To Boldly Look Where No One Has Looked Before: Identifying the Primary Photoproducts of Acetylacetone

Journal Article · Wed Jun 05 00:00:00 EDT 2019 · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

DOI:https://doi.org/10.1021/acs.jpca.9b04640· OSTI ID:1528999

Antonov, Ivan O. ^[1]; Voronova, Krisztina ^[2]; Chen, Ming -Wei ^[3]; Sztaray, Balint ^[4]; Hemberger, Patrick ^[5]; Bodi, Andras ^[5]; Osborn, David L. ^[1]; Sheps, Leonid ^[1]

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1. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
2. Univ. of the Pacific, Stockton, CA (United States); Univ. of Nevada, Reno, NV (United States)
3. Sandia National Lab. (SNL-CA), Livermore, CA (United States); KLA-Tencor Corp., Milpitas, CA (United States)
4. Univ. of the Pacific, Stockton, CA (United States)
5. Paul Scherrer Inst. (PSI), Villigen (Switzerland)

We investigate the gas-phase photochemistry of the enolone tautomer of acetylacetone (pentane-2,4-dione) following S2(ππ*) ← S_o excitation at λ = 266 and 248 nm, using three complementary time-resolved spectroscopic methods. Contrary to earlier reports, which claimed to study 1-photon excitation of acetylacetone and found OH and CH₃ as the only important gas-phase products, we detect fifteen unique primary photoproducts and demonstrate that five of them, including OH and CH₃, arise solely by multiphoton excitation. We assign the 1-photon products to six photochemical channels and show that the most significant pathway is tautomerization to the diketone form, which is likely an intermediate in several of the other product channels. Furthermore, we measure the equilibrium constant of this keto-enol tautomerization on S0 from 320 to 600 K and extract ΔH = 4.1 ± 0.3 kcal∙mol^–1, and ΔS = 6.8 ± 0.5 cal∙mol^–1∙K^–1 using a van’t Hoff analysis. We correct the C–OH bond dissociation energy in acetylacetone, previously determined as 90 kcal∙mol^–1 by theory and experiment, to a new value of 121.7 kcal∙mol^–1. Here, our experiments and electronic structure calculations provide evidence that some of the product channels, including phototautomerization, occur on S_o, while others likely occur on excited triplet surfaces. Although the strong oscillator strength of the S₂ ← S_o transition results from the (ππ*) excitation of the C=C–C=O backbone, similar to conjugated polyenes, the participation of triplets in the dissociation pathways of acetylacetone appears to have more in common with ketone photochemistry.

• Research Organization: Sandia National Laboratories (SNL-CA), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC04-94AL85000
• OSTI ID: 1528999
• Report Number(s): SAND--2019-3492J; 673899
• Journal Information: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Journal Name: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory Journal Issue: 26 Vol. 123; ISSN 1089-5639
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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