopy and computational photochemistry to determine that the adiabatic S₁-T₁ energy gap of pyruvic acid is 0.29 ± 0.04 eV. This study provides a reference value for the singlet-triplet energy gap of pyruvic acid and validates an approach that combines theory and experiment to determine energy gaps for volatile organic compounds of atmospheric interest.

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(a) Schematic representation of the deactivation pathways of pyruvic acid (PA) in the gas phase , upon absorption of UVA light and the photodetachment of the radical anion of pyruvic acid (PA^•–) in its doublet ground state (D₀). Vertical solid arrows show the photodetachment of PA^•– and the formation of neutral PA in S₀, S₁, or T₁ upon electron loss. (b) Schematic of anion photoelectron spectroscopy at two different photon energies (hν). The asterisk identifies the 0–0 transition, which has higher resolution at lower hv.

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(a) Experimental photoelectron spectrum of PA^•– for hν = 2.50 (left) and 4.66 eV (right) in black. (b) Calculated vibrationally resolved photoelectron spectra for the D₀ → S₀, D₀ → T₁, and D₀ → S₁ transitions of PA^•– are shown in red, green, and blue, respectively, as stick spectra. Calculated spectra were shifted to align with the experimental spectra. Included in (a) are the calculated spectra convolved with experimental resolution. Highly contributing transitions are identified with a label n^y where n is the vibrational mode of the corresponding state of the neutral system, and y is the excited quanta. Relevant normal modes of PA are shown with arrows representing atomic displacements. The inset panels show photoelectron spectra acquired using hν slightly above the 0–0 transition energy for (c) D₀ → S₀ and (d) D₀ → T₁. The asterisk highlights the peak assigned to the 0–0 adiabatic detachment energy.